Transcriber’s Notes
Text printed in italics has been transcribed _between underscores_,
bold face text ~between tildes~. Small capitals have been replaced
with ALL CAPITALS. _{text} and ^{text} represent subscript and
superscript texts respectively.
More Transcriber’s Notes may be found at the end of this text.
THE RARE EARTHS
THE RARE EARTHS
THEIR OCCURRENCE, CHEMISTRY,
AND TECHNOLOGY
BY
S. I. LEVY
B.A. (CANTAB.), B.SC. (LOND.), A.I.C.
LATE HUTCHINSON RESEARCH STUDENT OF ST. JOHN’S COLLEGE
CAMBRIDGE
WITH ILLUSTRATIONS
LONDON
EDWARD ARNOLD
1915
[All rights reserved]
PREFACE
During the thirty years which have elapsed since Dr. Auer’s application
of the rare earths to the production of artificial light, the
incandescent mantle industry has developed to an extent which gives it a
prominent place among those chemical industries which may be considered
essential to modern civilisation. This technical development has in turn
assisted and stimulated the scientific examination of the elements of
this group, with the result that ordered and accurate knowledge is
beginning to replace the confused and uncertain data which had been
collected by earlier workers in the field. These advances have served to
emphasise the scientific interest and importance of the rare earth
group, and the difficulty of bringing it into relation with the other
elements. The relatively scant attention devoted to the study of this
province of inorganic chemistry by teachers and students in England is
probably due no less to the difficulty in classification, and the
uncertainty with regard to the homogeneity and individuality of the
various members of the family--an uncertainty by no means entirely
removed even now--than to the fact that the very extensive literature on
the subject is somewhat confused and difficult of access, especially to
those unfamiliar with the French and German languages.
The present work is intended to give a general but fairly comprehensive
account of the rare earth group. In accordance with general usage, the
elements zirconium and thorium have been included, though these are now
recognised as falling outside the limits of the rare earth group proper.
The inclusion of titanium, which chemically is so far removed from the
cerium and yttrium elements, has been considered desirable, not only on
account of its general occurrence in the rare earth minerals, and its
position in Group IVB with zirconium, cerium, and thorium, but also on
account of its increasing chemical and technical interest, and its use
in the ordinary quantitative laboratory operations.
Though the nature of the matter embraced has rendered the division into
three parts desirable, the whole subject has been treated primarily from
the chemical standpoint. In view, however, of the occurrence of
considerable quantities of monazite within the British Empire, and of
the possibility that in the near future the Brazilian fields will not
remain the sole source of thorium nitrate, stress has been laid on the
technical aspect, which is more especially developed as regards the
production of monazite and the incandescent mantle industry in Chapters
VII and XVII-XX.
In the preparation of Part I full use has been made of Dana’s
indispensable ‘System of Mineralogy,’ as well as of the encyclopædic
‘Handbuch’ of Hintze, whilst for Part II the excellent monograph of R.
J. Meyer, in Abegg’s ‘Handbuch,’ Vol. III, Div. I, and the work of the
same author and Hauser, ‘Die Analyse der seltenen Erden und der
Erdsäuren,’ Vols. XIV-XV of ‘Die Chemische Analyse,’ have been of
service.
I have great pleasure in expressing my gratitude to Mr. A. Hutchinson,
of Pembroke College, Cambridge, who has kindly read for me the
manuscript of Part I, and suggested improvements; to Dr. H. J. H.
Fenton, of Christ’s College, who has given me similar assistance in Part
II; and to Dr. S. Ruhemann, of Gonville and Caius College, who has read
Parts II and III. I am also greatly indebted to Mr. E. J. Holmyard, of
Sidney Sussex College, who helped me with the preparation of Part II;
and to Mr. H. M. Spiers, of Gonville and Caius College, who read the
proofs for me with special thoroughness and care.
I have also to thank Professor Soddy and his publishers, Messrs.
Longmans, Green & Co., for kind permission to reproduce from ‘The
Chemistry of the Radio-Elements’ the diagram on p. 138.
S. I. LEVY.
CONTENTS
PART I
_OCCURRENCE OF THE RARE EARTHS_
CHAPTER PAGE
I. THE NATURE OF THE MINERALS AND THEIR MODE OF OCCURRENCE 1
II. THE SILICATES 30
(_a_) _Silicates of Yttrium and Cerium Metals_--Cerite;
Gadolinite, Glowing of Minerals; Allanite,
Hellandite, Thalénite and Thortveitite; etc.
(_b_) _Silicates of Thorium and Zirconium_--Thorite,
Zircon, Naegite; etc.
(_c_) _Mixed Silicates_--Eudialyte, Beckelite; etc.
III. THE TITANO-SILICATES AND TITANATES 52
(_a_) _Titano-silicates_--Yttrotitanite, Titanite; etc.
(_b_) _Titanates_--Yttrocrasite, Delorenzite, Ilmenite;
etc.
IV. THE TANTALO-COLUMBATES 60
(_a_) _Containing no Titanium Dioxide_--Samarskite
(Annerödite), Plumboniobite, Yttrotantalite,
Fergusonite, Sipylite; etc.
(_b_) _Containing Titanium Dioxide_--Æschynite; the
isodimorphous series Euxenite, Polycrase,
Blomstrandine, Priorite; Risörite, Wiikite; etc.
V. THE OXIDES AND CARBONATES 72
(_a_) _Oxides_--Uraninite, Thorianite, Baddeleyite;
Rutile, Anatase and Brookite; etc.
(_b_) _Carbonates_--Lanthanite; Parisite (Synchisite),
Cordylite; etc.
VI. THE PHOSPHATES AND HALIDES 82
(_a_) _Phosphates_--Monazite, Xenotime (Hussakite); etc.
(_b_) _Halides_--Yttrocerite, Yttrofluorite; etc.
VII. THE MONAZITE SANDS 90
VIII. RADIOACTIVITY OF THE MINERALS 99
PART II
_THE CHEMISTRY OF THE ELEMENTS_
IX. GENERAL PROPERTIES OF THE CERIUM AND YTTRIUM GROUPS 111
X. GENERAL METHODS OF SEPARATION 142
XI. THE CERIUM GROUP--CERIUM 156
XII. THE CERIUM GROUP (CONTINUED)--LANTHANUM, PRASEODYMIUM,
NEODYMIUM, AND SAMARIUM 168
XIII. THE TERBIUM GROUP 184
XIV. THE ERBIUM AND YTTERBIUM GROUPS--YTTRIUM AND SCANDIUM 194
XV. THE GROUP IVA ELEMENTS--TITANIUM 219
XVI. THE GROUP IVA ELEMENTS (CONTINUED)--ZIRCONIUM AND
THORIUM 238
PART III
_THE TECHNOLOGY OF THE ELEMENTS_
XVII. THE INCANDESCENT MANTLE INDUSTRY--HISTORICAL AND GENERAL
INTRODUCTION 265
XVIII. THE CHEMICAL TREATMENT OF MONAZITE 275
XIX. THE MANUFACTURE OF MANTLES FROM COTTON AND RAMIE 291
XX. ARTIFICIAL SILK--ITS PRODUCTION AND USE IN THE MANTLE
INDUSTRY 301
XXI. OTHER TECHNOLOGICAL USES OF THE CERIUM AND YTTRIUM
ELEMENTS, ZIRCONIUM AND THORIUM 313
XXII. THE INDUSTRIAL APPLICATIONS OF TITANIUM AND ITS COMPOUNDS 325
INDEX 342
TABLE OF ABBREVIATIONS EMPLOYED IN THE REFERENCES
_Abstr. Chem. Soc._ Abstracts in Journal of the Chemical
Society.
_Amer. Chem. J._ American Chemical Journal.
_Amer. J. Sci._ American Journal of Science.
_Annalen_ Justus Liebig’s Annalen der Chemie.
_Ann. Chim. Phys._ Annales de Chimie et de Physique.
_Astrophys. J._ Astrophysical Journal.
_Ber._ Berichte der Deutschen chemischen
Gesellschaft.
_Berz. Jahres._ Berzelius’ Jahresbericht über die
Fortschritte der Chemie und Mineralogie.
_Bull. Imp. Inst._ Bulletin of the Imperial Institute.
_Bull. Soc. chim._ Bulletin de la Société chimique de
France.
_Bull. Soc. franc. Min._ Bulletin de la Société française de
Minéralogie.
_Bull. Soc. franc. Photog._ Bulletin de la Société française de
Photographie.
_Bull. U. S. Geol. Survey_ Bulletin of the United States Geological
Survey.
_Cass. Mag._ Cassier’s Magazine.
_Centr. Min._ Centralblatt für Mineralogie, Geologie
und Paläontologie.
_Chem. Eng._ Chemical Engineer.
_Chem. Ind._ Chemische Industrie.
_Chem. News_ Chemical News.
_Chem. Zeitg._ Chemiker Zeitung.
_Chem. Zentr._ Chemisches Zentralblatt.
_Compt. rend._ Comptes rendus hebdomadaires des Séances
de l’Académie des Sciences.
_Dingl. Polyt. J._ Dingler’s Polytechnisches Journal.
_D. R. P._ Deutsche Reichspatentschrift.
_E._ English Patent Specification.
_Elect. chem. Ind._ Electrochemical Industry (since 1904,
Electrochemical and Metallurgical
Industry).
_F._ Brevet d’Invention de la République
Française.
_J. Amer. Chem. Soc._ Journal of the American Chemical
Society.
_J. Gasbel._ Journal für Gasbeleuchtung.
_J. Gaslighting_ Journal of Gaslighting.
_J. Ind. Eng. Chem._ Journal of Industrial and Engineering
Chemistry.
_J. pr. Chem._ Journal für practische Chemie.
_J. Russ. Phys. Chem. Soc._ Journal of the Physical and Chemical
Society of Russia.
_J. Soc. Chem. Ind._ Journal of the Society of Chemical
Industry.
_Met. Chem. Eng._ Metallurgical and Chemical Engineering.
_Min. Mag._ Mineralogical Magazine and Journal of
the Mineralogical Society.
_Monats._ Monatshefte für Chemie und verwandte
Theile anderer Wissenschaften.
_Phil. Mag._ Philosophical Magazine.
_Phil. Trans._ Philosophical Transactions of the Royal
Society of London.
_Pogg. Ann._ Poggendorff’s Annalen der Physik und
Chemie.
_Proc. Amer. Acad._ Proceedings of the American Academy.
_Proc. Chem. Soc._ Proceedings of the Chemical Society.
_Proc. Roy. Soc._ Proceedings of the Royal Society.
_Prog. Age_ Progressive Age (now Gas Age).
_Publ. Astrophys. Observ. Publikationen des Astrophysikalischen
Potsdam_ Observatoriums zu Potsdam.
_Schweigg._ J. Schweigger’s Journal für Chemie und
Physik.
_Sitzungsber. kaiserl. Akad. Sitzungsberichte der kaiserlich Akademie
Wiss. Wien_ der Wissenschaften zu Wien.
_Sitzungsber. königl. Akad. Sitzungsberichte der königlich Akademie
Preussischen Wiss. Berlin_ der Wissenschaften zu Berlin.
_Stahl Eisen_ Stahl und Eisen.
_Trans. Amer. El. chem. Soc._ Transactions of the American
Electrochemical Society.
_Trans. Amer. Inst. Min. Transactions of the American Institute
Eng._ of Mining Engineers.
_Trans. Chem. Soc._ Transactions of the Chemical Society.
_Tsch. Min. Mitt._ Tschermak’s Mineralogische
Mittheilungen.
_U. S. Geol. Survey_ United States Geological Survey--Mineral
Resources of the United States.
_U. S. P._ United States Patent Specification.
_Zeitsch. anal. Chem._ Zeitschrift für analytische Chemie.
_Zeitsch. angew. Chem._ Zeitschrift für angewandte Chemie.
_Zeitsch. anorg. Chem._ Zeitschrift für anorganische Chemie.
_Zeitsch. Elektrochem._ Zeitschrift für Elektrochemie.
_Zeitsch. Kryst. Min._ Zeitschrift für Krystallographie und
Mineralogie.
_Zeitsch. physikal. Chem._ Zeitschrift für physikalische Chemie,
Stöchiometrie und Verwandtschaftslehre.
_Zeitsch. pr. Geol._ Zeitschrift für praktische Geologie.
_Zeitsch. wiss. Photochem._ Zeitschrift für wissenschaftliche
Photographie, Photophysik und
Photochemie.
INTRODUCTION
By SIR WILLIAM CROOKES, O.M., F.E.S.
The mysterious group of substances to which have been given the title of
“rare earths” has long been the subject of my special study, and no one
knows better the magnitude of the difficulties encountered in the
investigation, or realises more clearly the comparative insignificance
of the knowledge we have acquired. The rare earths constitute the most
striking example of the association of chemical substances with others
which are closely allied to themselves, and from which they are
separable only with extreme difficulty. They form a group to themselves,
sharply demarcated from the other elements, and it is my belief that by
following the study of them to the utmost limits, we may arrive at the
explanation of what the chemical elements really are and how they
originated, and discover the reasons for their properties and mutual
relations. When this knowledge has been wrested from Nature chemistry
will be established upon an entirely new basis. We shall be set free
from the need for experiment, knowing _a priori_ what the result of each
and every experiment must be; and our knowledge then will as much
transcend our present scientific systems as the knowledge of the skilled
mathematician of the present day exceeds that of primitive man, counting
upon his fingers. The great problem of the nature and genesis of the
elements is approaching solution, and when the consummation is reached
it will undoubtedly be found that the study of the rare earths has been
an important factor in bringing it about.
There has long been a need for a work in the English language dealing
historically and descriptively with these substances, and Mr. Levy’s
book is well fitted to fill the gap. The chapters on the technical
applications of the rare earths are particularly valuable, and the
chemical aspect of the incandescent lighting industry is admirably
treated. The author is to be congratulated upon having successfully
achieved an important and useful piece of work.
WILLIAM CROOKES.
_December 1914._
THE RARE EARTHS
PART I
OCCURRENCE OF THE RARE EARTHS
CHAPTER I
THE NATURE OF THE MINERALS AND THEIR MODE OF OCCURRENCE
The history of the rare earth minerals begins in the year 1751, when the
Swedish mineralogist Cronstedt described a new mineral, which he had
found intimately mixed with chalcopyrite[1] in the quarry of Bastnäs,
near Ryddarhyttan, in the province of Westmannland, Sweden. Cronstedt
gave the mineral the name Tung-sten (heavy stone); but as the name
Tenn-spat (heavy spar, or heavy mineral) had already been selected by
Wallerius (1747) for a new species from Bohemia, believed to contain
tin, the choice was not a happy one. More than fifty years after its
discovery, a new earth, now known as ceria, was isolated from
Cronstedt’s mineral, for which at the same time the name Cerite was
proposed.[2] Meanwhile, however, the Finnish chemist Johann Gadolin had
observed, in the year 1794, a new earth in a mineral discovered by
Arrhenius at Ytterby in Sweden in 1788; he called the new oxide
Ytterbia, and the mineral in which he observed it, Ytterbite. The
discovery was confirmed in 1797 by Ekeberg, who suggested the names
Yttria and Gadolinite for the oxide and mineral respectively; these
names were accepted by Klaproth, and soon came into general use.[3]
Whilst then Cerite was the first of the rare earth minerals to be
discovered, it was in Gadolinite that new elements were first
recognised, and the chemistry of the rare earths began in 1794 with
Gadolin’s observation.
[1] Chalcopyrite, or Copper pyrites, is a mixed sulphide of iron and
copper, of the approximate formula CuFeS₂.
[2] For the history of the name Tungsten, see under the mineral
Cerite, Ch. II.
[3] The history of these names will be found somewhat more fully under
Gadolinite, Ch. II.
During the nineteenth century a considerable number of rare earth
minerals was discovered and analysed; the quantities of the minerals
observed, however, were so small that the name ‘Rare earths,’ applied to
the new oxides found, was in every sense justified. Until the year 1885,
though by that time the scientific interest of the group had been fully
demonstrated by the discovery of several new elements, it was supposed
that the minerals were almost entirely confined to a few scattered
localities in Scandinavia and the Ural mountains. In that year Dr. Auer
von Welsbach announced his application of the rare earths to the
manufacture of incandescent mantles. Immediately there was a great
demand for raw material for the preparation of thoria and ceria. The
agents of the Welsbach Company visited all the important mining centres
of Europe and America, intent on a search which shortly made it clear
that the metals of the so-called ‘rare earths’ are really quite widely
distributed in nature. The chief commercial deposits are the monazite
sands of the Carolinas, the Idaho basin, and Brazil, the gem-gravels of
Ceylon, and the remarkable deposits of gadolinite and allied minerals at
Barringer Hill in Texas.
Whilst deposits of commercial importance are not very common, improved
scientific methods and more careful search have shown that in traces the
rare earths are of exceedingly wide distribution. Sir William Crookes
has shown that yttria earths are often present in calcite and in coral;
whilst Headden[4] noted that quite considerable amounts (up to 0·03 per
cent.) were present in a yellow phosphorescent variety of calcite from
Colorado. Similarly Humphreys[5] found that fluorspar usually contains
traces of yttrium, whilst one or two phosphorescent varieties contain
quantities varying up to 0·05 per cent. The presence of yttria elements
in phosphorescent varieties of calcite is interesting, and some
connection has been suggested; there is, however, no positive ground for
the belief in such a relation.
[4] _Amer. J. Sci._, 1906, [iv.], ~21~, 301.
[5] _Astrophys. J._, 1904, ~20~, 266.
More recently Eberhard[6] has found very considerable quantities of rare
earths in cassiterite (tin dioxide, SnO₂) and wolframite [an iron
manganese tungstate, (Fe,Mn)WO₄]. A specimen of wolframite from the
Erzgebirge was found to contain nearly 0·4 per cent. of rare earths,
over half of this quantity being scandium oxide. A process which is
readily susceptible of commercial application has been worked out by R.
J. Meyer,[7] for the extraction of scandia and the yttria earths from
the mixed oxides left after the treatment of wolframite for tungstic
acid.
[6] _Sitzungsber. königl. Akad. Wiss. Berlin_, 1908, 851; 1910, 404.
[7] Meyer, _Zeitsch. anorg. Chem._, 1908, ~60~, 134. Meyer und Winter,
_ibid._, 1910, ~67~, 398.
Using the spectroscopic method, which is capable of detecting one part
of scandia in twenty thousand, Eberhard (_loc. cit._) has found that
minute quantities of scandia and yttria earths are present in almost all
the commoner rocks and minerals. The minerals richest in scandium were
beryl, cassiterite, wolfram, the zircon minerals, and the titanates and
columbates of the ceria and yttria oxides. These results are in
agreement with the observations of Sir William Crookes,[8] who has made
the study of scandium especially his own. From the fact that scandium
was often observed unaccompanied by any other member of the rare earth
group, Eberhard rather favours Urbain’s conclusion[9] that scandium may
not be a member of the rare earth family. Spectroscopic examination has
also shown the existence of some of the rare earth elements in the sun
and stars (see Europium, p. 189).
[8] _Phil. Trans._ 1910, A, ~210~, 359.
[9] See under Scandium in Pt. II.
In view of this extraordinarily wide distribution of the rare earths in
the mineral world, it is but natural that they should be found also in
the vegetable and animal kingdoms. Tschernik[10] found 10 per cent. of
rare earths in the ash of a coal from Kutais, in the Caucasus, and
smaller quantities have been found in the ashes of various plants;
members of the group have also been identified in the human body.
[10] See Abstr. in _Zeitsch. Kryst. Min._, 1899, ~31~, 513.
Apart from the general occurrence in traces throughout the mineral
kingdom, the minerals in which the rare earths occur are not very
common; and though of fairly wide distribution, they are found usually
only in small quantities. The earliest known locality, and the most
fruitful in regard to number of species, has been the southern part of
the Scandinavian peninsula;[11] the minerals occur in the numerous
pegmatite veins traversing the granitic country-rock. The mining
district round Miask, in the Ural mountains, has also long been known as
a fruitful source. Other districts in Europe are the Harz and
Erzgebirge, the Laacher See in Prussia, Joachimsthal in Bohemia,
Dauphiné, Cornwall, etc. In the United States numerous localities are
known; the chief are in the Carolinas and Georgia, Idaho, Oregon,
California, Texas, Colorado, Virginia, Pennsylvania and Connecticut.
Many of the southern provinces of Brazil also furnish important sources;
the famous diamond fields of Minas Geraes, Matto-Grosso, Goyaz and the
surrounding provinces yield numerous species, whilst the sands along the
southern coasts of Bahia are rich in monazite, and form to-day the most
important source of the mineral. Monazite, as well as other rare earth
minerals, occurs also in South Africa. An interesting species,
plumboniobite (_q.v._), has recently been found in German East Africa.
From Australia numerous occurrences are reported, whilst in Canada only
a few districts are known to yield members of the group. In Asia
important localities are Ceylon--the famous gem-gravels being the most
accessible source--and one or two districts in Japan; monazite has been
reported recently in considerable quantities near Travancore, India.[12]
A more extended search will doubtless show that they occur in many other
places.
[11] See Brögger, _Die Mineralien der Süd-Norwegische
Granit-Pegmatitgänge_, Christiania, 1906.
[12] _Bull. Imp. Inst._, 1911, vol. ~ix~; No. 2, p. 103.
For several reasons, the rare earth minerals[13] form a group of the
highest scientific interest. In the first place, they are generally of
very complex composition, more especially with regard to their rare
earth content. Thus, whilst it sometimes happens that one or other of
the two groups of oxides (the ceria and yttria groups) may predominate
to the complete exclusion of the second, it is no uncommon thing for a
species to contain almost all the elements of the rare earth family. On
the other hand, it is very uncommon for as much as 50 per cent. of the
rare earth content to consist of any one oxide. The usual case is that a
mineral contains chiefly yttria earths with some ceria earths, or _vice
versâ_, the two sub-groups being almost always complex mixtures of
several oxides, in which occasionally one may predominate. The
remarkable similarity in chemical behaviour of the rare earth elements,
and the difficulty of separating them, correspond to this peculiarity in
their occurrence.
[13] The phrase ‘rare earth minerals’ will be used whenever it is
desired to indicate collectively those minerals of which the yttria
and ceria earths form an important constituent, as contrasted to those
in which only traces of these oxides occur. Such minerals may often
contain titanium, zirconium, or thorium, and, for convenience, the
term may be taken to include the commoner zirconium and thorium
minerals, but not the commoner titanium minerals.
A second point of even greater interest is that the rare earth minerals
are as a general rule strongly radio-active; further, it only
occasionally happens that any mineral in which the rare earths do not
form an important constituent has more than the feeblest activity; the
exceptions being, of course, those uranium minerals which do not contain
rare earths. The connection may be pushed even further; for whilst it
appears that hardly any rock or mineral possesses absolutely no
radio-activity, it is equally worthy of notice that traces of the rare
earths, if not quite universal in the mineral world, are yet normally
found in the majority of common minerals. As a natural consequence of
their activity, the rare earth minerals are also as a rule rich in
helium. These facts and the problems which they open up will be treated
more fully in a later chapter.
A point of further interest is that of the age of the rare earth
minerals. Except in a few cases where they are obviously of secondary
formation, these minerals are among the oldest known to us. They occur
usually in igneous rocks, particularly in granites which have been
considerably metamorphosed. Where erosion has occurred, they are found
in deposits of such a nature as to leave very little doubt that the
original rock was of plutonic formation and of very considerable age.
Whilst it is true, however, that the rare earth minerals are generally
of very great antiquity (none of the primary minerals being of more
recent date than the palæozoic age), Eberhard has pointed out that the
age and nature of common rocks seem to have absolutely no influence on
the traces of scandia and yttria oxides which they contain. The
geological evidence shows that the rare earth minerals are on the whole
exceedingly stable, and that they have been generally formed during the
pegmatitic alteration of granites. As early as the year 1840, Scheerer
drew attention to these facts, and to the extreme age of the rare earth
minerals; but so far his observation seems to have attracted little
attention, and no explanation has been put forward.
* * * * *
In the following chapters no attempt is made to treat the rare earth
minerals fully. An alphabetical list of all the minerals of any
importance which contain rare earths, titanium, zirconium or thorium is
given, and of these several are selected for fuller treatment. The basis
of selection has been somewhat arbitrary. Those species which are of
mineralogical importance, as well as those to which any special
historical, scientific or commercial interest attaches, have of course
been singled out; in addition, the more recently discovered species have
occasionally been considered worthy of separate mention.[14]
[14] A full list of the minerals containing rare earths known up to
1904, with an account of their properties and very full references,
will be found in the work of Dr. J. Schilling, _Das Vorkommen der
Seltenen Erden im Mineralreiche_, 1904.
It is now being realised that some knowledge of crystallography is
essential to the chemist, and for this reason short accounts of the
crystallography of the selected types have been given. Apart from this,
every effort has been made to render the mineralogy intelligible to the
student of chemistry who has devoted no attention previously to this
subject, and also to stimulate an interest in the problems of mineral
chemistry, unfortunately too often ignored by our present-day teachers.
The rare earth minerals afford good examples of some phenomena of great
interest to the chemist, as, _e.g._ Isomorphism and Solid Solution,
Dimorphism, Isodimorphism, and Molecular Change, and in one or two cases
these are treated rather fully.
No special advantages are claimed for the system of classification,
which is merely one of convenience. The minerals are divided into five
groups:--
(1) The Silicates, which are grouped into three sub-divisions.
(2) The Titano-silicates and the Titanates.
(3) The Tantalo-columbates, sub-divided into those free from titanium
and those in which titanium is present.
(4) The Oxides and Carbonates.
(5) The Halides and Phosphates.
A separate chapter has been devoted to the monazite sands, and another
to the radio-active properties of the minerals.
ALPHABETICAL LIST OF MINERALS CONTAINING TITANIUM, ZIRCONIUM, THORIUM,
OR ELEMENTS OF THE CERIUM AND YTTRIUM GROUPS.
The following list contains all but a few entirely unimportant members
of these classes of minerals. The names of those species selected for
fuller treatment are printed in heavy type, whilst names of those not
so selected, which for convenience are included under the generic term
‘Rare earth mineral,’ _i.e._ roughly all those containing Thorium, or
elements of the Cerium and Yttrium groups, and the commoner Zirconium
minerals, as distinguished from minerals containing Titanium, are
printed in italics. (See footnote on p. 4.) Their properties are given
in the following order:--
Chemical Composition and Rare Earth Content.
Crystallographic Data.
Physical Properties.
Locality, etc.
The following contractions are employed:
E = any element or elements of the cerium or yttrium groups.
Cer = oxides of the cerium metals.
Yttr = oxides of the yttrium metals.
G = Specific Gravity.
H = Hardness.
Aenigmatite.
A Titanosilicate of Fe´´ and Na, with small proportions of Fe´´´ and
Al´´´. Closely allied to the amphiboles. TiO₂ = 7-8%.
Anorthic. Habit prismatic.
G = 3·80-3·86. H = 5¹⁄₂. Black; pleochroism strong.
Greenland and S. Norway.
~Aeschynite~
A Titanocolumbate of Cerium metals, with Th, Fe, Ca, Mn, aq. Cer =
19·4-24·1; Yttr = 1·1-3·1; ThO₂ = 15·7-17·6; TiO₂ = 21-22%.
Rhombic, holosymmetric. Habit prismatic or tabular.
G = 4·9-5·7. H = 5·6. Black; opaque.
Hitterö, Norway; Miask, Urals; also in Germany and Brazil.
~Allanite~ (Orthite).
H₂O, 4R´´O, 3R´´´₂O₃, 6SiO₂, where R´´ = Ca, Fe´´, Be, and R´´´ = Al,
Fe´´´, E. An epidote containing rare earths. Cer = 3·6-51 (usually
10-25); Yttr = 0-8 (usually < 3); ThO₂ = 0-3·5%.
Monoclinic; isomorphous with epidote.
G = 3·5-4·2. H = 5¹⁄₂-6. Brown to black; opaque.
Widely distributed in Greenland and Scandinavia.
_Alvite_ (Anderbergite).
Silicate of Zr and E, with Ca, Mg, Be, Al, Cu, Zn, and aq. in small
quantities. Cer → 3·98; Yttr → 22; ZrO₂ = 30·5-61·4%.
Tetragonal; optically isotropic. Pseudomorphous after zircon.
G = 3·3-4·3. H = 5-6. Yellowish brown; transparent.
Ytterby, Sweden; Arendal, Norway; various localities in N. America.
~Anatase~ (Octahedrite).
Titanium dioxide. TiO₂ = 97-100%.
Tetragonal; habit octahedral.
G = 3·82-3·95. H = 5¹⁄₂-6. Transparent to opaque; brown to black.
Dauphiné; Bavaria; Cornwall; Norway; Brazil, etc.
_Ancylite._
4Ce(OH)CO₃ + 3SrCO₃ + 3H₂O; with Fe, Mn, Ca, F, traces. Cer = 46·3%.
Rhombic; prismatic.
G = 3·95. H = 4¹⁄₂. Brown; translucent.
Plain of Narsarsuk, Greenland.
_Annerödite._
A parallel growth of Columbite on Samarskite, once believed to be a
new species.
Corresponding to Columbite.
Arfvedsonite.
Metasilicate of Na, Ca, Fe´´, Zr; approximately
4Na₂O,3CaO,14FeO,(Al,Fe)₂O₃,21SiO₂. ZrO₂ = 1-6%.
Monoclinic--an amphibole.
G = 3·44. H = 6. Black; pleochroism strong.
S. Greenland and S. Norway.
Arizonite.
Ferric metatitanate, Fe₂O₃,3TiO₂ or Fe₂(TiO₃)₃. TiO₂ = 36·7%.
Uncertain; apparently monoclinic.
G = 4·25. H = 6-7. Dark steel-grey; opaque.
Hackberry, Arizona.
_Arrhenite._
Silico-tantalate of Yttrium metals, with Ce, Al, Fe, Ca, Be, aq. Yttr
= 33·2; Cer = 2·6; ZrO₂ = 3·4%.
Amorphous.
G = 3·68. Red; translucent to opaque.
Ytterby, Sweden.
Astrophyllite.
Titano-silicate of Fe, Al, Mn, Zr, K, Na, with aq. ZrO₂ = 1·2-4·5;
TiO₂ = 7-14%.
Rhombic. Cleavage (010) perfect.
G = 3·2-3·4. H = 3. Golden to bronze yellow; strongly pleochroic.
Brevik, Norway; El Caso Co., Colorado; Greenland.
_Auerbachite._
An impure hydrated form of Zircon, ZrSiO₄. ZrO₂ = 55·2%.
Tetragonal; isotropic. Pseudomorphous after zircon.
G = 4·06. H = 6. Brownish-grey; translucent to opaque.
Alexandrovsk, Russia.
_Auerlite._
3ThO₂,[3SiO₂,P₂O₅]6H₂O; traces of Fe, Ca, Mg, Al, CO₂, etc. SiO₂
replaced by P₂O₅/3? ThO₂ = 69·2-72·2%.
Tetragonal; probably a pseudomorph after Thorite.
G = 4·4-4·8. H = 2-3. Yellowish to orange-red.
Henderson Co., N. Carolina.
~Baddeleyite.~
ZrO₂, with small amounts of SiO₂, Fe₂O₃, Al₂O₃, CaO, etc. ZrO₂ =
96·5%.
Monoclinic.
G = 4·4-6·0. H = 6¹⁄₂. Brown; pleochroic.
São Paulo, Brazil; Rakwana, Ceylon.
_Bagrationite._
A variety of Allanite (orthite) with no important chemical difference.
Monoclinic; habit prismatic.
G = 3·84. H = 6¹⁄₂. Black; translucent to opaque.
Achmatovsk, Urals.
_Bastnäsite_ (Harmatite).
Hydrated fluocarbonate of Cerium metals, E(F)CO₃. Cer = 64-93·5; ThO₂
= 0-10%.
Hexagonal prisms, pseudomorphous after Tysonite (_q.v._); or massive.
G = 4·9-5·2. H = 4-4¹⁄₂. Yellow to brown; transparent.
Bastnäs, Sweden; Pike’s Peak, Colorado.
~Beckelite.~
Zirconosilicate of rare earths and lime, Ca₃E₄(Si,Zr)₃O₁₅. Cer = 59·7;
Yttr = 2·8; ZrO₂ = 2·5%.
Cubic, in octahedra and dodecahedra. Cubic cleavage.
G = 4·15. Brown; transparent.
Near Sea of Azov, Russia.
Benitoite.
A Titano-silicate of barium, BaTiSi₃O₉. TiO₂ = 20·1%.
Rhombohedral.
H = 6¹⁄₂-7. Colourless to blue; transparent; pleochroism strong.
Source of San Benito River, California.
~Blomstrandine.~
Dimorphous with Polycrase (_q.v._), and of same composition.
Orthorhombic; isomorphous with priorite (_q.v._).
G = 4·5-5·0; H = 6¹⁄₂. Bright black; translucent.
Hitterö and Arendal, Norway.
Blomstrandite.
Hydrated titano-columbate of U, with some Fe and Ca. TiO₂ = 10·7%.
Massive.
G = 4·17-4·25. H = 5¹⁄₂. Black; opaque.
Nohl, Sweden.
_Bodenite._
A variety of Allanite (_q.v._), rich in Al and Ca, with no Be. Yttr =
17; Cer = 18%.
Monoclinic.
As Allanite.
Boden, near Marienburg.
_Britholite._
A basic phosphosilicate of cerium metals, with Fe, Ca, Mg, Na, F. Cer
= 60·5-60·9%.
Hexagonal; habit prismatic.
G = 4·446. H = 5¹⁄₂. Brown; transparent.
Naujakasik, Greenland.
_Bröggerite._
A variety of Uraninite (_q.v._), with rare earths, Th, Pb, Fe, Ca, Si,
aq., etc. Cer = 0·4; Yttr = 1·4-4·3; ThO₂ = 4·7-6·1%. Traces of ZrO₂.
Cubic, in octahedra and dodecahedra.
G = 8·7-9·0. H = 5-6. Black; translucent to opaque.
Anneröd, near Moos, Norway.
~Brookite.~
Titanium dioxide, TiO₂ = 99-100%; trimorphous with Anatase and Rutile.
Orthorhombic.
G = 3·87-4·01. H = 5¹⁄₂-6. Brown; opaque.
Dauphiné; Urals; Switzerland; Magnet Cove, Arkansas.
_Calciothorite._
A variety of Thorite containing lime--5ThSiO₄,2Ca₂SiO₄ + 10H₂O. ThO₂ =
59·3%.
Completely amorphous.
G = 4·114. H = 4¹⁄₂. Deep red; translucent.
Islands of Läven and Arö, Langesund Fiord, Norway.
_Cappelenite._
A borosilicate of rare earth metals and barium, with traces of Th, Ca,
K, Na, aq. Approximately BaSiO₃, YBO₃. Cer = 4·2; Yttr = 52·5%.
Hexagonal; habit prismatic.
G = 4·407. H = 6-6¹⁄₂. Greenish brown; translucent.
Island of Klein-Arö, Langesund Fiord, Norway.
_Caryocerite_ (Karyocerite).
Complex fluosilicate of E, with Ta, Th, Ca; also CO₂, P₂O₅, B, Al, Fe,
Mn, U, Mg, Na, aq., etc. Approaching Melanocerite, (_q.v._), but
richer in Th. Very complex. Cer = 41·8; Yttr = 2·2; ThO₂ = 13·6; ZrO₂
= 0·5%.
Rhombohedral, but isotropic; apparently a pseudomorph after
Melanocerite (_q.v._)
G = 4·295. H = 5-6. Nut brown; translucent. Faces very brilliant, but
striated. Lustre vitreous to resinous.
Various rocks and shoals round Arö Island, Langesund Fiord, Norway.
_Castelnaudite._
A variety of Xenotime (_q.v._) containing Zr. Yttr = 60·4; ZrO₂ =
7·4%.
Tetragonal.
G = 4·5. H = 4-5. Greyish white to pale yellow.
Diamond sands of Brazil.
_Cataplejite_ (Kataplejite).
H₄(Na₂,Ca)ZrSi₃O₁₁. ZrO₂ = 29·6-40% (usually 30-33%).
Monoclinic, pseudohexagonal. Becomes truly hexagonal at 140°C.
G = 2·8. H = 6. Yellow to brown; transparent to opaque.
A blue variety is known which contains no calcium.
Islands of Langesund Fiord, Norway; Narsarsuk, Greenland.
~Cerite.~
A basic silicate of Cerium metals, with Ca and Fe. Approximately
H₃(Ca,Fe)Ce₃Si₃O₁₃. Cer = 50·7-71·8%. In a variety from Batoum,
Tschermak reports Yttr = 7·6 and ZrO₂ = 11·7%.
Orthorhombic; usually massive or granular.
G = 4·9. H = 5-6. Brown to red; translucent to opaque.
Ryddarhyttan, Sweden; Batoum, Caucasus?
_Chalcolamprite._
A silico-columbate of E, Zr, Ca, Fe, Na, K; R₂Cb₂F₂SiO₉, where R
represents various metals. E = 3·41; ZrO₂ = 5·7%.
Cubic, in small octahedra.
G = 3·77. H = 5¹⁄₂. Greenish brown; opaque. Metallic lustre (χαλκός =
Copper, λαμπρός = lustre).
Narsarsuk, S. Greenland.
_Churchite._
Hydrous phosphate of Cerium metals and Ca; Cer = 51·87%.
Monoclinic? Allegations only.
G = 3·14. H = 3¹⁄₂. Greyish; transparent to translucent.
Cornwall.
_Cleveite._
A variety of Uraninite (_q.v._) rich in rare earths and helium. Cer =
2·3-2·9; Yttr = 10·0-10·3; ThO₂ = 4·6-4·8%.
Cubic; usually massive.
G = 7·49. H = 5¹⁄₂. Black; opaque.
Arendal, Norway.
~Cordylite.~
Fluocarbonate of Cerium metals and Ba; E₂F₂Ba(CO₃)₃. Cer = 49·4%.
Hexagonal; isomorphous with Parisite (_q.v._).
G = 4·31. H = 4¹⁄₂. Yellow; transparent.
Plain of Narsarsuk, Greenland.
Cossyrite.
A variety of Aenigmatite (_q.v._) of very complex composition, TiO₂ =
6-8%.
Anorthic.
G = 3·74. H = 5. Black; opaque.
Island of Pantellaria (formerly Cossyra).
_Cyrtolite._
A pseudomorph after zircon, allied to Alvite (_q.v._).
Tetragonal.
_See_ Alvite.
Various localities in Scandinavia, and U.S.A.
_Davidite._
A Titanate of Fe, U, V, Cr, and E--uncertain formula. TiO₂ > 50; E₂O₃
= 5-10%.
Cubic--in grains and rounded crystals.
G = 4 about. Black, with brilliant lustre.
Olary, S. Australia.
~Delorenzite.~
2FeO,UO₂,2Y₂O₃,24TiO₂. Yttr = 14·63; TiO₂ = 55%.
Rhombic; habit prismatic.
G = 4·7. H = 5¹⁄₂-6. Black; translucent to opaque; lustrous.
Craveggia, Piedmont, Italy.
Derbylite.
FeO,Sb₂O₅ + 5FeO,TiO₂? TiO₂ = 35% about.
Orthorhombic; habit prismatic.
G = 4·53. H = 5. Pitch black; opaque; lustre resinous.
Tripuhy, Minas Geraes, Brazil.
Dysanalyte (Perovskite).
Approximately 6RTiO₃,R(Cb,Ta)₂O₆, where R = Ca, Fe´´. Believed by
Hauser to be merely an impure Perovskite (_q.v._). Cer = 0-5·1; TiO₂ =
41·5-59·3%.
Cubic.
G = 4·13. H = 5-6. Black; opaque.
Vogtsburg, near Baden, Germany.
Elpidite.
Na₂Zr(Si₂O₅)₃, 1¹⁄₂H₂O. ZrO₂ = 20·5%.
Orthorhombic.
G = 2·52-2·56. H = 7-8. Colourless to red; translucent.
Various localities in Greenland.
_Endeiolite._
R´´Cb₂O₆(OH)₂ + R´´SiO₃ (cf. Chalcolamprite). E₂O₃ = 4·43; ZrO₂ =
3·78%.
Cubic.
G = 3·44. H = 4. Dark chocolate-brown; transparent.
Narsarsuk, Greenland.
_Erdmannite_ (Michaelsonite).
A silicate of E and Ca, with Zr, Be, Th, Al, Fe, aq., etc. An altered
Homilite? Cer = 17·7-34·9; Yttr = 1·4-2·1; ThO₂ + ZrO₂ = 0-12%.
Amorphous; isotropic.
G = 3·01-3·39. H = 4¹⁄₂. Brown to leek-green.
Near Brevig, Norway.
_Erikite._
A phosphosilicate of E, Ca, Al, K and Na, with ThO₂, H₂O, etc. Cer =
40·5; ThO₂ = 3·3%.
Orthorhombic.
G = 3·473. H = 5¹⁄₂-6. Brown; opaque.
Julianehaab, Greenland.
~Eucolyte.~
R´₄R´´₃Zr(SiO₃)₇, where R´ = K, Na, H, and R´´ = Ce(OH), Fe, Mn, Ca,
and Zr(OCl) may replace SiO₂? A very complex mineral. ZrO₂ = 10·9-20;
Cer = 0-5·2%.
Rhombohedral.
G = 3·0-3·1. H = 5-5¹⁄₂. Red to brown; translucent. Double Refraction
strong, -ve.
Various localities in Norway.
_Eucrasite._
An altered Thorite (_q.v._) containing E, Ca, Fe, Mn, Na, Ti, H₂O,
etc. Cer = 14; Yttr = 5·9; ThO₂ = 36·0; ZrO₂ = 0·6%.
Rhombic (Paijkull). Amorphous, isotropic (Brögger).
G = 4·39. H = 4¹⁄₂-5. Brownish black; opaque.
Near Brevig, Norway.
~Eudialite.~
A variety of Eucolyte (_q.v._) of the same composition.
As Eucolyte.
G = 2·92. Double Refraction strong, +ve. Otherwise as Eucolyte.
Greenland; Lapland; Arkansas, U.S.A.
~Euxenite.~
E(CbO₃)₃,E₂(TiO₃)₃,1¹⁄₂H₂O; with U and Zr. Cer = 2·3-8·4; Yttr =
13·2-34·6; TiO₂ = 20-23%. ThO₂ + ZrO₂ usually in traces.
Orthorhombic; usually massive.
G = 4·6-5·0. H = 6¹⁄₂. Brownish-black; translucent to opaque.
Hitterö, Brevig, Jolster, Arendal, Norway; Cooglegong, Australia; N.
Carolina.
~Fergusonite.~
Approximately E₂O₃, (Cb,Ta)₂O₅, with U, Fe, Ca. Cer = 0·5-13·9; Yttr =
27·9-47·1; ThO₂ + ZrO₂ = 0-7%. [Berzelius found Cer = 36·3; Yttr = 0%
in one specimen.]
Tetragonal, polar.
G = 5·84-4·3 when largely hydrated. H = 5·6. Brown to black.
Norway; Australia; Texas, etc.
_Florencite._
A silico-phosphate of E and Al. Cer = 28% approximately.
G = 3·6. H = 5. Yellow to red. Resinous lustre.
Minas Geraes and diamond localities in Brazil.
_Fluocerite._
Basic fluoride of rare earth metals, E₂O₃,4EF₃. Cer = 81·4-82·6; Yttr
= 1·1-4·3%.
Massive. Original hexagonal mineral of Berzelius and Haidinger,
probably Tysonite (_q.v._).
G = 5·7-5·9. H = 4. Reddish yellow; opaque.
Österby, Sweden.
_Freyalite._
Silicate of E and Th, with Al, Fe, Mn, Na, aq., etc. Cer = 31·3; ThO₂
= 28·4; ZrO₂ = 6·3%.
Amorphous.
G = 4·06-4·17. H = 6. Brown; opaque; lustre resinous.
Brevig, Norway.
~Gadolinite.~
FeO, 2BeO, Y₂O₃, 2SiO₂, where Y = yttrium metals. Cer = 3·4-51·5
(usual 6-20); Yttr = 5-60 (usual 35-48)%.
Monoclinic; habit prismatic. Often amorphous and isotropic.
G = 4·0-4·5. H = 6¹⁄₂-7. Brown and green. Double Refraction strong,
+ve.
Ytterby and Fahlun, Sweden; Hitterö and Malö, Norway; Llano Co.,
Texas; Colorado, etc.
Geikielite.
(Mg,Fe´´)TiO₃. TiO₂ = 56·1-64·8%. Specimens rich in iron are called
Picroilmenite.
Massive.
G = 4 about. H = 6. Purplish or brownish black.
Ceylon.
Gorceixite.
An alumino-phosphate of alkaline and ceria earths. Cer = 0-3%.
Microcrystalline.
G = 3. H = 6. White to brown. Translucent.
Diamond sands of Brazil.
Guarinite.
Formerly supposed to be dimorphous with Titanite (_q.v._); shown by
Zambonini and Prior (1909) to be identical with Hiortdahlite (_q.v._).
Hainite.
Tantalo-silicate and titanate of Zr, Ca, Na. ZrO₂ = 29-32%.
Anorthic.
G = 3·2. H = 5. Colourless to yellow; transparent.
Bohemia.
~Hellandite.~
3H₂0, 2R´´O, 3R´´´₂O₃, 4SiO₂, where R´´ = Ca, Mg, Th/2; R´´´ = E, Al,
Fe, Mn. E₂O₃ = 40%.
Monoclinic; habit prismatic.
G = 3·70. H = 5¹⁄₂. Reddish-brown when fresh.
Lindvikskollan and Kragerö, Norway.
Hiortdahlite.
3CaSiO₃,Ca(F,OH)NaZrO₃. ZrO₂ = 21·5; TiO₂ = 1·5%.
Anorthic; habit tabular.
G = 3·27; H = 5-5¹⁄₂. Yellow, with weak pleochroism.
Island of Läven, Langesund Fiord, Norway.
_Hjelmite_ (Hielmite).
A stanno-tantalate of Ca, Mn, Fe, E, related to Yttrotantalite
(_q.v._). E₂O₃ = 1-6%.
Orthorhombic.
G = 5·82. H = 5. Black; lustre metallic.
Fahlun, Sweden.
Homilite.
(Ca,Fe)₃(BO)₂(SiO₄)₂. Sometimes with ceria earths, 0-2·6%.
Monoclinic--isomorphous with Gadolinite (? Brögger).
G = 3·34-3·38. H = 4¹⁄₂-5. Black; pleochroic.
Islands of Lanegsund Fiord, Norway.
_Hussakite_ (Xenotime).
A prismatic form of Xenotime (_q.v._), erroneously supposed to contain
> 6% SO₃.
Diamond sands of Brazil.
Hydrotitanite.
An altered Perovskite (_q.v._) with Fe´´´ and aq. TiO₂ = 82·8%.
Amorphous.
G = 3·68. H = 1-2. Yellowish grey.
Magnet Cove, Arkansas.
~Ilmenite.~
FeTiO₃; composition varies widely. TiO₂ = 3·5-52·3%.
Rhombohedral.
G = 4·5-5. H = 5-6. Black; opaque. Slightly magnetic.
Norway; Dauphiné; Bohemia; Cornwall, etc.
Ilmenorutile.
FeO,Nb₂O₅,5TiO₂? TiO₂ = 66-75%.
Tetragonal, very near to Rutile (_q.v._).
G = 4·3-5·0. H = 6-7. Brown to black; opaque.
Ilmen Mountains, Russia.
_Johnstrupite._
Silico-titanate of E, Al, Mg, Ca, Na, etc., with F and aq. Cer = 13·5;
Yttr = 1·1; TiO₂ = 7-8; ThO₂ + ZrO₂ = 3·6%.
Monoclinic, very close to Epidote.
G = 3·19-3·29. H = 5. Brownish green; weakly pleochroic.
Islands of the Langesund Fiord, Norway.
_Kainosite_ (Cenosite).
CaY₂(SiO₃)₄,CaCO₃,2H₂O, where Y = Yttrium metals. Yttr = 30-37%.
Uncertain; pseudo-hexagonal.
G = 3·38-3·41. H = 5-6. Yellowish brown.
Hitterö and province of Nordmark, Norway.
~Keilhauite~ (Yttrotitanite).
An isomorphous mixture of Titanite (_q.v._) with (E,Al,Fe)SiO₅. E₂O₃ =
5-12; TiO₂ = 26-30%.
Monoclinic; isomorphous with Titanite.
G = 3·52-3·77. H = 6¹⁄₂. Brown to black.
Various localities in Norway.
_Kischtimite._
A fluocarbonate of the Cerium metals, near Parisite (_q.v._). Cer =
74·2%.
Massive.
G = 4·78. H = 4¹⁄₂. Yellowish brown; translucent.
Barsovka River, Kyshtymsk, Urals.
_Knopite._
A variety of Perovskite (_q.v._) containing E and Fe. Cer = 4-7; TiO₂
= 55%.
Pseudo-cubic.
G = 4·2. H = 5¹⁄₂. Grey; opaque; lustre metallic.
Alnö, Sweden.
_Kochelite._
A columbate of E, Fe, Zr; with ThO₂, SiO₂, Ca, aq., etc. Allied to
Fergusonite (_q.v._). Yttr = 17·22; ZrO₂ = 12·8; ThO₂ = 1·23%.
Doubtful; may be tetragonal.
G = 3·74. H = 3-3¹⁄₂. Brown to honey yellow; translucent.
The Kochelweise, near Schreiberhau, Silesia.
_Koppite._
Columbate of E, Ca, Fe, Th, K, Na, etc. Near Pyrochlore (_q.v._). Cer
= 4-10; ZrO₂ = 0-5%.
Cubic; in dodecahedra.
G = 4·45-4·46. H = 5-6. Brown; transparent.
Schelingen, Black Forest Mountains, Germany.
~Lanthanite.~
Hydrated carbonate of Cerium metals, especially La; E₂(CO₃)₃,9aq. Cer
= 54·9%.
Orthorhombic; habit tabular.
G = 2·6-2·7. H = 2. White; opaque.
With Cerite (_q.v._) at Bastnäs, Sweden; Bethlehem, Pennsylvania,
U.S.A.
Lavenite.
(Mn,Ca,Fe)(ZrOF)Na(SiO₃)₂? ZrO₂ = 28·8-31·6%.
Monoclinic; habit prismatic.
G = 3·51-3·55. H = 6. Brown to yellow; translucent.
Langesund Fiord, Norway; the Ardennes, France.
Leucosphenite.
BaO,2Na₂O,2(Ti,Zr)O₂,10SiO₂. TiO₂ = 13·2; ZrO₂ = 3·5%.
Monoclinic; wedge-shaped.
G = 3·05. H = 6¹⁄₂. White; transparent.
Narsarsuk, Greenland.
Lewisite.
3R´´Sb₂O₆,2R´´TiO₃, where R = Ca, Fe´´ and Mn. TiO₂ = 11-12%.
Cubic; in small octahedra.
G = 4·95. H = 5¹⁄₂. Yellow to brown; translucent.
Tripuhy, Minas Geraes, Brazil.
_Loranskite._
Tantalate of E, Zr, Fe, etc. Yttr = 10; Cer = 3; ZrO₂ = 20%.
Massive.
G = 4·6. H = 5. Black; opaque. Metallic lustre.
Finland.
Lorenzenite.
Titano-silicate of Na and Zr; TiO₂ = 35; ZrO₂ = 12%.
Orthorhombic; acicular.
G = 3·4. H = 6. Colourless; transparent.
South Greenland.
_Mackintoshite._
Mixture of oxides, chiefly of Th and U; also Fe, Ca, Mg, Pb, Na, B,
Ta, etc. Composition very complex. ThO₂ = 45·3; E₂O₃ = 1·9; ZrO₂ = 1%.
Tetragonal, resembling thorite (_q.v._).
G = 5·42. H = 5¹⁄₂. Black; opaque.
Bluffton, Llano Co., Texas.
_Malacone._
An altered Zircon (_q.v._), with E, Ca, Fe, H₂O, etc. ZrO₂ = 47-67%.
Tetragonal; pseudomorphous.
G = 3·9-4·1. H = 6. Brown, often dull white internally.
Hitterö, Norway; Haute Loire, France; and in U.S.A.
Mauzeliite.
Very similar to Lewisite (_q.v._), with Pb. TiO₂ = 8%.
Cubic.
G = 5·11. H = 5-6. Brown; translucent.
Jakobsberg, Sweden.
_Melanocerite._
Very complex fluosilicate of E and Ca, chiefly. Cer = 48; Yttr = 9·2;
ThO₂ + ZrO₂ = 2%.
Rhombohedral; habit tabular.
G = 4·13. H = 5-6. Deep brown to black. Transparent.
Langesund Fiord, Norway.
_Microlite._
Complex columbate of Ca, E, Fe, etc., with F and H₂O. E₂O₃→ 8%.
Cubic; habit octahedral.
G = 5·48-5·56. H = 5-5¹⁄₂. Red to yellow.
Stockholm, Sweden; Island of Elba; and in U.S.A.
Molengraafite.
Titano-silicate of Ca, Na, Fe, Al, Mn, etc. TiO₂ = 28%.
Monoclinic; in small prisms.
Yellow. High refraction and birefringence.
Pilandsberg, Transvaal.
~Monazite.~
Phosphate of E, with Th and SiO₂. Cer = 49-74; Yttr = 1-4; ThO₂ =
1-20%.
Monoclinic.
G = 4·9-5·3. H = 5-5¹⁄₂. Red to brown and yellow; translucent.
The Carolinas; Idaho; Brazil; Scandinavia, etc.
_Mosandrite._
In composition identical with Johnstrupite (_q.v._).
Isomeric with Johnstrupite (_q.v._).
G = 2·93-3·03. H = 4. Reddish brown; translucent.
Langesund Fiord, Norway.
_Muromontite._
A variety of Allanite (_q.v._), rich in yttria earths and Be, but poor
in Al and ceria earths. Cer = 9·1; Yttr = 37·1%.
_See_ Allanite.
G = 4·263. H = 7. Black to greenish black.
Mauersberg, Erzgebirge, Saxony.
~Naegite.~
A silicate of Zr, ZrSiO₄, with E, Th, U, Cb, etc. ZrO₂ = 55·2; Yttr =
9·12; ThO₂ = 5·01%.
Tetragonal; in globular aggregates.
Gr = 4·091. H = 7¹⁄₂. Dark green or brown; dull.
Gravel-tin of Japan.
Narsarsukite.
Na₆FeTi₂Si₁₂O₃₂F. TiO₂ = 14%.
Tetragonal. Habit tabular.
Gr = 2·75. H = 7-7¹⁄₂. Yellow to reddish-brown; pleochroic.
Plain of Narsarsuk, Greenland.
Neptunite.
(K,Na)₂(Fe,Mg,Ca,)₂(Ti,Si)₄O₁₂. TiO₂ = 18%.
Monoclinic. Habit prismatic.
G = 3·23. H = 5¹⁄₂. Black, red in flakes. Translucent to opaque.
Narsarsuk, Greenland.
_Nivenite._
A variety of Cleveite (_q.v._), readily soluble in dilute acids.
Cubic; crystallisation indistinct.
G = 8·01. H = 5¹⁄₂. Velvet black; opaque.
Bluffton, Llano Co., Texas.
_Nohlite._
A variety of Samarskite (_q.v._) containing water (→ 4·6%).
Massive, without cleavage.
G = 5·04. H = 4¹⁄₂-5. Brownish black; opaque.
Nohl, near Kongelf, Sweden.
_Oerstedite._
A variety of Zircon (_q.v._), poor in SiO₂. ZrO₂ = 69%.
Tetragonal; angles exactly those of Zircon.
G = 3·629. H = 5¹⁄₂. Reddish-brown; adamantine lustre.
Arendal, Norway.
~Orangite.~
ThSiO₄, usually with Fe, Ca, H₂O in traces. ThO₂ = 71·2-73·8%.
Tetragonal. Habit prismatic.
G = 5·19-5·40. H = 4¹⁄₂-5. Orange yellow; lustrous.
_See under_ Thorite.
~Parisite.~
E₂CaF₂(CO₃)₃. Cer = 50·8-64·4; Yttr = 0-2·5%.
Hexagonal. Habit pyramidal.
G = 4·36. H = 4¹⁄₂. Yellow to red; transparent.
Muso Valley, Columbia; Montana, U.S.A.; Greenland; Norway; the Urals,
etc.
Perovskite.
CaTiO₃, with traces of Fe´´. TiO₂ = 58·9%.
Pseudo-cubic? Optically biaxial.
G = 4·017. H = 5¹⁄₂. Yellow; transparent to opaque.
The Urals; Switzerland; Tyrol, etc.
_Pilbarite._
PbO,UO₃,ThO₂,2SiO₂,2H₂0 + 2aq. ThO₂ = 31·3%. Cer and Yttr--traces.
Amorphous.
G = 4·4-4·7. H = 2¹⁄₂-3. Bright yellow; opaque.
Pilbara goldfields, West Australia.
_Pitchblende._
A mixture of oxides, chiefly UO₂ and UO₃, but without E₂O₃ or ThO₂.
Amorphous.
G = 5-6·5. H = 3-4. Black; resinous lustre.
Bohemia; Cornwall; Carolina; Norway, etc.
_Plumboniobite._
A variety of Samarskite (_q.v._) containing Pb; R´´₂Cb₂O₇,
R´´´₄(Cb₂O₇)₃, where R´´ = Fe, Pb, Ca, UO, R´´´ = E, Al. Yttr = 14·3%.
Massive, isotropic.
G = 4·80-4·81. H = 5-5¹⁄₂. Dark brown to black.
Morogoro, Uluguru Mountains, German E. Africa.
~Polycrase.~
A titano-columbate of E and U; Yttr = 19·5-32·5; TiO₂ = 25-33%. Cer
and ThO₂ traces. Isomorphous with Euxenite.
Orthorhombic.
G = 4·0-4·8. H = 6. Black; vitreous lustre.
Norway.
~Priorite.~
Dimorphous with Euxenite (_q.v._).
Orthorhombic; isomorphous with Blomstrandine.
G = 4·6-5·0. H = 6. Black; transparent in flakes.
Swaziland, S. Africa.
Pseudobrookite.
Fe₄(TiO₄)₃, ferric orthotitanate. TiO₂ = 44-53%.
Orthorhombic.
G = 4·39-4·98. H = 6. Dark brown to black.
Norway; France.
_Pyrochlore._
A columbate of Ca and E, with Th, Fe, Ti, F, etc. E₂O₃ → 18; TiO₂ =
5-14%.
Cubic.
G = 4·2-4·36. H = 5-5¹⁄₂. Dark brown.
Scandinavia; the Urals; Tasmania, etc.
Pyrophanite.
MnTiO₃, with traces of SiO₂. TiO₂ = 50-53%.
Rhombohedral; isomorphous with Ilmenite.
G = 4·537. H = 5. Deep blood-red; translucent; lustrous.
Pajsberg, Sweden.
_Retzian._
Hydrated arsenate of Mn´´, Ca, E. Cer + Yttr = 8-11%.
Orthorhombic, usually in prisms.
G = 4·15. H = 4. Brown; pleochroic; transparent.
Province of Nordmarken, Sweden.
_Rhabdophane_ (Scovillite).
Hydrated phosphate of E, Al, Fe, Mg, etc., with SiO₂. Cer = 53·8-57;
Yttr = 2·1-10·0%.
Massive.
G = 3·94-4·01. H = 3¹⁄₂. Brown to yellow; translucent.
Cornwall; Scoville, Connecticut, U.S.A.
Rhönite.
(Na,K,H)₃Ca₃(Fe´´,Mg)₁₅(Al,Fe´´´)₁₆(Si,Ti)₂₁O₉₀. TiO₂ = 9·5%.
Anorthic, isomorphous with Aenigmatite.
G = 3·5-4·3. Brown, with strong pleochroism.
Rhön Mountains, Saxony.
_Rinkite._
A titanosilicate closely allied to Mosandrite and Johnstrupite
(_q.v._)--Na₉Ca₁₁Ce₃(Ti,Th)₄₁Si₁₂O₄₆? Cer = 21; Yttr = 0·4-1·4; TiO₂ =
13-14%.
Monoclinic, very close to Johnstrupite.
G = 3·46. H = 5. Yellow, pleochroic; translucent.
Kangerdluarsuk, Greenland.
~Risörite.~
An yttria columbate, near Fergusonite, but with no U and considerable
TiO₂; Yttr = 37; Cer = 2·9-4·0; TiO₂ = 6·5%.
No data yet determined. Isotropic.
G = 4·179. H = 5¹⁄₂. Yellowish brown.
Norway.
_Rogersite._
Hydrated yttria columbate. Yttr = 60·12%. A weathered Samarskite?
Amorphous, mamillary.
G = 3·313. H = 3¹⁄₂. White.
Mitchell Co., N. Carolina.
_Rosenbuschite._
Titanosilicate of Ca, Zr, Na, E, Fe, Mn, with F. ZrO₂ = 18·7-20; Cer =
0·3-2·4%.
Monoclinic, in spherical aggregates.
G = 3·30-3·31. H = 5-6. Orange-grey.
Near Brevik, Sweden.
_Rowlandite._
Silicate of E, with Th, Ti, Fe, etc.--2Y₂O₃, 3SiO₂. Cer = 14·4; Yttr =
47·7; ThO₂ = 0·6%.
Massive.
G = 4·515. H = 6. Pale dull green.
Llano Co., Texas.
~Rutile.~
Titanium dioxide; TiO₂ = 98-100%
Tetragonal; habit prismatic.
G = 4·18-4·25. H = 6-6¹⁄₂. Reddish-brown to black.
Very widely in Europe and America.
~Samarskite.~
R´´₃R´´´₂(Cb,Ta)₆O₂₁, where R´´ = Fe, Ca, UO₂; R´´´ = E. Cer =
1·2-6·4; Yttr = 4·72-21·2; ThO₂ + ZrO₂ → 7%.
Orthorhombic; usually massive.
G = 5·6-5·8. H = 5-6. Deep velvet black; opaque.
Miask; Urals; Mitchell Co., N. Carolina.
Schorlomite.
A titaniferous Garnet--3CaO,(Fe,Ti)₂O₃,3(Si,Ti)O₂. TiO₂ = 12·5-22%.
Cubic; usually massive.
G = 3·81-3·88. H = 7-7¹⁄₂. Black; transparent in flakes.
Magnet Cove, Arkansas.
Senaite.
(Fe,Mn,Pb)O,TiO₂, cf. Ilmenite. TiO₂ = 49-52%.
Rhombohedral; isomorphous with Ilmenite, Geikielite, etc.
G = 5·3 (to 4·2 when weathered). H = 6¹⁄₂. Black.
Diamantina, Minas Geraes, Brazil.
~Sipylite.~
Columbate of E, Zr, Fe, U, Sn, etc.; near Fergusonite (_q.v._).
Cubic, in octahedra. Usually granular.
G = 4·89. H = 6. Brownish-black; translucent.
Amhurst Co., Virginia.
_Steenstrupine._
A silicate of E, Fe, Na, Th, Mn, Al, Ti, H₂O, etc.; near Melanocerite.
Cer = 14·4-32·5; Yttr = 0-15·9; ThO₂ = 2·1-7·1%.
Rhombohedral.
G = 3·38. H = 4. Brown; faces dull.
Kangerdluarsuk, Greenland.
Strüverite.
FeO,(Nb,Ta)₂O₅,4TiO₂. TiO₂ = 69-71%.
Tetragonal; angles very close to those of rutile.
G = 5·0. H = 6-7. Black; opaque.
Craveggia, Piedmont, Italy; and in Madagascar.
_Tachyaphaltite._
An altered zircon, containing H₂O. ZrO₂ = 40-50%.
Tetragonal; very close to Zircon.
G = 3·6. H = 5¹⁄₂. Dark brown.
Kragerö, Norway.
_Tengerite._
Hydrated carbonate of E, Be, Ca, etc.; a weathered Gadolinite
(_q.v._). E₂O₃ = 39·2-47·8%.
Amorphous.
White; opaque; very soft.
Llano Co., Texas.
~Thalenite.~
H₂E₄Si₄O₁₅, with traces of Fe´´´ and Al. Yttr = 58·6-63·9%.
Monoclinic.
G = 4·23. H = 6¹⁄₂. Bright red and yellow,
Österby, Sweden.
~Thorianite.~
Mixed ThO₂ + UO₂, with E, Pb, Zr, Si, Fe, etc. ThO₂ = 72-79; Cer =
1-8%.
Rhombohedral; pseudocubic.
G = 8·0-9·7. H = 7. Jet black; bright resinous lustre.
Gem-gravels of Ceylon.
~Thorite.~
ThSiO₄, with H₂O, U, Fe, E, Ca, Al, etc. ThO₂ = 41·4-57·9; E₂O₃ =
0-6%.
Tetragonal; habit prismatic.
G = 4·4-4·8; H = 4¹⁄₂-5. Brown to black.
Various localities in Scandinavia.
_Thorogummite._
UO₃,3ThO₂,3SiO₂,6H₂O? An altered Mackintoshite (_q.v._)? ThO₂ = 41·4;
E₂O₃ = 6·7%.
Usually massive; sometimes in crystals resembling Zircon.
G = 4·43-4·54. H = 4-4¹⁄₂. Dull brown; opaque.
Llano Co., Texas.
~Thortveitite.~
E₂O₃,2SiO₂, with Fe´´´, Al, Mn´´´ traces; E = chiefly Sc. Yttr =
54·5%.
Orthorhombic, in radial aggregates.
G = 3·571. H = 6-7. Greyish green; translucent.
Iveland, Sätersdalen, Norway.
~Titanite~ (Sphene, Grothite).
CaSiTiO₅, with Fe´´, Mn´´. TiO₂ = 34-45% (usually 41%).
Monoclinic; wedge-shaped.
G = 3·40-3·56. H = 5-5¹⁄₂. Yellow, green, or brown; pleochroism
strong; lustre resinous.
Widely distributed in Europe and N. America.
Titanium Olivine.
(H₂,Fe´´,Mg)₂(Si,Ti)O₄; Mn and F in traces. TiO₂ = 3-12%.
Orthorhombic.
G = 3·25-3·27. H = 6¹⁄₂-7. Deep red to yellow; pleochroic.
Pfunders, Tyrol; Zermatt, Switzerland.
_Tritomite._
A fluo-borosilicate of E, Th, Ca, with Zr, Na, H₂O, etc. Cer =
44·2-59·2; Yttr = 0·4-4·6; ThO₂ + ZrO₂ = 0-10·6%.
Rhombohedral; in crystals resembling regular tetrahedra.
G = 4·15-4·25. H = 5¹⁄₂. Dark brown; transparent to opaque.
Langesund Fiord, Norway.
_Tscheffkinite._
Titano-silicate of E, Th, Fe, Ca, etc. Cer = 23-47; Yttr = 0-3·4; ThO₂
+ ZrO₂ = 0-20; TiO₂ = 16-21%.
Massive, amorphous.
G = 4·26-4·55. H = 5-5¹⁄₂. Velvet black.
Ilmen Mountains; Nelson Co. and Bedford Co., Virginia, U.S.A.
_Tysonite._
Fluoride of E, with Th, H₂O, CO₂, etc. Cer = 69·2-70·6; ThO₂ = 0-31%.
Hexagonal; in thick prisms.
G = 6·12-6·14. H = 4¹⁄₂-5. Wax yellow; transparent to translucent.
Fahlun and Österby, Sweden; Pike’s Peak, Colorado.
_Uhligite._
Titanate of Zr, Ca, Al; Ca(Zr,Ti)O₃ + Al(Ti,Al)O₃? TiO₂ = 48; ZrO₂ =
22%.
Cubic; near to Perovskite (_q.v._).
H = 5-6. Black. Transparent in flakes.
Lake Magad, E. Africa.
~Uraninite.~
Oxides of U (60-75%), with PbO₂, ThO₂, ZrO₂, E₂O₃, Fe₂O₃, etc. Cer. =
0-2·7; Yttr = 0-10·2; ThO₂ = 1·6-11·1; ZrO₂ = 0-8·1%.
Cubic, usually massive; alters to amorphous pitchblende.
G = → 6·4 (massive); → 9·7 (crystalline). H = 5¹⁄₂. Black; transparent
in splinters.
Norway; Bohemia; Saxony; Cornwall; Carolina, etc.
_Vietinghofite._
A hydrated ferruginous samarskite (_q.v._). E₂O₃ = 8·2; ZrO₂ = 1·0%.
Amorphous.
G = 5·53. H = 5¹⁄₂-6. Dull black; opaque.
Lake Baikal, Siberia.
Warwickite.
6MgO,FeO,2TiO₂,3B₂O₃? TiO₂ = 23·5%.
Orthorhombic; habit prismatic, elongated.
G = 3·35-3·36. H = 3-4. Dark brown to black; pleochroic. Double
refraction strong, +ve.
Edenville, New York State.
_Weibyite._
Carbonate of E, with Ca, Sr, F, and H₂O; allied to Bastnäsite
(_q.v._). Cer = 66·96%?
Orthorhombic; in pyramids resembling those of Zircon.
Crystals are small, and covered with a thin yellow crust; they are
intergrown with Parisite (_q.v._)
Langesund Fiord, Norway.
~Wiikite.~
Titano-tantalo-silicate of Zr, Th, E, Fe, U, with Cb₂O₅, H₂O, etc. Cer
= 2·5; Yttr = 7·6; Sc₂O₃ = 1·2; ThO₂ = 5·5; ZrO₂ + TiO₂ = 23·4%.
Perfectly amorphous.
G = 4·85. H = 6. Black; opaque; infusible.
Impilaks, Lake Ladoga, Finland.
_Wöhlerite._
Silicate and columbate of Ca, Zr, Na; Si₁₀Zr₃Cb₂O₄₂F₃Ca₁₀Na₅? ZrO₂ =
15·2-22·7%. Cer, traces.
Monoclinic; prismatic or tabular habit.
G = 3·41-3·44. H = = 5¹⁄₂-6. Light yellow; pleochroic.
Langesund Fiord.
~Xenotime.~
Phosphate of E, with ThO₂, SiO₂, Zr, etc. Cer = 0-11; Yttr =
54·1-64·7; ThO₂ = 1-5%.
Tetragonal; isomorphous with Zircon?
G = 4·45-4·56. H = 4-5. Brown to yellow; opaque.
Diamond sands of Brazil; Norway.
_Yttrialite_ (Green Gadolinite).
A weathered gadolinite (_q.v._)--E₂O₃,2SiO₂. Cer = 6·6-8·2; Yttr =
43·4-46·5; ThO₂ = 10·8-12·8%.
Amorphous, massive.
G = 4·6. H = 5¹⁄₂. Green to brown; translucent.
Bluffton, Llano Co., Texas.
~Yttrocerite.~
Ca₃E₂F₁₂, 1¹⁄₂H₂O. Cer = 9·3-18·2; Yttr = 8·1-29·4%.
Massive, granular.
G = 3·45. H = 4¹⁄₂. White to violet blue or brown.
Various localities in Scandinavia.
_Yttrocrasite._
(Ca,Pb)O,(Th,U)O₂,3E₂O₃,16TiO₂,6H₂O. Yttr = 25·7; Cer = 2·9; ThO₂ =
8·7; TiO₂ = 49·7%.
Orthorhombic; axial ratios unknown.
G = 4·80. H = 5¹⁄₂-6. Black; lustrous.
Burnet Co. Texas.
~Yttrofluorite.~
_n_CaF₂ + _m_YF₃ in isomorphous mixture? Yttr = 20-25; Cer = 1-2%.
Cubic.
G = 3·54-3·56. H = 4¹⁄₂. Closely resembles fluorspar, except in
badness of cleavage.
Northern Norway.
_Yttrogarnet._
A variety of garnet with E and Zr. Yttr = 1-6·7; ZrO₂ = 0-3%.
Cubic (cf. Garnet).
Dark reddish brown (cf. Garnet).
Stockö, Norway; Schreiberhau, Germany.
_Yttrogummite._
UO₃, 3ThO₂, 3SiO₂, 6H₂O? E₂O₃ = 6·7; ThO₂ = 41·4%.
Tetragonal; angles near Zircon. Usually massive.
G = 4·43-4·54. H = 4-4¹⁄₂. Yellowish brown.
Llano Co., Texas.
_Yttrotantalite._
R´´R´´´₂(Cb,Ta)₄O₁₄ + 4H₂O; R´´ = Fe´´, Ca; R´´´ = E; Cer = 0-2·4;
Yttr = 17·2-38·3%.
Orthorhombic; isomorphous with Samarskite (_q.v._).
G = 5·5-5·8. H = 5-6. Yellow to black.
Ytterby, Sweden; South Norway.
~Zircon.~
ZrSiO₄, with Fe, Th, etc., in traces. ZrO₂ = 61·0-70·0%.
Tetragonal; habit prismatic.
G = 4·68-4·70. varying considerably. H = 7¹⁄₂. Colour very variable.
Widely distributed as a rock mineral, in sands, etc.
_Zirkelite._
(Ca,Fe)(Zr,Ti,Th)₂O₅, with E, U, Mg, etc. ZrO₂ = 48·9-52·9; ThO₂ =
0-7·3; TiO₂ = 14-15; E₂O₃ = 0-3%.
Cubic; in twinned octahedra.
G = 4·7. H = 5. Black; transparent in thin flakes.
Jacupiranga, São Paulo, Brazil.
CHAPTER II
THE SILICATES
(_a_) SILICATES OF THE YTTRIUM AND CERIUM METALS
~Cerite.~--Cerite is a silicate of the cerium metals, with small amounts
of lime, ferrous oxide and water. Hintze gives the formula
H₃(Ca,Fe)Ce₃Si₃O₁₃,[15] which Groth interprets as a basic metasilicate
(Ca,Fe)[CeO]Ce₂(OH)₃(SiO₃)₃, _i.e._ a basic salt of the acid H₆Si₃O₉, a
polymer of metasilicic acid, H₂SiO₃.
[15] The symbol (Ca,Fe) here indicates that the iron and calcium occur
in variable proportions, the variation however occurring in such a way
that the equivalent of the two taken together is always the same,
_i.e._ the iron can replace the calcium, or _vice versa_, atom by
atom. The recognition of this possibility of ‘Vicarious Replacement’
between similar elements first brought order into the confused field
of mineral chemistry, and allowed a systematic classification of
minerals according to chemical composition to be made. Iron and
calcium, or, according to the more convenient nomenclature of the
mineralogists, lime and ferrous oxide, are here vicarious
constituents.
The symbol Ce here stands for elements of the cerium group, which are
never found singly.
Crystals are not very common, the mineral usually occurring granular or
massive.
Crystals, orthorhombic, holosymmetric; _a_ : _b_ : _c_ = 0·9988 : 1 :
0·8127. Usual forms--the Pinakoids _a_, _b_, and _c_ {100}, {010} and
{001}, prisms _m_ {110} and _q_ {130}, domes _u_ {101}, _t_ {301} and
_n_ {011}, and some pyramids {hkl}.
Angles, _a_ ∧ _m_ = 44° 58´, _u_ ∧ _c_ = 39° 8´, _n_ ∧ _c_ = 39° 6´.
The crystals usually occur as short prisms. No cleavage. Optical
constants unknown. In flakes the absorption spectrum of didymium can
be observed.
The mineral is brittle; hardness 5 to 6 on Mohs’ scale; sp. gr. varies
a little about 4·9. Fracture splintery; lustre dull, resinous. Colour
brown to red and greyish-red, streak greyish-white. The mineral is
almost opaque.
Cerite is infusible before the blowpipe. It is attacked readily by
sulphuric acid, less easily by hydrochloric acid, with which it gives a
gelatinous mass. Rammelsberg[16] found that the silica left behind on
treatment of the powdered granular variety with the latter acid
contained a variable proportion of bases, which he obtained and
estimated after fusing the siliceous residue with sodium carbonate. From
the different proportions of the earths in the part attacked by the acid
and that left in the silica, he remarks, ‘It would almost appear that
Cerite is a mixture of silicates which are not all attacked with the
same ease by hydrochloric acid.’ Apparently without previous knowledge
of this observation, Welsbach[17] noticed the same thing in 1884. He
concluded that ordinary granular ‘cerite’ is a mixture of several
minerals, among which there are at least two which contain rare earths.
Of these, one, the chief constituent of the aggregate, is probably
identical with the crystallised mineral, and is characterised by the
readiness and completeness with which it is attacked by hydrochloric
acid. The other does not react, with hydrochloric, but is readily
attacked by sulphuric acid; it contains yttria earths, in addition to
the ceria earths. In the extraction of ceria earths from the mineral
aggregate, Welsbach used hydrochloric acid, so leaving this second
mineral unchanged; but to avoid loss of the rare earths, sulphuric acid
is more commonly employed for the decomposition.
[16] _Pogg. Ann._, 1859, ~107~, 631.
[17] _Monats._, 1884, ~5~, 512.
Though of great historical interest, cerite is of very small importance
for the extraction of rare earths at the present time, on account of its
very rare occurrence. The mineral seems to be almost entirely confined
to the Bastnäs quarry near Ryddarhyttan, Sweden, where it is found with
the rare earth silicate allanite (_q.v._), biotite, hornblende, bismuth
glance, chalcopyrite, etc. Here it was observed in 1751 by Cronstedt,
who called it Tungsten (_vide supra_, p. 1). In 1781 Scheele examined a
specimen of Wallerius’s ‘Tenn-spat’ from Bipsberg, Dalecarlia, and found
Tungstic Oxide (Acid), WO₃, in it.[18] After Scheele’s work, the
Ryddarhyttan mineral was known as Red Tungsten, until Bergmann (1780)
and d’Elhuyar (1784) showed that the two minerals were chemically
distinct. They considered the red variety to be a silicate of iron and
calcium, the rare earths being mistaken for lime. In 1804 Klaproth
examined it, and found a new earth; he called the mineral ‘Ochroite,’
from its colour. In the same year, but independently of Klaproth,
Berzelius and Hisinger made the same discovery; they called the mineral
Cerite and the new metal Cerium, in honour of the discovery of the minor
planet Ceres by Piazzi in 1801.
[18] This mineral, which Scheele knew as Tungstein, is now called
Scheelite.
The analyses of cerite made in the earlier part of the nineteenth
century resulted in some confusion. Klaproth in 1807 found 34·5 per
cent. SiO₂ in a specimen (his Ochroite); Vauquelin in 1805, and Hisinger
in 1810, found 17·0 and 18·0 per cent. respectively.[19] Hermann[20]
called attention to this discrepancy in 1843 (and again in 1861), and
declared that the two could not be the same. For Klaproth’s mineral he
proposed to revive the name Ochroite, whilst from his own analyses he
proposed for the cerite of Berzelius the name Lanthanocerite, having
found carbon dioxide and lanthanum, with much less cerium, in the
latter.[21] In 1861 Kenngott partly explained these results by showing
that the sample of cerite which Hermann had analysed contained
Lanthanite[22]; but the extraordinarily high percentage of silica
obtained by Klaproth remained unexplained. It may have been due to
impurities of high silica content in the specimen he examined.
[19] _Vide_ Hintze, _Handbuch der Mineralogie_, Leipzig, 1897, ~ii.~,
1329.
[20] Hermann, _J. pr. Chem._ 1843, ~30~, 194, and 1861, ~82~, 406.
[21] The announcement of the discovery of Lanthanum by Mosander was
made in 1839.
[22] Lanthanite (see list) is an hydrated carbonate, R₂O₃,3CO₂,9H₂O,
where R = cerium metals, chiefly Lanthanum.
Cerite contains from 59·4 to 71·8 per cent. of rare earths (oxides), the
amount and nature of which vary with the precise locality. The oxides
consist chiefly of ceria, lanthana, and didymia (praseodymia and
neodymia), the complexity of the so-called ceria having been shown by
Mosander in the case of ceria separated from gadolinite as well as from
cerite; but yttria earths are also found to a small extent in the
mineral.
It is remarkable that neither thorium nor uranium has been found in
cerite, which is thus practically unique among the rare earth minerals.
This anomaly becomes even more marked in view of the very high
percentage of inert gases found by Tschernik[23] in a related mineral
from Batoum. This is a very complex mineral in which the basic part is
represented by rare earths, chiefly ceria earths (50·8 per cent.) with
water (3·4 per cent.), and oxides of iron, calcium and copper (6·8 per
cent.); the acidic oxides being silica (6·6 per cent.), zirconia (11·6
per cent.), and titanium dioxide (14·7 per cent.), with phosphorus
pentoxide (3·2 per cent.), and sulphuric anhydride (1·7 per cent.).
Traces of thoria are present, but no uranium; very considerable
quantities (up to 1 per cent.?) of helium were found.
[23] G. Tschernik, _J. Russ. Phys. Chem. Soc._ 1896, ~28~, 345; 1897,
~29~, 291. Abstracts in _Zeitsch. Kryst. Min._ 1899, ~31~, 513 and
514.
It is somewhat heavier than cerite (sp. gr. 5·08), but otherwise
resembles it closely.
~Gadolinite~ (Ytterbite).--Gadolinite is a silicate of iron, beryllium,
and the yttria earths, of the formula 2BeO,FeO,Y₂O₃,2SiO₂, which may be
written FeBe₂Y₂Si₂O₁₀. According to Groth, it is a basic orthosilicate,
Be₂Fe(YO)₂(SiO₄)₂, derived from the acid H₈Si₂O₈. The beryllium content
varies considerably, and some authors recognise two varieties of the
mineral, one rich, and one poor in beryllium; but Scheerer pointed out
in 1840 that iron and beryllium are probably vicarious constituents.
Analysis gives silica 21·8 to 25·3 per cent.; yttria earths 22 to 47 per
cent.; ceria earths 5 to 31 per cent. In a variety from Ytterby, the
rare earth Scandia was first found, forming up to 0·02 per cent. of the
mineral. Small quantities of thoria, ThO₂ may be present, and traces of
helium were found by Ramsay, Collie, and Travers. According to Strutt it
contains also uranium and radium. Like cerite, it does not often occur
crystalline, being usually found in amorphous masses.
The crystals are monoclinic; _a_ : _b_ : _c_ = 0·6273 : 1 : 1·3215; β
= 89° 26¹⁄₂´.
Common forms are--Ortho-, clino-, and basal pinakoids, _a_ {100}, _b_
{010}, and _c_ {001}, hemi-prisms _m_ {110}, _v_ {120}, clino-prisms
_w_ {012}, _q_ {011}, and many others; and various hemi-pyramids {hkl}
and {h̅kl}.
Angles _a_ ∧ _m_ = 32° 6´, _c_ ∧ _q_ = 52° 53´, _c_ ∧ (101) = 64° 9´.
Crystals commonly prismatic, terminated by _c_. Faces rough and
coarse; lustre vitreous to greasy, seen only on freshly-broken
surfaces. Brittle. No cleavage. Fracture conchoidal to splintery.
Hardness 6¹⁄₂-7; sp. gr. 4·0-4·5.
Colour black, greenish- and brownish-black; green and transparent in
flakes. The crystalline variety has strong positive birefringence,
with the plane of the optic axes parallel to (_b_), the plane of
symmetry; the amorphous variety is of course isotropic. The brown
variety shows very distinct pleochroism, _i.e._ the colour as seen by
transmitted light varies with the direction in which the light
traverses the crystal; the green kinds have much weaker pleochroism.
Gadolinite is of common occurrence in the pegmatite veins of the
Scandinavian granite. It was first found in a felspar quarry on the
island of Ytterby, near Stockholm, by a Lieutenant Arrhenius[24]; it is
also found, together with a large number of other rare earth minerals,
at Fahlun. It occurs in Norway on the islands of Hitterö and Malö, and
in Germany in the Riesengebirge and the Harz. Probably the largest
deposit is that in Texas, at Barringer Hill, near Bluffton, on the west
bank of the Colorado River, Llano County, now owned and worked by the
Nernst Light Company of Pittsburg; in 1904 a mass of very pure
gadolinite weighing 200 lb. was found here.[25]
[24] _Vide_ Geijer, _Crell’s Chemische Annalen_, 1788, ~1~, 229.
[25] See _U.S. Geol. Survey_ (_Minerals_), 1904, 1213.
In the same place a decomposition product of gadolinite was discovered
by Hidden and Mackintosh in 1889. They named it Yttrialite or Green
Gadolinite. It contains no beryllium, and twice as much silica as the
parent mineral, and approximates to the formula R₂O₃,2SiO₂, where R₂O₃
is chiefly yttria oxides; it is thus similar in composition to the newly
found scandium silicate, Thortveitite (_q.v._). It is amorphous and
massive; and is often found in continuous growth with gadolinite. Pieces
up to 10 lb. in weight have been obtained.
As stated above, Gadolinite was discovered by Arrhenius in 1788. Geijer
examined it in the same year, and described it as a black zeolite. In
1794 it was analysed by Gadolin, who declared it to be a silicate of
iron, aluminium, and a new element which he called Ytterbium. In 1797
Ekeberg examined it, and confirmed the discovery. He proposed the name
Gadolinite for the mineral, and Yttria for the new earth; these names
were accepted by Klaproth, who examined it with Vauquelin in 1800, and
by the French crystallographer Haüy. In 1802 Ekeberg showed that the
oxide originally taken for alumina was in reality beryllia; in 1816
Berzelius showed that ceria was present with the yttria.[26] About 1838
Mosander began his classical work on the earths in gadolinite. In that
year he announced the separation of Lanthana,[27] and in 1842 that of
Didymia, which he had actually discovered eighteen months earlier. In
the latter year he announced[28] the separation of erbia and terbia. In
1842 also Scheerer[29] declared that the yttria from gadolinite was a
mixture of earths, from its different behaviour on heating in closed and
open vessels; but when Mosander announced the discovery of didymia (the
announcement appears to have been hastened indeed by Scheerer’s
observation) it was agreed that the colouration observed was probably
due to that earth. The further history of these earths must be continued
elsewhere (_vide_ p. 111).
[26] _Schweigg. J._, 1816, ~16~, 405.
[27] Berzelius (a letter to Pelouze), _Pogg. Ann._, 1839, ~46~, 648.
[28] _Berz. Jahres._, ~23~, 145; ~24~, 105.
[29] _Pogg. Ann._, 1842, ~56~, 483.
The behaviour of gadolinite on heating is of great interest. When heated
uniformly, in closed or open vessels, the mineral suddenly glows very
strongly at a definite temperature (according to Hofmann and Zerban[30]
at 430°C.), with considerable alteration in properties. The amorphous
variety exhibits the phenomenon much more markedly than the crystalline
form. The change in the two cases is entirely distinct, the only effect
in common being that both varieties are rendered insoluble in acids
after the glowing. The amorphous variety, in the act of glowing, changes
to the crystalline form.
[30] _Ber._, 1903, ~36~, 3095.
This phenomenon of phosphorescence, or glowing, on heating, with a
change in properties, was first observed by Berzelius in 1816. He found
that the oxides of many metals, _e.g._ chromium, tantalum, and rhodium,
became denser and insoluble in acids after being heated. Later in the
same year he observed the glowing, with a similar change in properties,
in the case of a gadolinite from Fahlun.[31] Apparently without
knowledge of this observation, Wollaston published a similar account of
the glowing of a gadolinite in 1825. In 1840 Scheerer noted an almost
identical change in the case of the mineral allanite (_q.v._). Scheerer
made a careful study of the phenomena in the cases of allanite and
gadolinite.[32] In each case he found that the variety of lower specific
gravity showed, on heating, a very strong phosphorescence, accompanied
by change of colour and optical properties, and a marked increase of
specific gravity. Gadolinite suffered no appreciable loss of weight, but
allanite had lost a little water after the change. Careful measurement
of the specific gravity before and after the change showed, in the case
of two varieties of gadolinite and one of allanite, that the volume had
decreased in the ratio 1 : 0·94. Scheerer assumed that this ratio was
constant for all such cases, and advanced a general explanation. We know
now that numerous cases of similar phenomena occur, in which the change
of volume is quite different; but Scheerer’s explanation is so
ingenious, and so foreshadows some modern theories, that it is given
here in full.
[31] _Schweigg. J._, 1816, ~16~, 405.
[32] _Pogg. Ann._, 1840, ~51~, 493.
He ascribes the alteration to ‘interatomic change, involving change of
relative position of atoms and decrease of interatomic distances.’
(Scheerer and the chemists of that period understood by atoms the
ultimate particles of a body, making no distinction between elements and
compounds; in this case he meant by atoms what we mean by molecules, and
the word ‘molecule’ has therefore been substituted for ‘atom’ in what
follows.) The change is simply one of closer packing of the molecules,
which take up a more stable position with liberation of energy as heat
and light. He imagines his molecules as uniform spheres arranged in
horizontal layers, as shown in Fig. 1. In placing one layer vertically
over another there are three possible arrangements, of which only two
concern us. In the arrangement for closest packing, B, say, a molecule
of any one layer touches three molecules in each of the layers above and
below, which with the six it touches in its own layer make twelve
altogether. In the next closest arrangement, A, say, a molecule of any
one layer touches only two molecules in each of the layers above and
below it, so that one molecule is in contact with ten others altogether.
[Illustration: FIG. 1]
Now it can be shown that the volumes of equal numbers of molecules in
the arrangements A and B will be to one another as the height, H, of an
equilateral triangle, to the height, h, of a regular tetrahedron whose
edges are equal to the sides of the triangle, a length R (which will be
equal to the diameter of a molecule).
Then H = ¹⁄₂R√3, _h_ = R√²⁄₃.
Then vol. in arrangement A : vol. in arr. B ∷ H : _h_
_i.e._ ∷ √3/2 : √²⁄₃
∷ 1 : 0·943.
That is, the volume changes in the ratio 1 to 0·943, the amorphous
variety of gadolinite consisting of molecules in arrangement A, which go
over to the closer packed arrangement B in the change to the crystalline
form.
More extended work has shown that this ingenious and interesting
explanation is not of general application. Thus H. Rose[33] found that
samarskite (_q.v._) exhibited the phenomenon of glowing, but that the
specific gravity was actually less after the change than it was before,
_i.e._ there was an increase of volume. Damour observed glowing in the
case of zircon from Ceylon (_q.v._) with increase of density, the volume
change being from 1 to 0·922, _i.e._ even greater than for gadolinite.
Again, Hauser[34] observed in the case of his new rare earth mineral
risörite a sudden change at a red heat, the mineral losing water,
becoming very brittle, and increasing very considerably in specific
gravity (the volume changing from 1 to 0·90 approximately), but without
glowing. Ramsay and Travers[35] found that fergusonite (_q.v._) glowed
strongly when heated to 500°-600°, with decrease of specific gravity
(5·62 before to 5·37 after), evolution of all its helium, and very
considerable evolution of heat; they suggested that helium was present
in combination, in an endothermic compound decomposed by heat, but in
view of the properties of helium, this hypothesis seems hardly tenable.
[33] _J. pr. Chem._ 1858, ~73~, 391.
[34] _Ber._ 1907, ~40~, 3118.
[35] _Zeitsch. physikal. Chem._ 1898, ~25~, 568.
It appears unlikely that any one explanation can cover all these
interesting facts; there are in each case peculiar factors to be taken
into account. In 1841, Regnault,[36] considering the case of the oxides
observed by Berzelius, inferred that the development of light and heat
denoted that the bodies possessed a lower specific heat after the change
than before. The experimental difficulties encountered in attempting to
dry the oxides prevented him from confirming this view. He measured the
specific heats of the minerals calcite and aragonite (CaCO₃), and of the
two allotropic modifications of phosphorus, but could observe no
appreciable differences. H. Rose (_vide supra_) showed by experiment
that considerable heat was evolved on the glowing of gadolinite, with a
decrease of about one-fourteenth in the specific heat. In the case of
samarskite there was, however, no appreciable evolution of heat, nor
could he determine any difference in the specific heats before and after
glowing.
[36] _Pogg. Ann._ 1841, ~53~, 249.
Probably the only inference that can be safely drawn is that in most
cases the change is due to some molecular re-arrangement. The evolution
of water, helium, etc., in some cases, may possibly be due to
intramolecular change, but on the one hand the current view at present
is that the helium is mechanically held in radio-active minerals, and on
the other hand it is not known that the water evolved is water of
constitution; in an intermolecular change at fairly high temperature,
these might be evolved without disruption of the true mineral molecules.
The question of the energy involved, and consequently of the specific
heats, appears to depend on factors peculiar to each case, of which at
present no accurate conception can be formed; and the change in specific
gravity is probably bound up with these. The loss of solubility in acids
is a factor not always connected with glowing, as it is frequently
observed in the laboratory after ignition of compounds, but here again
no adequate explanation is forthcoming.
The possibility of chemical change in one or two cases, however, must
not be ignored. Thus ammonium magnesium phosphate, NH₄MgPO₄, on heating
glows, and is converted to magnesium pyrophosphate, according to the
equation:
2NH₄MgPO₄ = Mg₂P₂O₇ + H₂O + 2NH₃
A case possibly analogous to this is that of the mineral sipylite
(_q.v._), R´´´₂Cb₂O₈, with ‘basic water’ (_i.e._ R´´´ partially replaced
by H). Before the blowpipe this decrepitates with loss of water, and
glows brilliantly. The specific gravity after the change does not appear
to have been determined. Mallet explains the glow as due to a change to
the pyrocolumbate.
Similar explanations may possibly hold in the cases of allanite and
risörite, but it must be remembered that we are really ignorant of the
part played by the water in these minerals.
~Allanite.~--Allanite, or Orthite, as it is often called, is a mineral
of the epidote family, containing rare earths. The general formula for
Epidote is H₂O,4R´´O,3R´´´´₂O₃,6SiO₂, where R´´ is a divalent and R´´´ a
trivalent metal, or vicarious series of metals. In the case of Allanite,
R´´ = (Fe´´,Ca), R´´´ = (Al,Fe´´´,E), where E stands for metals of the
cerium and yttrium groups (Engström’s formula). Groth formulates it as
a basic salt, R´´´₃(OH)R´´₂Si₃O₁₂, of the acid H₁₂Si₃O₁₂ (= 3H₄SiO₄).
Crystals are fairly common, but the mineral usually occurs massive or in
rounded grains.
Crystals--Monoclinic, holosymmetric; _a_ : _b_ : _c_ = 1·5509 : 1 :
1·7691, β = 64° 59´.
Common forms--Ortho- and basal pinakoids _a_ {100} and _c_ {001}; _m_
{110} and other prisms, _e_ {101} and other hemi-ortho-prisms, _o_
{011}, _d_ {111} and other hemi-pyramids.
Angles, (100) ∧ (110) = 54° 34´; (001) ∧ (101) = 63° 24´; (001) ∧
(011) = 58° 3´.
Tabular, parallel to _a_, or long and slender by elongation parallel
to axis _b_.
Birefringence weak, variable. Refraction strong. Colour brown to
brownish-black; almost opaque. In flakes very strongly pleochroic, the
colours for light parallel to the three vibration directions ~c~, ~b~
and ~a~ being brownish-yellow, reddish-brown, and greenish-brown
respectively.
Brittle. Hardness 5¹⁄₂-6; sp. gr. 3·5-4·2.
On heating, allanite becomes amorphous and isotropic with increase of
specific gravity (cf. Gadolinite). Before the blowpipe it loses water,
and melts to a black magnetic glass, many varieties phosphorescing
strongly (_vide supra_). With hydrochloric acid it gelatinises, unless
previously heated strongly, in which case it is not attacked.
Analyses show that the rare earth content varies considerably
(vicariously as regards ferric iron and aluminium), ceria earths varying
from 3·6 to 51·1 per cent. and yttria earths from traces up to 4·7 per
cent.[37] Thoria is usually present, 0 to 3·5 per cent. In 1909
Fromme[38] found small quantities of beryllia in the mineral, and in
1911 Meyer[39] found amounts of scandium oxide up to 1 per cent. It
contains traces of uranium, and is weakly radioactive. Ramsay, Collie
and Travers found no helium (1895), but in 1905 Strutt found radium in
it, so that the presence of helium seems _a priori_ probable.
[37] _Vide_ Schilling, pp. 70-75 for analyses of this mineral.
[38] Fromme, _Tsch. Min. Mitt._ 1909, ~28~.
[39] Meyer, _Sitzungsber. königl. Akad. Wiss. Berlin_, 1911, 379.
Many varieties of the mineral are known, differing in habit, colour,
water content, specific gravity, etc., and the percentage composition
varies very much by reason of vicarious replacement of the bases.
Goldschmidt[40] has found ‘Epidote-orthites’ which are isomorphous
mixtures of orthite with an iron epidote; he concludes that most
orthites are probably similar solid solutions, and in this way accounts
to a large extent for the varying composition.
[40] _Centr. Min._ 1911, 4.
Allanite is of very wide distribution, though it is not often found in
large quantities. The usual occurrence in pegmatitic veins in granites,
syenites and other acid plutonic rocks has been often noted, _e.g._ in
many parts of Sweden and Norway. It is found also in the extinct crater
now forming the Laacher See, near Coblenz, Germany, and at Impilaks,
near Lake Ladoga, on the border of Finland; a mass of the pure mineral
weighing 300 lb. was recently discovered at Barringer Hill, (cf. under
Gadolinite), and it occurs in large quantities in Amherst Co., Virginia.
It is an accessory constituent of many acid volcanic and hypabyssal
rocks, and has been found also in limestone, and in magnetic iron ores.
On account of its exceedingly wide distribution, and the variations in
appearance and composition, it has been repeatedly described under
various names, varieties being constantly mistaken for new mineral
species.
Its history is rather curious.[41] In 1806 the Danish mineralogist
Giesecke made a protracted voyage to Greenland, collecting minerals and
rocks; he remained there until 1813. In 1808 he sent off his first
collection by ship to Copenhagen; on the voyage the ship was taken by an
English privateer, and the cargo landed and sold at Leith. The minerals
were bought by Allan, a Scotch mineralogist, who recognised, that they
were from Greenland by the presence of cryolite, at that time only known
to occur in Greenland. He mistook the mineral subsequently named after
him for gadolinite, and sent it to Thomson for analysis.[42] Thomson
recognised it as a new mineral, and named it Allanite (1810). In 1815
Hisinger described a mineral from Ryddarhyttan, Sweden, which he called
Cerin; Leonhard (1821) and Hauy (1822) showed that this was identical
with Allanite. In 1818 Berzelius described two varieties of a mineral
from Finbo, near Fahlun, Sweden, which he called Orthite, and
Pyrorthite; these were eventually shown by Scheerer (1844) to be
varieties of Allanite. In 1824 the French mineralogist Lévy described a
mineral from Arendal, Norway, which he named Bucklandite, in honour of
the English naturalist; in 1825 this was identified with a ‘black
zeolite’ from the Laacher See by G. Rose, and in 1828 both were shown by
Hermann to have the same composition as orthite or allanite. The list
might be extended at will; the Tautolite of Kokscharow (1847), the
Bodenite of Breithaupt (1844), the Muromontite of Kemdt (1848), and the
Vasite of Bahr (1863) have all been shown to be varieties of the same
bewildering mineral.
[41] _Vide_ Schilling, pp. 75-76, where full references are given.
[42] See Kobell’s _Geschichte der Mineralogie_, 1864, p. 679.
~Hellandite.~--Hellandite[43] is a mixed silicate of rare earths
with lime, magnesia, alumina, ferric and manganic oxides, with
considerable quantities of water. The formula approximates to
3H₂O,2R´´O,3R´´´´₂O₃,4SiO₂, where R´´ = (Ca,Mg,Th/2)--Thorium being able
to replace two atoms of calcium or magnesium--and R´´´ = (Al,Fe´´´,Mn´´´
and rare earth metals). This may be written as a basic orthosilicate,
R´´₂[R´´´´(OH)]₆(SiO₄)₄, a basic salt of the acid H₁₆Si₄O₁₆ (= 4H₄SiO₄).
This composition puts it in the class containing topaz and some rarer
silicates.
[43] Brögger, _Zeitsch. Kryst. Min._ 1906, ~42~, 417.
The mineral is crystalline, the crystals being well developed, but often
dull and opaque by alteration (hydration).
Crystal system--Monoclinic, holosymmetric, _a_ : _b_ : _c_ = 2·0646 :
1 : 2·507. β = 109° 45´. Habit usually prismatic, with {100}, {010},
and several prisms {_hko_}, terminated by various pyramid forms.
Angles (100) ∧ (001) = 70° 32´; (100) ∧ (110) = 62° 22´; (010) ∧ (110)
= 27° 14´; (110) ∧ (11̅0) = 125° 0´.
Twinned on (001), twin plane (001), forming knee-shaped twins.
Hardness varies from 5¹⁄₂ in the least altered to 1 in the most
altered specimens; sp. gr. 3·70 in least altered specimens, decreasing
with hydration. Colour of fresh crystals, reddish-brown; on alteration
they become brownish-black, yellow, or even white.
The mineral dissolves easily in hydrochloric acid, with evolution of
chlorine; it is less soluble in nitric and sulphuric acids. It readily
fuses to a yellow mass.
It was first discovered by Brögger at Lindvikskollan, in 1903, and
later, in larger quantities, at Kragerö in Norway. It occurs in
pegmatite veins in granite.
~Thalénite.~[44]--A silicate of yttria earths with water and small
quantities of alumina, ferric oxide, carbon dioxide and alkalies. The
ratio of rare earths to silica gives the formula R₂O₃,2SiO₂, or R₂Si₂O₇;
if the water be included, the formula becomes H₂R₄Si₄O₁₅. The presence
of both water and carbon dioxide indicates, however, that the mineral
has been somewhat altered, and the simpler formula R₂Si₂O₇, (cf.
Thortveitite, below) probably expresses the composition of the original
mineral. It contains considerable quantities of nitrogen and helium,
though uranium and thorium appear to be absent.
[44] Benedicts, Abstract in _Zeitsch. Kryst. Min._ 1900, ~32~, 614.
Monoclinic; _a_ : _b_ : _c_ = 1·154 : 1 : 0·602. β = 80° 12´.
Common forms are the pinakoids {100} and {010}, hemi-prism {110},
hemi-pyramids {111} and {111̅}, and others, and the hemi-dome {021}.
Angles, (100) ∧ (010) = 91° 0´; (100) ∧ (110) = 48° 9´; (100) : (111)
= 59° 4´.
Double refraction weak. No cleavage. Brittle. Hardness 6¹⁄₂. Colour,
bright flesh-red; translucent, with greasy lustre; sp. gr. 4·227,
increasing to 4·29 after ignition. A yellow variety has sp. gr.
4·11-4·16, and is transparent.
The ‘average atomic weight’ of the rare earth metals is 99, from which
it appears that these consist chiefly of yttrium, with a smaller
quantity of the metals of higher atomic weight.
It was discovered in 1898 by Benedicts, accompanying fluocerite (_q.v._)
in a quartz quarry at Oesterby in Dalekarlia.
~Thortveitite.~[45]--A silicate of yttria earths, chiefly scandia, of
the formula R₂O₃,2SiO₂. Scandia forms about 37 per cent. of the whole
(R. J. Meyer); yttria with small quantities of the other yttria earths
forms the bulk of the remainder of the bases, the ceria group being
almost completely absent. Ferric oxide (with traces of manganic oxide
and alumina) forms about 3 per cent. Thorium is present only in traces,
and radioactivity is barely perceptible.
[45] J. Schetelig, _Centr. Min._ 1911, 721.
Thortveitite is the first mineral to be discovered in which the content
of scandia is greater than 2 per cent.; in 1908 Crookes[46] examined a
very large number of yttria minerals for scandia, and finally chose for
extraction of the earth Wiikite (_q.v._) which has a scandia content of
1·2 per cent.[47]
[46] _Phil. Trans._ 1908, A, ~209~, 15.
[47] According to Eberhard, some varieties of Wiikite have a much
lower scandia content.
Thortveitite is orthorhombic; _a_ : _b_ : _c_ = 0·7456 : 1 : 1·4912;
commonly combinations of pyramids _o_ {111} and _s_ {211} with prism
_m_ {110}, in radial aggregates of crystals elongated parallel to the
_c_ axis. Cleavage parallel to _m_, fair. Twin plane _m_ (110),
twinning very common.
Refraction strong; birefringence strong, negative. Acute bisectrix
perpendicular to (001), plane of the optic axes (010). Hardness, 6-7;
sp. gr. 3·571. Extremely brittle; lustre brilliant, vitreous to
adamantine. Colour, greyish-green, white to reddish-grey on
alteration; in transmitted light yellowish-green, after ignition,
reddish; the change being probably due to presence of oxides of iron.
It is fusible with difficulty, and only partially attacked by
hydrochloric acid. It was found by Thortveit, in 1910, in a pegmatite
vein in granite, at Iveland, Sätersdalen, S. Norway, accompanied by
euxenite, monazite, beryl, and the usual vein-materials (quartz,
felspar, etc.). It was analysed and recognised as a new mineral by
Schetelig (_loc. cit._).
* * * * *
The following minerals, of which particulars will be found in the
alphabetical list, also belong to this class:
_Bagrationite_, _Bodenite_, and _Muromontite_, varieties of allanite
with differences in composition and physical properties.
_Yttrialite_, a weathered variety of gadolinite.
_Elpidite_, _Erdmannite_ and _Cainosite_, more complex silicates.
_Rowlandite_, a comparatively simple silicate of the yttrium metals.
_Yttrogarnet_, a variety of garnet containing yttrium metals.
(_b_) SILICATES OF THORIUM AND ZIRCONIUM
~Thorite.~--Thorite and its variety Orangite are somewhat altered forms
of a pure silicate of thorium, ThSiO₄, containing also small quantities
of water, usually uranium, and often rare earths, with iron, lead,
calcium, and aluminium. Orangite differs from thorite in its beautiful
orange colour and greater specific gravity. Both varieties are
radio-active.
When unaltered, the crystals are tetragonal and uniaxial, the pure
mineral ThSiO₄ being isomorphous with zircon, ZrSiO₄ (_q.v._). By
alteration they become isotropic.
Crystals are tetragonal, holosymmetric; _c_ = 0·6402; _p_ ∧ _p_´ = 56°
40´.
Common forms are the prism _m_ {110} with the pyramids _p_ {111} and
_z_ {311}.
Hardness 4¹⁄₂-5; sp. gr. 4·4 to 4·8 for thorite, 5·2 to 5·4 for
orangite.
Thorite contains from 1·4 to 3·1 per cent. of rare earths. According to
Nilson and Blomstrand, the uranium is present as uranium dioxide, UO₂
replacing thoria, ThO₂, but Dunstan and Blake state that the two oxides
are isomorphous (see under Thorianite, p. 74), and so they might be
expected to be vicarious. Thorite was discovered by Esmark in 1828, and
first analysed by Berzelius,[48] who announced the discovery of a new
earth in it in 1829. The name Thorite is from Thor, the god of
Scandinavian mythology.
[48] _Pogg. Ann._, 1829, ~16~, 385.
Thorite is a member of a peculiarly interesting series of isomorphous
minerals, which includes Cassiterite (SnO₂), Rutile (TiO₂), Zircon
(ZrSiO₄), and most probably the allied silicate Naegite, and the rare
earth phosphate Xenotime (_q.v._), which are very similar in forms and
angles. The oxide TiO₂ is itself trimorphous, being known in the three
crystallographically different forms, Rutile, Anatase, and Brookite
(_q.v._). On account of the isomorphism of cassiterite and rutile with
the two silicates, it has been suggested that the oxide formulæ be
doubled and written Sn(SnO₄) and Ti(TiO₄) respectively,[49] to show the
analogy with Th(SiO₄) and Zr(SiO₄). Consideration of the molecular
volumes (obtained by dividing molecular weight by specific gravity,
_i.e._ multiplying by specific volume) lends a certain amount of support
to this view. It has often been observed that isomorphous compounds, and
many compounds which occur in parallel growth to one another, have
nearly equal molecular volumes; there are, however, many exceptions.
Taking molecular volumes for the series under consideration, we have,
using approximate numbers only--
Mol. Wt. Sp. Gr. Mol. Vol.
Cassiterite, SnO₂ 151 6·9 22
Rutile, TiO₂ 80 4·2 19
Zircon, ZrSiO₄ 182 4·7 39
Thorite, ThSiO₄ 325 5·4 (Orangite) 60
Xenotime, XPO₄ 184 4·5 41
[49] This isomorphous series has recently been extended by Zambonini,
and also by Schaller, by the inclusion of minerals containing
Columbium and Tantalum; see under Ilmenorutile and Strüverite, end of
Ch. IV., p. 71.
It will be seen that if the numbers for cassiterite and rutile be
doubled, four out of the five show very fair approximation to the
constant value 40. The number 60 for thorite is quite irreconcilable
with the values obtained from the other members; of course pure silicate
of thorium, ThSiO₄, is not known as a mineral, but it is most unlikely
that the relatively small amount of impurity in the densest specimens of
orangite should have depressed the specific gravity by over two units,
as would be required if the molecular volume of thorite were to show
even the most approximate semblance of agreement with the others. It
cannot be too often remarked, however, that very little indeed is known
of the molecular formulas of minerals, and that very little reliance can
be placed on such figures as the above. On the contrary, it is hardly
conceivable that amphoteric oxides like those of tin and titanium,
occurring in the form of heavy crystalline minerals, should have
molecular formulæ only double the empirical formulæ. Where agreements of
the kind do occur, they must be taken as indicating approximately equal
degrees of molecular complexity in the minerals concerned, rather than
as affording any real insight into the molecular condition.
~Zircon.~--Zircon is a silicate of zirconium, ZrSiO₄, with small
quantities of other elements. Most varieties contain ferric oxide and
thoria; more rarely small proportions of the yttria earths may be
present. All varieties contain traces of a large number of the common
metals. Traces of radium are usually present, with helium and neon,[50]
and the mineral is strongly radioactive.
[50] Strutt, _Nature_, 1906, 102.
System tetragonal, holosymmetric sub-class. _c_ = 0·6404; (001) ∧
(101) = 32° 38´.
Usual forms--Prisms _a_ {100} and _m_ {110}; pyramids _e_ {101}, _p_
{111}, _u_ {221} and _x_ {311}, etc. The basal pinakoid _c_ {001} is
rare. The usual combination is one or both of the prisms _a_, _m_,
with one or two pyramids. Twinning is rare, the twin plane being _e_
(101), giving knee-shaped twins similar to those so characteristic of
cassiterite and rutile. Cleavage ∥ _m_ imperfect, ∥ _p_ bad.
Brittle; conchoidal fracture. Hardness 7¹⁄₂; sp. gr. usually
4·68-4·70, but varying from 4·2 to 4·86. Adamantine lustre. Clear and
colourless to yellow-, red- or greenish-brown. Transparent to opaque.
Refraction and double refraction strong, double refraction positive (ω
= 1·924, ε = 1·968, for sodium light); on heating it becomes biaxial,
and occasionally is found biaxial in nature. By alteration it becomes
isotropic.
It is infusible before the blowpipe, but loses its colour; some
varieties glow and increase in density (see p. 38). In some varieties
also the colour changes or disappears rapidly on exposure to sunlight,
and is often restored on keeping in the dark. These phenomena of colour
change have been attributed variously to alteration in the state of
oxidation of the iron present, and to the presence of organic matter. It
seems probable that either cause or even both may be at the root of the
change in particular cases.
On account of the hardness, unalterability, and strong refraction and
double refraction, good crystals of zircon are used as gems. The two gem
varieties, Hyacinth and Jargon, are found chiefly in the gem gravels of
Ceylon. It was in a zircon from Ceylon that Klaproth discovered the new
earth, Zirconia, in 1789.[51] In 1795 he found the same earth in
hyacinth, and so showed the two to be identical.
[51] _Schriften der Gesellschaft naturforschender Freunde in Berlin_,
1789, vol. 9.
Artificial crystals of zircon have been obtained by the action of
silicon tetrachloride and silicon tetrafluoride on zirconia, and by the
action of zirconium tetrafluoride on silica at high temperatures.
Zircon is one of the most widely distributed minerals known, though
usually it occurs in very small quantities. Good crystals have been
found in New Zealand, in Ceylon, at Miask in the Urals, and in North
Carolina. This last deposit has been worked commercially for the
extraction of zirconia for Nernst lamps (_vide_ p. 320). It occurs in a
decomposed felspar in a pegmatite dyke in the Archæan gneiss near
Zirconia, Henderson Co., and can be easily extracted by picking or
washing, after crushing if necessary. Should there ever be a
considerable demand for zirconia, it could doubtless be saved as a
by-product in the extraction of thoria from monazite sands (_q.v._),
zircon being very generally found in those sands (see below).
Zircon is common in crystalline rocks, limestones, schists, syenites,
granites, etc. It is a constant accessory constituent in the acid
igneous rocks, especially in the more acid eruptive rocks. It is readily
detected under the microscope by the pleochroic haloes with which the
tiny crystals are surrounded; these have been shown by Joly to be due to
alteration of the surrounding rock by the radiations emitted by the
radio-active constituents of the zircon. It also occurs as a constituent
of those sands which are formed by the erosion of the igneous rocks in
which it is enclosed, and hence it almost invariably accompanies
monazite in the so-called monazite sands.
Zircon is one of the least easily altered minerals; by the prolonged
action of chalybeate and other waters, during many geological ages,
however, it gradually changes, losing silica and gaining lime, oxides of
iron, and water. Some of these altered varieties have received special
names, as, _e.g._ Auerbachite, Malacone, Cyrtolite, and Alvite; but none
of them is of special interest.
~Naegite.~[52]--This rare mineral is a silicate closely related to
zircon, but of rather more complex composition. It may be represented as
silicate of zirconium, ZrSiO₄ (zirconia = 55·3, silica = 20·6 per
cent.), with rare earths (chiefly yttria, 9·1 per cent.), uranium (UO₃ =
3 per cent.), and thorium (ThO₂ = 5·0 per cent.), partly as silicates,
partly as columbates and tantalates ((Cb,Ta)₂O₅ = 7·7 per cent.).[53]
[52] _Beiträge zur Mineralogie von Japan_, 1906, ~2~, 23.
[53] An earlier analysis (_Abstr. Chem. Soc._ 1905, ~88~, [ii.], 177)
gave over 20 per cent. of uranous oxide, UO₂; the greater part of this
appears to have been zirconia, ZiO₂.
It is tetragonal, usually occurring in globular aggregates of crystals.
The measurable angles are extremely close to those of zircon, and it is
probable that naegite is isomorphous with the series mentioned above
under Thorite.
The hardness is 7¹⁄₂, the sp. gr. 4·091. The colour is dark green or
brown, becoming dull by weathering. The double refraction is extremely
weak.
So far it has only been found in the ‘placer’ tin deposits or ‘gravel
tin’ of Japan.
* * * * *
The following minerals (see list) are also to be included in this
sub-class:
_Alvite_ (Anderbergite or Cyrtolite), _Auerbachite_, _Malacone_,
_Oerstedite_ and _Tachyaphaltite_, altered varieties of zircon.
_Calciothorite_, _Eucrasite_ and _Freyalite_, altered varieties of
Thorite.
_Pilbarite_, _Thorogummite_ and _Yttrogummite_, hydrated silicates of
thorium with uranium and other metals.
(_c_) COMPLEX SILICATES
~Eudialyte~ (Eucolyte).--This is a complex silicate of alkalies, lime,
ferrous oxide, rare earths, etc., containing chlorine and a high
proportion (up to 17 per cent.) of zirconia. The empirical formula is
given by Dana as Na₁₃(Ca,Fe)₆Cl(Si,Zr)₂₀O₅₂. Brögger gives the simpler
metasilicate formula R´₄R´´₃Zr(SiO₃)₇, where R = (Na,K,H), R´´ =
(Ca,Fe,Mn,CeOH), and Zr(OCl) may partly function as an acid in place of
SiO₂. The true formula, however, is quite uncertain, as the zirconia may
function either as an acidic or basic oxide. The fact that a mineral of
such exceedingly complex composition occurs in perfectly well-defined
crystals indicates the intricate nature of the problems to be solved in
mineral chemistry.
The crystals are rhombohedral, _a_ : _c_ = 1 : 2·1116.
Common forms are--the pinakoid _c_ {111}, prisms _a_ {101}, and _m_
{211}, and pyramids _r_ {100} and _e_ {110}. _c_ ∧ _r_ = 31° 22´.
Habit tabular parallel to _c_, rhombohedral with _e_ prominent, or
prismatic with _a_ prominent.
Cleavage ∥ _c_ very good, ∥ _a_ difficult.
The colour is brown or red to brownish- or bluish-red. Brittle.
Hardness 5 to 5¹⁄₂; sp. gr. 2·92 for eudialyte, 3·0 to 3·1 for
eucolyte.
The double refraction is strong, being positive for eudialyte, negative
for the Norwegian variety, eucolyte. From careful microscopic
examination, Ramsay has found that zones of positive and negative
birefringence, as well as isotropic (singly-refracting) zones can occur
on the same crystal, and he suggests that the mineral is really composed
of two isomorphous compounds forming mixtures. In view of the continuous
variation of optical properties in an isomorphous series like the
felspars, such an explanation seems doubtful. The optical behaviour of
minerals is very often anomalous, and the phenomena in this case are
probably due to repeated twinning, with some alteration in the double
refraction, or to the lamellar intergrowth of two varieties having
slightly different optical properties.
On heating, the mineral evolves moisture and readily fuses. It is easily
attacked even by dilute acids, being named by Strohmeyer (1819) on
account of this property. The dilute hydrochloric acid solution reddens
turmeric paper--a test for the presence of zirconium.
It is found in Greenland, usually embedded in felspar, in Norway, in
Lapland and in Arkansas, being generally associated with minerals rich
in alkalies, _e.g._ ægirine, ælæolite, nepheline, sodalite,
arfvedsonite, etc.
~Beckelite.~--This is a mineral similar in composition to eudialyte,
though not so complex, and of more recent discovery.[54] It is a
silicate of ceria earths and lime, in which zirconia replaces silica;
the oxygen ratio (_i.e._ ratio of oxygen in basic oxides to oxygen in
acid oxides) is 3 : 1, and the formula Ca₃R´´´₄(Si,Zr)₃O₁₅, where R =
rare earth metals, chiefly of the cerium group. It is thus a salt of an
acid H₁₈Si₃O₁₅ [= 3H₆SiO₅ = 3(3H₂O,SiO₂)] with zirconium and silicon
vicarious.
[54] _Abstr. Chem. Soc._ 1905, ~88~, ii, 177.
The crystals appear to belong to the cubic system, occurring in cuboid
grains, and in octahedra and dodecahedra. It is brown, and isotropic,
with cubic cleavage. Sp. gr. = 4·15.
It is soluble in hot hydrochloric acid, even after ignition; the
solution gives the turmeric test for zirconium.
It was found in a dyke in an ælæolite syenite, near the Sea of Azov.
* * * * *
The following minerals (see list) are also to be placed in the class of
mixed silicates:
_Arfvedsonite_ and _cataplejite_, complex zircono-silicates.
_Hiortdahlite_ (Guarinite) and _Lavenite_, zircono-silicates with
fluorine.
_Caryocerite_, _Melanocerite_ and _Steenstrupine_, complex
fluosilicates.
_Auerlite_, _Britholite_, _Erikite_ and _Florencite_, phospho-silicates.
_Cappelenite_, _Homilite_ and _Tritomite_, boro-silicates.
CHAPTER III
THE TITANO-SILICATES AND TITANATES
(_a_) THE TITANO-SILICATES
~Yttrotitanite or Keilhauite.~--A titano-silicate of calcium, aluminium,
iron and yttrium metals. The mineral is isomorphous with titanite,
CaO,TiO₂,SiO₂ (_q.v._), and is itself probably an isomorphous mixture of
titanite with the silicate (Y,Al,Fe)₂SiO₅, where Y = yttrium metals. Its
composition will then be represented by the formula _m_ (Y,Al,Fe)₂(SiO₅)
+ _n_ CaTi(SiO₅).
It is monoclinic, with axial ratios and angles very close to those of
titanite. Usual forms--pinakoids _a_ {100} and _c_ {001}, hemi-prism
_m_ {110}, hemi-pyramids _n_ {111}, _e_ {1̅11} and _l_ {1̅12}.
Cleavage ∥ _n_ distinct. Birefringence weak, +ve. Colour brown to
brownish-black. Hardness 6¹⁄₂; sp. gr. 3·52 to 3·77.
The mineral is fusible before the blowpipe, and is decomposed by
hydrochloric acid.
It was named by Scheerer in 1844 from its composition, and by Ekeberg in
the same year in honour of the Norwegian geologist Keilhau.
~Titanite or Sphene.~--This species, important as an accessory mineral
of many rocks, is a titano-silicate of calcium, generally containing
small quantities of aluminium and iron. The approximate formula usually
given, CaTiSiO₅, is unsatisfactory; some specimens contain as much as 7
per cent. of ferric oxide, others up to 2 per cent. of manganese, whilst
the percentage of titanium oxide, TiO₂, varies very considerably (30 to
45 per cent.). Zambonini and Nickolan have independently analysed
specimens for which no satisfactory formulæ could be deduced. For
specimens containing trivalent metals, Groth considers the mineral to be
an isomorphous mixture of CaTiSiO₅ and R´´´₂SiO₅ (see under
Yttrotitanite, above); Blomstrand, however, advances the formula
2(R´´R´´´₂O₂,TiO)O,SiO₂, where TiO is basic, and the trivalent metals
occur in the divalent group R´´´₂O₂; this formula is also supported by
Zambonini.
More recently the problem of the constitution has been attacked by
Bruckmoser, using Tschermak’s method of determining the nature of the
salts present in silicates. In this method, the mineral is digested with
hydrochloric acid, at a temperature not greater than 60°, until
decomposition is complete; the silicic acid formed is washed by
decantation, and dried in air at a constant temperature; it is weighed
at regular intervals until the weight is constant. It is stated that if
a curve of times and weights be plotted, a break is observed at the
point where drying ceases (for the acid is of course wet) and
decomposition begins; the composition at this point, which is taken as
the composition of the acid required, can be determined from the weight
of the acid, and the weight of anhydrous silica present, which is
determined by ignition after the weight has become constant.
Employing this method in the case of titanite, Bruckmoser claims to have
obtained the acids H₂Si₂O₅ and H₂Ti₂O₅. He therefore concludes that the
constitution of the mineral is represented by the formula Si₂O₅,Ti₂O₅Ca,
which presumably may be written Ca(Ti,Si)₂O₅.
Crystal system--monoclinic; _a_ : _b_ : _c_ = 0·7547 : 1 : 0·8543. β =
60° 17´.
Common forms (Des Cloizeaux’s orientation)--the pinakoids _a_ {100}
and _c_ {001}, with _m_ {110}, _s_ {021}, _x_ {102}, _n_ {111}, and
many others.
(100) ∧ (110) = 38° 14¹⁄₂´; (001) ∧ (1̅01) = 65° 57´; (001) ∧ (011) =
36° 34´.
The habit is very varied, the commonest being the wedge form,
elongated ∥ _c_. Twinning is fairly common, especially on the
law--Twin plane ∥ _a_, which gives both contact and interpenetrant
twins. Cleavage ∥ _m_, fairly distinct. Hardness 5 to 5¹⁄₂; sp. gr.
3·40 to 3·56. Lustre adamantine to resinous. The colour varies very
much, doubtless with the content of iron and manganese; it is commonly
yellow, green, or brown. Pleochroism is very distinct. The refraction
and dispersion are very high, giving the facetted stone a ‘fire’
inferior only to that of diamond. Birefringence positive, strong; the
axial angles vary very widely in different specimens.
It is fusible with difficulty before the blowpipe. Hot concentrated
hydrochloric acid decomposes it partially, with separation of silica;
boiling sulphuric acid, or, better, fused potassium hydrogen sulphate,
decomposes it completely.
On account of the high dispersion and refractive index, clear specimens
of sphene make very beautiful gems, but the stone is not sufficiently
hard to stand much wear.
The mineral was discovered in Chamouni by Pictet in 1787, and was named
Pictite by Delamètherie (1797). In 1795 Klaproth analysed a specimen
from Passau, and, observing the presence of titanium (which he had just
discovered in rutile), proposed the name Titanite. The mineral described
by de Saussure (1796) as ‘Schorl rayonnante,’ and afterwards by Hauy
(1801) as Sphene (σφήν = a wedge), was shown to be identical in
composition with titanite by Cordier, and also by Klaproth (1810); the
crystallographic identity was proved by G. Rose (1820).
On account of the difference in colour and composition, a large number
of varieties are distinguished. The ordinary yellow and brown varieties
are known indifferently as sphene or titanite. _Ligurite_ has an
apple-green colour; _Semeline_ is a greenish form named from a fancied
resemblance to flax seed. _Lederite_ is a brown variety of tabular
habit; _Greenovite_ is rose-coloured, and contains manganese.
_Alshedite_ and _Eucolite-Titanite_ are rich in the trivalent metals;
_Grothite_ is a brown variety containing a considerable percentage of
ferric iron. _Yttrotitanite_, which contains a high proportion of rare
earths, is usually treated as a separate species (see above).
_Titanomorphite_ and _Leucoxene_ are white amorphous varieties chiefly
produced by alteration of rutile and ilmenite.
Titanite is a fairly widespread mineral; as an accessory rock
constituent it is common in the massive plutonic rocks in tiny crystals,
readily distinguished under the microscope by the high refraction and
birefringence, whilst in large embedded crystals it occurs in many
granular limestones, and in plutonic acid, as well as in some
metamorphic rocks. In good crystals it is found in many parts of
Switzerland and the Alps, in Dauphiné, the Tyrol, Piedmont, the Urals,
South Norway, and other European localities; it is also widely
distributed in the United States and Canada.
The mineral is important as a valuable source of titanium.
* * * * *
The class of Titano-silicates is a very large one, and might be extended
almost at will by the inclusion of the numerous silicates which contain
titanium. Owing to the frequency with which small quantities of silica
are replaced by titanium dioxide, almost all the commoner silicate
minerals contain the latter oxide, so that titanium is one of the most
widely distributed of the elements. Relatively very few, however, of the
titanium-bearing minerals contain the element in considerable
quantities, and only two or three have any importance as commercial
sources of titanium compounds.
Only those additional titano-silicates which contain titanium as an
important constituent are mentioned below; short accounts will be found
in the alphabetical list.
_Johnstrupite_, _Mosandrite_, _Rinkite_, _Rosenbuschite_ and
_Tscheffkinite_ are complex titano-silicates containing yttrium or
cerium metals.
_Astrophyllite_, _Leucosphenite_, _Molengraafite_, _Neptunite_ and
_Rhönite_ are complex titano-silicates free from rare earth elements.
_Benitoite_ is a simple titano-silicate of barium; _Ænigmatite_ and
_Narsarsukite_ contain iron and sodium; _Lorenzenite_ has sodium and
zirconium. _Schorlomite_ is a titaniferous garnet. A variety of olivine
rich in titanium (_Titanium Olivine_) is also known.
(_b_) THE TITANATES
~Yttrocrasite.~[55]--This is a complex titanate of rare earths (chiefly
yttria earths) with lime, thoria, and oxides of lead, iron, uranium,
etc.; it has a considerable water content. An approximate formula is
R´´O,R^{iv}O₂,3R´´´₂O₃,16TiO₂,6H₂O, where R´´ = (Ca,Pb,Fe), R^{iv} =
(Th,U), and R´´´₂O₃ = rare earths. No constitutional formula can be
given; it will be noticed that the amount of titanium dioxide is
considerably more than is required to combine with the bases present
(cf. also Delorenzite below). It is radioactive.
[55] Hidden and Warren, _Amer. J. Sci._ 1906, [iv.], ~22~, 515; also
_Zeitsch. Kryst. Min._ 1907, ~43~, 18.
Imperfect crystals only were found, apparently belonging to the
orthorhombic system. No crystallographic data could be determined.
The mineral is black, closely resembling polycrase and euxenite
(_q.v._) in appearance. Hardness 5¹⁄₂-6; sp. gr. 4·80.
It is infusible, and not easily soluble in acids. Hydrofluoric acid
decomposes it, and the powdered mineral is also slowly attacked by
boiling concentrated sulphuric acid.
It was found in 1904 by Barringer, in Burnet Co., Texas.
~Delorenzite.~[56]--A compound similar to the above, but even richer in
titanium dioxide, which amounts to 66 per cent. Tin dioxide is also
present, with traces of columbic anhydride. The bases are the yttria
earths (almost free from ceria earths), uranium dioxide, and some
ferrous oxide, the formula being 2FeO,UO₂,2Y₂O₃,24TiO₂, with a little
SnO₂ replacing TiO₂. It is strongly radioactive. Its closest chemical
neighbour is yttrocrasite, but in appearance and angles it closely
resembles polycrase (_q.v._). Its discoverer, Zambonini, therefore
formulates it as a metatitanate with titanium acting also as a
base--polycrase is a mixed metatitanate and metacolumbate--thus, 2FeTiO₃
+ U(TiO₃)₂ + 2Y₂(TiO₃)₃ + 7(TiO)TiO₃.
[56] Zambonini, _Zeitsch. Kryst. Min._ 1908, ~45~, 76.
The crystals occur in aggregates of numerous individuals in
sub-parallel growth. The system is orthorhombic; _a_ : _b_ : _c_ =
0·3375 : 1 : 0·3412. Usual forms--the pinakoids _a_ {100} and _b_
{010} with prism _m_ {110}, dome _d_ {201}, etc. Habit prismatic,
elongated ∥ c axis. Hardness 5¹⁄₂-6; sp. gr. about 4·7.
It was found with struvite in a pegmatite at Craveggia, Piedmont, Italy.
~Ilmenite or Menaccanite~ (Specular Iron Ore, Titaniferous Ironstone,
etc.).--This is a titanate of iron, usually written FeTiO₃. Its
constitution has given rise to very considerable discussion[57]; not
only do the relative proportions of iron and titanium vary greatly, but
the iron is undoubtedly present in both the ferrous and the ferric
states, and in the former state is partly replaced in some specimens by
manganese and magnesium. In 1829 Mosander put forward the view that the
mineral consisted of FeTiO₃, ferrous titanate, with varying proportions
of ferric oxide, the forms and angles of ilmenite being very similar to
those of hæmatite, Fe₂O₃. This view was disputed by H. Rose, who
concluded that the mineral must have been originally an isomorphous
mixture of ferric oxide, Fe₂O₃, and titanic oxide, Ti₂O₃, which on
exposure to high temperature in the earth’s crust would change according
to the equation
Fe₂O₃ + Ti₂O₃ = 2TiO₂ + 2FeO
so that the proportion of ferrous iron increases with the proportion of
titanium dioxide, as is actually found to be the case. This condition,
however, is also satisfied by Mosander’s view. The latter view was also
supported by Rammelsberg, who pointed out that the presence of magnesium
indicated the existence of ferrous iron as a primary constituent.
Additional support is lent to this view by the discovery of Pyrophanite,
MnTiO₃ (see list), which is found to be isomorphous with ilmenite, so
that there can be little doubt that MgTiO₃, which can be only a
titanate, would, if it existed in the crystalline form (see Geikielite
in list), also be isomorphous with ilmenite. Friedel and Guérin (1876)
prepared artificial titanium sesquioxide, Ti₂O₃, and found it to be
isomorphous with hæmatite, Fe₂O₃; they concluded that FeFeO₃, FeTiO₃ and
TiTiO₃ formed an isomorphous series, and that ilmenite was a mixture of
the second with the other two. In 1890 Hamberg pointed out that there
was no reason to suppose that hæmatite, Fe₂O₃, contains ferrous iron,
_i.e._ has the constitution Fe´´Fe^{iv}O₃, analogous to Fe´´Ti^{iv}O₃,
since in corundum, the analogous compound of aluminium, Al₂O₃, divalent
aluminium can hardly exist; nevertheless, strict analogy of constitution
is not necessary for isomorphism, as shown by the case of potassium
nitrate, KNO₃, and aragonite, CaCO₃, so that hæmatite, Fe₂O₃, and
ferrous titanate, FeTiO₃, might form solid solutions in varying
proportions without the strictly analogous formula FeFeO₃ being true for
the former. The balance of opinion inclines to the constitution
(_m_FeTiO₃ + _n_Fe₂O₃ in isomorphous mixture) originally proposed by
Mosander. The evidence in support of this view has been greatly
strengthened by the recent work of Manchot,[58] which has proved the
absence of titanium sesquioxide, Ti₂O₃; the mineral is therefore to be
regarded as a titanate.
[57] For a full account of the earlier work on the constitution of
ilmenite _vide_ Hintze, i. 1858 _et seq._
[58] _Zeitsch. anorg. Chem._ 1912, ~74~, 79.
Crystal system--rhombohedral; in forms and angles very close to
hæmatite, but the two differ in symmetry (hæmatite has _t_, 3δ, _c_,
3π; ilmenite has only _t_, _c_).
_c_ = 1·38458; (111) ∧ (100) = 57° 58¹⁄₂´; habit, tabular, thick; or
in thin laminæ. Usually in embedded grains or rolled crystals in sand.
Hardness 5 to 6; sp. gr. 4·5 to 5·0, increasing with percentage of
ferric oxide. Iron black, opaque; streak black to brownish-red. Lustre
sub-metallic. Slightly magnetic.
The mineral is infusible; when powdered, it dissolves slowly in boiling
hydrochloric acid, the filtered yellow solution giving the
characteristic blue colouration of titanium salts on addition of
tinfoil. In fused potassium hydrogen sulphate it dissolves readily. The
variation in composition can be judged from the following limits:
TiO₂ Fe₂O₃ FeO
3·5 93·6 3·3 per cent.
52·8 1·2 46·5 „
Ilmenite is a widely distributed mineral. In crystals it occurs chiefly
at Kragerö and Arendal in Norway, at Miask in the Ilmen mountains, in
Dauphiné, the St. Gothard, etc.; in the massive form at Bay St. Paul,
Quebec, and other localities in America; and in sands at Menaccan in
Cornwall, Iserwiese in Bohemia, Puy de Dôme, dép. Haute Loire, France,
and in Brazil, Australia, and New Zealand.
The mineral was discovered at Menaccan in Cornwall by McGregor, about
1790. He described it as containing iron and a new oxide; the unknown
oxide was obtained in 1795 from rutile by Klaproth, who gave the name
Titanium to the new metal it contained.
* * * * *
Short descriptions of the following titanates are also given (see list):
_Davidite_ and _Knopite_; these are complex titanates containing
elements of the cerium and yttrium groups.
_Arizonite_ and _Pseudobrookite_--ferric titanates.
_Perovskite_, calcium titanate, and its variety _Hydrotitanite_.
_Pyrophanite_, a manganese titanate isomorphous with ilmenite, and
_Senaite_, a species intermediate in composition between these two.
_Geikielite_, the magnesium analogue of ilmenite, with the variety
_Picroilmenite_, which is rich in iron.
_Uhligite_, a titanate of zirconium, calcium and aluminium.
_Derbylite_, _Lewisite_ and _Mauzeliite_, an interesting series of
titano-antimonates.
_Warwickite_, a boro-titanate.
CHAPTER IV
THE TANTALO-COLUMBATES
(_a_) TANTALO-COLUMBATES CONTAINING NO TITANIUM DIOXIDE
~Samarskite~, Yttro-ilmenite or Eytlandite
(Urano-tantalite).--Samarskite is a tantalo-columbate[59] of the rare
earth metals, with iron, calcium, and uranium.
[59] In this and all similar minerals, columbium (niobium) and
tantalum are to be regarded as vicarious; they replace each other in
all proportions. It seldom happens that a pure columbate is found free
from tantalum, or _vice versa_; one or other may predominate, but the
two are almost always found together.
Rammelsberg gives the formula R´´₃R´´´₂(Cb,Ta)₆O₂₁, where R´´ =
(Fe´´,Ca,UO₂), and R´´´ = rare earth metals. Groth regards it as
essentially a pyrocolumbate (tantalate) of rare earth metals
R₄[(Cb,Ta)₂O₇]₃ the iron, calcium and uranium being more or less
accessory constituents. Des Cloizeaux considers the formula indefinite.
The mineral has also been found to contain tin, thorium, germanium, and
helium. The yttria earths usually predominate (11·9 to 18·9 per cent.),
the percentage of ceria earths being low (2·4 to 5·2 per cent.). The
yttria earths contain the very rare oxide samaria.
The mineral is radio-active.
Crystal system--orthorhombic; _a_ : _b_ : _c_ = 0·5456 : 1 : 0·5178.
Forms--macro- and brachy-pinakoids _a_ {100} and _b_ {010}; prisms _m_
{110} and _h_ {120}, the macrodome _e_ {101}, and pyramids _p_ {111}
and _v_ {231}.
Angles--(100) ∧ (110) = 28° 37´; (001) ∧ (101) = 43° 30´; (001) ∧
(011) = 27° 22¹⁄₂´.
Habit usually prismatic, with _e_ prominent; sometimes tabular
parallel to _a_ or _b_. Cleavage ∥ _b_, imperfect. The faces are
usually rough. The mineral commonly occurs massive, and in flattened
grains embedded in granite. Conchoidal fracture. Brittle. Hardness 5
to 6; sp. gr. 5·6 to 5·8.
Colour velvet-black, streak reddish-brown. Opaque even in thin films.
Before the blowpipe it fuses at the edges; with borax it gives an iron
bead. It is decomposed by boiling concentrated sulphuric acid, better by
fusion with potassium hydrogen sulphate, and leaching the residue with
dilute hydrochloric acid--this leaves the insoluble oxides Cb₂O₅ and
Ta₂O₅. On heating it glows, with decrease in specific gravity (cf. p.
38).
Samarskite occurs with other columbo-tantalates in felspar, or in veins
in granite, near Miask in the Urals, near Quebec in Canada, and in
Mitchell County, North Carolina. From the last-named locality, masses up
to twenty pounds in weight have been obtained.
The mineral was first discovered in the Urals by Ewreinoff, captain of a
corps of Russian mountain engineers. He sent a specimen for
identification to the mineralogist Gustave Rose, who pronounced it to be
a tantalate of uranium containing manganese, and called it
Urano-tantalite.[60] In 1847 the chemist Heinrich Rose, brother of
Gustave, in the course of his researches on tantalic ‘acid’ (oxide),
analysed a specimen. He found the composition given above, and renamed
it Samarskite,[61] in honour of the Russian engineer who furnished him
with the specimen for analysis.
[60] _Pogg. Ann._ 1839, ~48~, 555.
[61] _Ibid._, 1847, ~71~, 157.
In 1907, Brögger[62] announced that _Annerödite_, of which he had
published an account as a new species in 1881, was a parallel growth of
the mineral columbite, (Fe,Mn)Cb₂O₆, on samarskite.
[62] _Abstr. Chem. Soc._, 1907, ~92~, ii. 885.
Both minerals are orthorhombic, but they are not isomorphous. The
mistake was due to the fact that whilst the crystallographic data were
determined from the upper crystals of columbite, the crystals of
samarskite were used for analysis.
~Plumboniobite.~[63]--This is a recently discovered mineral closely
related to samarskite and yttrotantalite (_q.v._). It is essentially a
columbate[64] of yttrium metals, lead and uranium, with water, ferrous
oxide, titanium dioxide, stannic oxide, alumina, lime, and cuprous
oxide. The formula given is R´´₂Cb₂O₇,R´´´´₄(Cb₂O₇)₃, where R´´ =
(Fe,Pb,Ca,UO), and R´´´ = Al and yttria metals, with isomorphous (?)
metatitanate. The mineral is radio-active, and gives considerable
quantities of gas on being heated with sulphuric acid (carbon dioxide
0·19, helium and nitrogen 0·22 per cent.). The yttria earths are rich in
the oxides of gadolinium and samarium, and the mineral should prove a
valuable source of these elements. It is remarkable that the ceria
earths are almost entirely absent.
[63] Hauser u. Finch, _Ber._ 1909, ~42~, 2270; Hauser, _ibid._, 1910,
~43~, 417.
[64] It is to be understood that small quantities of columbium are
replaced by tantalum.
The mineral is massive, with some indication of crystalline structure.
It is dark brown to black, transparent in flakes, and under the
microscope is seen to be isotropic, with doubly-refracting inclusions,
undoubtedly of a secondary nature. Hardness 5 to 5¹⁄₂; sp. gr. 4·80 to
4·81. Unlike samarskite, it does not glow on ignition.
It occurs with mica and pitchblende in pegmatite veins in granite, at
Morogoro, in the Uluguru Mountains, German East Africa.
~Yttrotantalite.~--This is a tantalo-columbate similar in composition to
Samarskite, and isomorphous with it; though, as the name implies, the
acidic oxide is chiefly tantalum pentoxide, the percentage of columbic
anhydride being much lower than in the latter mineral. It is a pyro-salt
of the formula R´´R´´´₂(Cb,Ta)₄O₁₄ + 4H₂O,[65] where R´´ = (Fe,Ca) and
R´´´ = rare earth (chiefly yttrium) metals (Rammelsberg). Strutt found
thorium and radium in it. The manner in which the water is combined in
this, as in many other minerals, is at present undetermined.
[65] Dana gives R´´R´´´₂(Cb,Ta)₄O₁₅ + 4H₂O; this appears to be an
error.
Crystal system--orthorhombic; _a_ : _b_ : _c_ = 0·5411 : 1 : 1·1330.
Common forms--pinakoids _b_ {010} and _c_ {001}, prisms _m_ {110}, _o_
{210}, _p_ {120}, domes _s_ {201} and β {011}. Habit, prismatic with
_m_ and _b_ prominent, or tabular parallel to _b_. Colour yellow to
black, white after strong ignition.
It is found at Ytterby in Sweden, and in South Norway.
~Fergusonite~, Tyrite, or Bragite.--A columbate and tantalate of the
rare earth metals, with uranium, iron, calcium, etc. The general formula
is that of an ortho-compound, R₂O₃,(Cb,Ta)₂O₅ or R(Cb,Ta)O₄, where R =
metals of the rare earths, chiefly of the yttrium group. Brögger
includes the other constituents in the more complex formula
(Th,U)(Si,Sn)O₄ + 12R(Cb,Ta)O₄; but the simpler formula agrees quite
well with specimens from the most widely separated localities, and is
usually adopted. The mineral is radio-active and contains helium.
Tetragonal, polar (with tetrad axis of symmetry only) _c_ = 1·4643.
(001) ∧ (101) = 55° 40´. Common forms--Basal pinakoid _c_ {001},
tetragonal prism _g_ {320}, pyramids _s_ {111}, _z_ {321}. Brittle.
Hardness 5 to 6; sp. gr. 5·84, decreasing on hydration. Lustre dull,
brilliantly vitreous on broken surfaces. Colour brownish-black.
Translucent to opaque.
Fergusonite was discovered by Hartwell. It occurs with samarskite, and
often with gadolinite and allanite, in Norway and Sweden, the Carolinas,
Texas, the Urals, W. Australia, etc.
On heating it glows suddenly between 500° and 600°C.,[66] losing all its
helium, and with decrease in density (5·619 to 5·375). At the same time
it gives out a considerable amount of heat--8·09 C.[67] for 1 gm. (see
p. 38).
[66] Ramsay and Travers, _Zeitsch. physikal. Chem._ 1898, ~25~, 568.
[67] The heat of combustion of a gram of hydrogen is 342 K.
~Sipylite.~--Essentially a columbate of rare earth metals, with oxides
of tantalum, tungsten, zirconium, uranium, iron and calcium, and some
water. Mallet, the discoverer, gives the formula as R₂O₃,Cb₂O₅, the
basic oxides including, besides the rare earths, Cb₂O₅ with Ta₂O₅ and
WO₃, and some water. An alternative formula, making it a complex
pyro-salt, is also given, but from its great similarity in form and
angles to fergusonite, the first formula is preferred. Strutt finds that
it contains not only uranium, radium and helium, but also thorium in
considerable quantity (ThO₂ = 4·9 per cent.), a fact which had been
overlooked by Mallet. The rare earths contain a high proportion of
erbia.
It is tetragonal, _c_ = 1·4767, (001) ∧ (101) = 55° 54´. The crystals
are octahedral, with the form _p_ {111}; _p_ ∧ _p_´ = 79° 15´, _p_ ∧
_p_´´ = 128° 50´. Cleavage distinct ∥ _p_. It is usually granular and
amorphous. Colour brownish-black to brownish-red, lustre resinous.
Brittle. Hardness 6; sp. gr. 4·89. Translucent.
Its behaviour on heating has been already mentioned (see p. 39); it is
infusible. Boiling hydrochloric acid partially dissolves it; the
solution gives the turmeric test for zirconium, and on diluting and
adding metallic tin a sapphire-blue colour is developed, due to the
columbium present. Boiling concentrated sulphuric acid decomposes it
slowly.
It is found in Amhurst Co., Virginia, adherent to the allanite which
occurs there in large quantities. It was discovered there by Mallet in
1877, who named it, on account of the columbium (niobium) present, from
Sipylus, one of the sons of Niobe.[68]
[68] See Mallet, _Amer. J. Sci._ 1877, [iii.], ~14~, 397.
* * * * *
In this class, also, are to be included the following minerals (see
list):
_Nohlite_ and _Vietinghofite_, varieties of Samarskite.
_Hjelmite_ and _Kochelite_, minerals closely related to Yttrotantalite
and Fergusonite respectively.
_Koppite_, _Loranskite_, _Microlite_ and _Rogersite_, complex
tantalo-columbates containing elements of the cerium or yttrium groups.
(_b_) TANTALO-COLUMBATES CONTAINING TITANIUM DIOXIDE
~Æschynite.~--A columbate and titanate of the cerium metals, with
thorium, calcium, iron, etc. From the results of an analysis on a
specimen from Hitterö, Norway, Tschernik proposed the rather formidable
formula
2(2Ce₂O₃,3TiO₂),4(ThO₂,TiO₂),Y₂(CbO₃)₆,3(CaO,TiO₂),3Fe(CbO₃)₂,
Fe(TaO₃)₂,6TiO₂.
This can be simplified to Y(CbO₃)₃ + ThTiO₄ + ³⁄₂TiO₂, in which Y
represents rare earth metals partially replaced (2 atoms) by ferrous
iron (3 atoms), whilst thorium can be partially replaced by (2 atoms of)
ferrous iron or calcium. Strutt found it to contain the uranium-radium
combination and helium.
The crystals are orthorhombic, holosymmetric; _a_ : _b_ : _c_ =
0·4866 : 1 : 0·6737.
Common forms--brachy- and basal pinakoids _b_ {010} and _c_ {001},
prisms _m_ {110} and _r_ {120}, domes _d_ {101} and _v_ {021}, with
pyramid _o_ {111}.
(100) ∧ (110) = 25° 57´; (001) ∧ (101) = 54° 9´; (001) ∧ (011) = 33°
58´.
Habit prismatic, vertically striated, or tabular parallel to b with
horizontal striations. Brittle. Hardness 5 to 6; sp. gr. 4·9 to 5·7.
Colour nearly black. Nearly opaque.
It occurs at Miask, in the Urals, at Hitterö in Norway, and at
Fredriksvarn. The variety from the last locality is called Polymignite;
it was shown by Rose to be probably identical with Æschynite. Æschynite
was discovered by Berzelius at Miask and named by him from the Greek
αίσχύνη, shame, from the fact that its composition could not at that
time be determined.
If the ceria earths be largely replaced by yttria earths, a variety very
similar in appearance and angles, but approximating to polycrase
(_q.v._) in composition, is obtained. This mineral was found in 1879,
and referred to Æschynite; analysis subsequently showed its true
composition, and it was named Blomstrandine (_q.v._) by Brögger in
1907.
_The Isodimorphous Series Euxenite, Polycrase, Blomstrandine, and
Priorite._
Euxenite and Polycrase are members of an isomorphous series and vary
considerably in composition. The composition of the series is that of
mixed columbates and titanates of yttria earths (with, as usual, some
ceria earths), with uranium and zirconium, and water. Before the
isomorphous relation was recognised, Rammelsberg gave for Euxenite the
formula R´´´(CbO₃)₃,R´´´₂(TiO₃)₃,1¹⁄₂H₂O. The ratio of the acidic
oxides, Cb₂O₅ : TiO₂, is here 1 : 2. This is the greatest value of the
ratio, which varies for the series between 1 : 2 and 1 : 5.[69] The end
members, the pure metacolumbate and pure metatitanate respectively, are
unknown; all the members occurring in nature are to be regarded as
mixtures of these within the limits set by the ratios ¹⁄₂ and ¹⁄₅.
Brögger[70] suggests that the name Euxenite be retained for all members
for which the ratio is between ¹⁄₂ and ¹⁄₃, whilst for those minerals in
which it is less than ¹⁄₄ the name Polycrase be kept; these views have
been supported by Lange, who has analysed members of the series.
[69] Lange (_Abstr. Chem. Soc. 1911_, ~100~, ii. 499) gives the limits
¹⁄₂ and ¹⁄₆.
[70] _Abstr. Chem. Soc. 1907_, ~92~, ii. 885.
The members of this isomorphous series, however, are themselves
dimorphous, that is, can each crystallise in two different ways. The
second form corresponding to the Euxenites is known as Priorite, whilst
that corresponding to Polycrase is known as Blomstrandine; and these
second forms are themselves members of a parallel isomorphous series of
the same chemical composition, of course, as the first series. It is,
perhaps, undesirable to cite this as a typical example of an
isodimorphous series, since no end members of unmixed composition are
known. A perfect example of such a series is furnished by the oxides of
antimony and arsenic. Each of these compounds exists in two distinct
crystalline varieties, antimony trioxide, Sb₂O₃, as Valentinite
(orthorhombic) and Senarmontite (cubic), arsenic trioxide, As₂O₃, as
Claudetite (orthorhombic) and Arsenolite (cubic); and these two
modifications are isomorphous with one another, senarmontite with
arsenolite, and valentinite with claudetite.
In the case we are considering, the name Euxenite is applied to one
crystalline modification (A) of a number of isomorphous compounds within
certain limits of composition, the name Priorite to the second
crystalline modification (B) of the same compounds; the name Polycrase
is applied to compounds having the crystal form A, and a composition
varying within a second set of limits in the same chemical series,
whilst this second set of compounds in the crystalline form B is known
as Blomstrandine.
Stated as concisely as possible, the relationship is as follows: Each
member of this chemical series of continuously varying composition can
crystallise in two forms, which are the same for every member. The two
varieties at one end of the series are called euxenite and priorite, at
the other end polycrase and blomstrandine.
Thus, whilst euxenite and priorite, at the one end, and polycrase and
blomstrandine at the other, have the same compositions, euxenite and
polycrase have the same crystalline form, whilst priorite and
blomstrandine have the same second crystalline form.
All four minerals have the same bright black appearance, and bright
conchoidal fracture; they are all four isotropic, probably as a result
of hydration. All are orthorhombic, but the measurements for euxenite
and polycrase are different from those for blomstrandine and priorite.
The two latter are not so widely distributed as the two former.
Blomstrandine occurs at Hitterö, Arendal, and other localities in
Norway; priorite is found in Swaziland, South Africa.
The crystal system of the Polycrase-Euxenite series is orthorhombic, but
Dana gives slightly different axial ratios for the two minerals. This,
though Brögger gives the same values for both, is by no means
incompatible with isomorphism, as a glance at the axial ratios for the
minerals aragonite, strontianite, witherite, etc., of the series of the
orthorhombic carbonates, will show.
Brögger’s ratios for the two are _a_ : _b_ : _c_ = 0·3789 : 1 : 0·3527;
Dana gives for polycrase 0·3462 : 1 : 0·3124, for euxenite 0·364 : 1 :
0·303.
~Euxenite.~
This species occurs usually in the massive form as a bright
brownish-black mineral, of hardness 6¹⁄₂, and sp. gr. 4·6 to 5·0. The
crystals are prismatic in habit; the common forms are the pinakoids
_a_ {100} and _b_ {010}, the prism _m_ {110}, the unit pyramid _p_
{111}, and the dome {201}. Ramsay, Collie and Travers found no helium
in it; Boltwood found uranium, radium and helium, and Strutt found in
addition to these thorium. As early as 1879, Blomstrand had observed
zirconium in euxenite.
The mineral is infusible and with difficulty soluble in acids. It occurs
in many localities in Scandinavia (Hitterö, Arendal, Brevig, etc.), in
North Carolina, South Australia, etc. It was discovered by Scheerer at
Jölster, in Norway, in 1839.
The Euxenite-Polycrase series was studied by Hauser and Wirth in
1909,[71] in an endeavour to establish their theory that the proportions
in which the various earths and acids occur in this group of minerals is
subject to definite laws beyond the ordinary laws of combination. Thus
of the erbia earths they state that the proportion of holmia and
dysprosia increases relatively to erbia as titanium dioxide increases,
_i.e._ as we pass from the euxenites to the polycrases; at the same time
scandia and yttria increase relatively to the other yttria earths (the
terbia group), whilst in the ceria group samaria and praseodymia
decrease relatively to the others. Thus samaria is found in appreciable
quantities only when the titanium content is low. The original paper
must be consulted for full details.
[71] _Ber._ 1909, ~42~, 4443.
It was stated above that zirconium was found in euxenite in 1879. In
1901 Hofmann and Prandtl[72] declared that zirconia was an unfailing
constituent of the mineral, and that it was always accompanied by a new
oxide, which they named Euxenia (‘Euxenerde’). This was characterised by
the solubility of its oxalate in acid solutions, the insolubility of
the precipitated hydroxide in excess of alkali, and the gradual
precipitation by hydrogen peroxide from a slightly acid solution of its
salts. In their paper quoted above, Hauser and Wirth state that zirconia
is never present in typical euxenites. In a second paper[73] they state
that after exhaustive treatment of every known zirconia mineral, they
can find no trace whatever of the ‘new earth,’ and conclude that Hofmann
and Prandtl must have made some experimental error. During this
examination, they observed radioactivity in some minerals which
contained no traces of uranium or thorium.
[72] _Ibid._ 1901, ~34~, 1064.
[73] _Ber._ 1910, ~43~, 1807.
~Risörite.~[74]--A columbate of yttria earths, with titanium; ferric
oxide, alumina, lime and lead monoxide are present in small quantities.
It resembles fergusonite in composition, but differs in the almost
complete absence of uranium, the high loss on ignition, and the amount
of titanium present, which is here considerable (TiO₂ = 6·5 per cent.).
Hauser regards it as an orthocolumbate, R´´´(Cb,Ta)O₄, with an
isomorphous admixture of metatitanate, R´´´₂(TiO₃)₃.
[74] Hauser, _Ber._ 1907, ~40~, 3118; _Zeitsch. anorg. Chem._ 1908,
~60~, 230.
The rare earths are chiefly yttria, with some erbia earths and a little
terbia; ceria, lanthana and didymia are also present. The mineral
contains a considerable amount of helium, which is remarkable in view of
the very small content of uranium and thorium (cf. Thalenite). It is
radioactive, the active constituent being precipitated with the lead
(and to a very small extent with the rare earths).
It is infusible, but at a red heat it loses much water, and becomes very
brittle, with increase of specific gravity; no glowing is observed. It
is attacked by boiling concentrated sulphuric acid, and by fused
potassium bisulphate; also by hydrofluoric acid (40 per cent.), with
separation of the insoluble rare earth fluorides.
No good crystals have been found, and no crystallographic data are
known; examined by polarised light it appears isotropic, but this may
be due to alteration. Colour, yellowish- to greenish-brown. Streak,
yellowish-white. Hardness 5¹⁄₂; sp. gr. 4·179, increasing to 4·678
after ignition (cf. p. 38).
The mineral was found in a granite-pegmatite at Risör, South Norway.
~Wiikite.~[75]--A mineral of very complex composition, for which no
definite formula can be assigned. Its chemical nature may be understood
from the following analytical data:
Columbic and tantalic anhydrides = 16·0; Dioxides of titanium and
zirconium = 23·4; Silica = 17·0; Ceria = 2·5; Yttria = 7·6; Scandia =
1·2; Thoria = 5·5; Ferrous oxide = 15·5; Uranic oxide = 3·6; water (and
gas) = 5·8 per cent.
[75] Crookes, _Phil. Trans._ 1908, A, ~209~, 15.
Traces of lime, magnesia, stannic oxide and sulphur are also present.
The mineral is infusible; on heating, helium, sulphuretted hydrogen and
water vapour are given off, and a white sublimate is formed. The
evolution of gas is almost explosive, the mineral breaking with a
curious fracture.
It is black and perfectly amorphous, showing no trace of crystalline
structure or action on polarised light. Hardness, 6; sp. gr. 4·85.
Wiikite is partially attacked by acids, readily by fused potassium
bisulphate. It is radioactive.
The mineral was found with monazite in a felspar quarry at Impilaks,
Lake Ladoga, Finland. It is important as the source of scandium used by
Sir William Crookes in his investigations of that element; some
specimens of the mineral contain over 1 per cent. of the oxide (see p.
44).
* * * * *
The following related minerals, of which descriptions are given in the
alphabetical list, are to be included here:
_Arrhenite_, _Chalcolamprite_, _Endeiolite_ and _Wöhlerite_, are complex
tantalo-columbates containing silica.
_Hainite_ contains both silicon and titanium.
_Dysanalyte_ is a titano-columbate believed by Hauser[76] to be merely
an impure form of perovskite (see p. 14).
[76] Vide _Zeitsch. anorg. Chem._ 1908, ~60~, 237.
_Ilmenorutile_ and _Strüverite_ are closely allied minerals believed by
Prior[77] and Schaller[78] to be isomorphous mixtures of rutile with
Tapiolite or Mossite (ferrous tantalo-columbates).
[77] _Min. Mag._ 1908, ~15~, 78.
[78] _Abstr. Chem. Soc._ 1912, ~102~, ii. 773.
_Pyrochlore_ is a complex titano-columbate containing elements of the
cerium or yttrium groups.
_Blomstrandite_ is an hydrated titano-columbate of rare earth elements,
with calcium and uranium; it must not be confused with blomstrandine.
CHAPTER V
THE OXIDES AND CARBONATES
(_a_) THE OXIDES
~Uraninite~ or Pitchblende.--Uraninite consists essentially of oxides of
uranium (UO₂ + UO₃ = 75 to 85 per cent.), associated with thoria,
zirconia, rare earths, beryllia, and oxides of lead. Traces of lime,
iron oxides, silica, bismuth, and arsenic are also sometimes present,
with water in widely varying quantities. Nitrogen and helium are always
found in it, and, of course, radium. Groth regards pitchblende as
uranous uranate U^{iv}(U^{vi}O₄)₂, the uranium in the acidic radicle
being hexavalent and in the basic radicle tetravalent, and in the latter
condition partially replaced by lead, thorium, and rare earths.
Szilard[79] regards it rather as a loose compound or even a solid
solution of oxides of thorium and uranium,[80] with small quantities of
other oxides, he having obtained apparently homogeneous (though
non-crystalline) bodies by dissolving thorium hydroxide in solutions of
uranium salts and evaporating to dryness.
[79] _Compt. rend._ 1907, ~145~, 463.
[80] See under Thorianite, _infra_.
The cubic form of the crystalline varieties has been taken as indicating
that the mineral is really a spinel,[81] but it is difficult to see how
the general formula of that group can be considered comparable to the
uranyl uranate formula, UO₂,UO₃, for pitchblende.
[81] The Spinels are an isomorphous family of cubic minerals of the
general formula R´´O,R´´´₂O₃, where R´´ = Be, Fe, Mg, Ca, etc., and
R´´´ = Fe, Al, Cr, etc.
Crystals are rare, and belong to the cubic system, the common forms
being the octahedron _o_ {111} and the dodecahedron _d_ {110}; the
cube _a_ {100} is sometimes present. The mineral is massive, usually
botryoidal. The crystalline or primary form is black, with hardness
5¹⁄₂, sp. gr. 9·0 to 9·7; the altered varieties are grey to greenish-
and brownish-black, sp. gr. 5·0 to 6·4.
It is infusible before the blowpipe, but readily soluble in nitric acid.
The mineral occurs both as a primary and secondary constituent of rocks;
as a primary mineral it is found in Norway, North Carolina, etc.; as a
secondary species it occurs in the massive and hydrated form, with ores
of lead, silver, tin, etc., in Saxony and Cornwall, and at the
celebrated mine of Joachimsthal, in Bohemia. The latter deposits,
consisting of the massive and altered varieties, for which the name
Pitchblende is generally reserved, have been much used as a source of
radium, especially those at Joachimsthal, and the Cornwall ore.
Several varieties of uraninite have been distinguished by special names.
Crystalline varieties from Anneröd and Arendal in Norway are known as
Bröggerite and Cleveite respectively; Nivenite is a third form. In these
varieties uranium oxides have been replaced to a considerable extent by
the rare earths and thoria. An amorphous variety of doubtful
composition, produced by alteration, is known as Gummite; Uranosphærite
is a similar altered form.
~Thorianite.~[82]--This interesting mineral consists chiefly of thoria,
ThO₂ (55-79 per cent.), with oxides of uranium (11-32 per cent.), and
ceria oxides (1-8 per cent.); oxides of lead and iron are also present
in small quantities, and zirconia with silica, probably due to
associated zircon.
[82] Dunstan and Blake, _Proc. Roy. Soc._ 1905, A, ~76~, 253; Dunstan
and Jones, _ibid._, 1906, A, ~77~, 546.
Helium is present, and the mineral is strongly radioactive. A careful
analysis by Hahn[83] shows traces of many metals; the same chemist has
also separated an extremely active component, 250,000 times as active
as thorium nitrate, which he calls Radiothorium.
[83] Hahn, _ibid._, 1907, A, ~78~, 385.
The composition has been accounted for (Dunstan and Jones, _loc. cit._)
on the hypothesis that thoria (ThO₂) and uranous oxide (UO₂) are
isomorphous, the mineral being really a solid solution. Whilst, however,
the crystal system of the natural body is really rhombohedral (_vide
infra_) the two pure oxides appear to be cubic. Thus Troost and
Ouvrard[84] obtained artificial thoria in minute octahedra; and,
similarly, Hillebrand[85] obtained uranous oxide in octahedra by
reduction of uranyl chloride, UO₂Cl₂, though his work seems to be open
to objection. On the other hand, the same author[86] found that uranous
oxide and thoria, fused together in almost any proportions, gave a
homogeneous body crystallising in octahedra (cf. Szilard, _Compt. rend._
1907, ~145~, 463, quoted under Uraninite). The probability of the
isomorphism of the oxides is strengthened by the observation of
isomorphism in the sulphates. As early as 1886, Rammelsberg showed that
uranous sulphate, U(SO₄)₂, crystallises with nine molecules of water and
is isomorphous with the corresponding thorium sulphate, Th(SO₄)₂,9H₂O;
and six years later, Hillebrand and Melville[87] obtained mixed crystals
of the two sulphates which were exceedingly close in forms and angles to
those of pure uranous sulphate. It is then at least probable that the
two oxides are isomorphous, though the point cannot be regarded as
satisfactorily proved, by reason of the anomalous crystal forms of the
naturally occurring mixtures, thorianite and uraninite. The recent
results of Kobayashi[88] point to the conclusion that different
varieties of thorianite may exist, in each of which the oxides of
thorium and uranium bear definite simple ratios to one another.
[84] _Compt. rend._ 1882, ~102~, 1422.
[85] _Zeitsch. anorg. Chem._ 1893, ~3~, 243.
[86] _Bull. U.S. Geol. Surv._ No. 113, 1893.
[87] _Ibid._ No. 90, 1892, p. 30.
[88] _Abstr. Chem. Soc._ 1912, ~102~, ii. 1181.
Thorianite occurs in jet-black crystals with a bright resinous lustre.
They are pseudocubic, and the twinning resembles that of the cubic
mineral fluorspar--interpenetrant cubes, twin axis a cube diagonal.
Close examination shows, however, that twinning can only take place
about one of the four diagonals, and an optical examination makes it
clear that the symmetry is really rhombohedral. The case is exactly
analogous to that of the mineral chabazite, a zeolite which occurs in
rhombohedra of which the angles differ but little from those of the
cube, and which also forms the interpenetrant twins. In view of the
fact that both uranous oxide and thoria have been obtained as
octahedra, whilst a fused mixture of the two on cooling forms cubic
crystals, it seems not unlikely that at high temperatures the
pseudocubic thorianite would become truly cubic; but no experiments in
this direction seem to have been tried.
The crystals are brittle; hardness 7; sp. gr. 8·0-9·7.
Thorianite is infusible, incandescing before the blowpipe. When
powdered, it dissolves readily in nitric and sulphuric acids, with
evolution of helium. Gray[89] has shown that the helium content can be
reduced by 28 per cent. by fine grinding, thus showing that part at
least of the gas must be mechanically held.
[89] _Proc. Roy. Soc._ 1908, A, ~82~, 306.
Thorianite was found in Ceylon, being originally mistaken for
pitchblende. A sample was supplied by the discoverer, Mr. Holland, to
the officers of the Mineral Survey, by whom it was sent to London for
examination. Its composition was determined by Dunstan, who named it. It
was found in the river gravels (gem-gravels), the matrix being a
pegmatite granite. It is a valuable source of thorium nitrate for
incandescent mantles, one ton of the mineral (with thoria content of 70
per cent.) having been sold for £1500; but the supply is small and
unreliable.
~Baddeleyite.~[90]-- Baddeleyite consists of almost pure zirconia (ZrO₂
= 96·5 per cent.) with small quantities of ferric oxide, alumina, lime,
magnesia, alkalies and silica. Thoria and rare earths are present in
traces, uranium is absent; the mineral is not radioactive, and contains
only traces of helium.
[90] _Vide_ Fletcher, _Min. Mag._ 1893, 46, ~10~, 148; Hussak,
_Zeitsch. Kryst. Min._ 1895, ~24~, 164, and ~25~, 298.
Monoclinic--_a_ : _b_ : _c_ = 0·9871 : 1 : 0·5114. β = 98° 45¹⁄₂´.
Common forms--all three pinakoids, _a_ {100}, _b_ {010}, and _c_
{001}, with the hemi-prisms _m_ {110}, _k_ {120}, and _l_ {230}, and
various pyramids and domes.
Angles--(100) ∧ (110) = 44° 17¹⁄₂´; (100) ∧ (001) = 81° 14¹⁄₂´; (100)
∧ (101) = 55° 33¹⁄₂´.
Cleavage ∥ _c_ and ∥ _b_, parting ∥ _m_ due to repeated twinning.
Twinning is exceedingly common; of many hundred crystals examined by
Hussak, only three were found untwinned. Twin planes _m_ (110), _a_
(100), and _x_ (201).
Colour brown, varying in zones by twinning, with distinct pleochroism.
Hardness; sp. gr. varies from 4·4 to 6·0, being about 5·5 to 5·6 for
fairly pure material. Double refraction negative, 2 E = 70-75°. Acute
bisectrix nearly coincident with _c_ axis, plane of the optic axes
_b_, (010).
The mineral is insoluble in acids, readily soluble in fused potassium
hydrogen sulphate. Before the blowpipe it is almost infusible; it
dissolves in the fused borax bead, rapid cooling causing separation of
crystals. If a bead containing zirconia be heated until the borax is
partially volatilised, zirconia crystallises on cooling in tetragonal
crystals, isomorphous with those of rutile.[91]
[91] Nordenskiöld, _Pogg. Ann._ 1861, ~114~, 625; for tetragonal
zirconia see also Troost and Ouvrard, _Compt. rend._ 1888, ~102~,
1422.
The mineral was discovered in 1892 by Hussak and L. Fletcher
independently. The former, who obtained it from the pyroxenite sand of
São Paulo, South Brazil, believed it to be a tantalo-columbate, and
called it Brasilite. Fletcher found it in a gem-gravel from Rakwana,
Ceylon, and named it Baddeleyite. An analysis by Blomstrand of Hussak’s
mineral showed it to be identical with the Ceylon mineral, and Hussak
withdrew his name and accepted Fletcher’s. It has recently been
found[92] in a corundum-syenite, near Bozeman, Montana, U.S.A.
[92] Rogers, _Amer. J. Sci._ 1912, [iv.], ~33~, 54.
The mineral now comes on the market in commercial quantities; pure
zirconia almost entirely free from iron can be obtained by leaching with
acids. The pure oxide is extraordinarily refractory, and promises to be
of great use for crucibles, furnace linings, etc. (_vide_ p. 324).
~Rutile.~--Titanium dioxide, TiO₂, occurs crystallised in nature in the
three minerals Rutile, Brookite, and Anatase (Octahedrite), which
therefore form a trimorphous series. They are all stable minerals,
though rutile appears the most stable, being occasionally found in
pseudomorphs after the other two. The family is remarkable in that it is
not unusual to find two of them occurring together--an uncommon
phenomenon with polymorphous minerals.
Rutile often contains small quantities of iron and chromium. The
ferriferous varieties are distinguished as Nigrine, which is black, with
2-3 per cent. ferric oxide, and Ilmenorutile, with up to 10 per cent. of
ferric oxide, and specific gravity up to 5·13.
Crystal system--tetragonal, holosymmetric; _c_ = 0·6442; (001) ∧ (101)
= 32° 47´.
Common forms--prisms _a_ {100}, _m_ {110}, and _l_ {310}; pyramids _e_
{101}, _s_ {111}, and many others. The basal pinakoid _c_ {001} is
very rare. Habit, prismatic, with vertical striations; or in slender
needles. Twinning very common and varied; usually on the cassiterite
law--twin plane _e_ (101)--forming the knee-shaped twins, and
irregular rosettes by repetition, and many contact twins. Contact
twins on the law--twin plane _v_ (301) are less common.
Cleavage ∥ _a_ (100) and _m_ (110), distinct. Hardness 6-6¹⁄₂; sp. gr.
4·18-4·25, and up to 5·2 if much iron is present. Colour reddish-brown
to black, with good metallic lustre; transparent to opaque. The
refraction and double refraction are very high--ω = 2·6158, ε = 2·9029
for sodium light--and allow the crystals to be readily distinguished
in rock-sections.
The mineral is insoluble in acids, but can be dissolved after fusion
with alkalies or alkali carbonates.
Rutile is a member of the isomorphous series, cassiterite, zircon, etc.
(see under Thorite), and in particular it has the colour, appearance,
and twinning of cassiterite, from which, however, it is readily
distinguished by its lower specific gravity. In this connection it is
interesting to note that an apparently pure specimen, quite free from
inclusions, was found (1904) to contain 1·7 per cent. of tin
dioxide.[93]
[93] Friedel et Grandjean, _Bull. Soc. franc. Min._ 1909, ~32~, 52.
As an accessory rock mineral, and also as an important constituent of
many sands, rutile is of very wide distribution. It occurs, usually
imbedded in quartz or felspar, in many granites, syenites, gneisses,
slates, and allied rocks; in acicular crystals penetrating quartz it
forms the ‘Veneris Crinis’ of Pliny. At Risör and other localities in
Norway, it is found in the massive form, and it is largely worked at
Risör as a source of titanium. It occurs in all the countries of Europe,
and largely in America. Arendal, Kragerö, and Risör, in Norway, the
Binnenthal, the Urals, the St. Gothard, Castile, Magnet Cove in
Arkansas, Alexander Co. in N. Carolina, Barre and Shelburne in
Massachusetts, and Chester Co. in Pennsylvania are the chief localities.
It was in this mineral that the element titanium was first recognised by
Klaproth (1795).
~Anatase~ (Octahedrite) is the second crystalline modification of
titanium dioxide.
Tetragonal _c_ = 1·7771. (001) ∧ (101) = 60° 38´, (111) ∧ (11̅1) = 82°
9´.
Common forms--Prisms _a_ {100} and _m_ {110}, pyramids _p_ {111}, _e_
{101}, and many other complex forms; the basal plane _c_ {001} is
occasionally found. Habit usually octahedral, with _p_ or _v_
prominent; sometimes tabular with _c_, more rarely prismatic with _a_
well developed. Cleavage ∥ _c_ and _p_ perfect. Hardness 5¹⁄₂-6; sp.
gr. 3·82-3·95, usually increasing after heating. Lustre adamantine, so
splendent that in Brazil detached crystals have been mistaken for
diamonds. Colour, some shade of bluish-black to brown; by transmitted
light, greenish-yellow. Transparent to opaque. Double refraction
negative, strong; for sodium light ω = 2·554, ε = 2·493.
It is found at Bourg d’Oisans in Dauphiné, and in Norway, the Urals,
Brazil, etc. In Switzerland it occurs as the variety Wiserine, which was
at one time believed to be xenotime. It was named Octahedrite by de
Saussure, in 1796, from the prevailing habit, and Oisanite, from its
occurrence in Dauphiné, by Delamètherie, in 1797. The name anatase
(ανατασις = erection) was proposed by Haüy, being intended to denote
that the vertical axis (_c_ : _a_) is greater than that of rutile, the
other tetragonal modification of the dioxide.
~Brookite~, the third form of this compound, is orthorhombic.
_a_ : _b_ : _c_ = 0·8416 : 1 : 0·9444.
Common forms--the three pinakoids _a_ {100}, _b_ {010}, and _c_ {001},
prisms _m_ {110}, _l_ {210}, pyramids _e_ {122}, _z_ {122}, and
numerous others.
Angles--(100) ∧ (110) = 40° 5´; (001) ∧ (100) = 48° 18´; (001) ∧ (011)
= 43° 22´.
The habit is varied; it occurs usually in bipyramids with _e_ and _m_
or prismatic with _m_, _a_, and terminating pyramids. Cleavage ∥ _m_
indistinct, ∥ _c_ very poor.
Hardness 5¹⁄₂-6; sp. gr. 3·87-4·01. Lustre metallic. Colour brown to
reddish- and yellowish-brown and black. The optical behaviour is
interesting. The acute bisectrix is perpendicular to _a_ (100), but
while for red light the plane of the optic axes is (001), for blue it
is (010); for an intermediate light, therefore (λ = 5550 µµ), the
mineral appears uniaxial.
The chief localities are Bourg d’Oisans, Miask, the St. Gothard, the
Tyrol, Magnet Cove in Arkansas, and Tremadoc in Wales.
Titanium dioxide can be obtained crystalline by the action of steam on
titanium tetrafluoride, TiF₄, at high temperatures; it is stated that by
varying the temperature of the reaction, any one of the three
crystalline modifications can be obtained.
* * * * *
The only other minerals which need be mentioned in this class (see list)
are:
_Zirkelite_, a complicated mixture of oxides, in which thoria, zirconia,
and titanium dioxide act as acidic oxides, and
_Mackintoshite_, a mixture of several oxides, of which those of thorium
and uranium are the most important.
(_b_) THE CARBONATES
~Lanthanite~, Hydrocerite.--This mineral is a carbonate of ceria earths,
chiefly lanthana, of the formula La₂(CO₃)₃,9H₂O.
Orthorhombic; _a_ : _b_ : _c_ = 0·9528 : 1 : 0·9023. Common forms--the
pinakoids _a_ {100} and _c_ {001}, with _m_ {110} and _o_ {111}.
Angles--(100) ∧ (110) = 43° 37´; (001) ∧ (101) = 43° 26¹⁄₂´; (001) ∧
(011) = 42° 3¹⁄₂´.
Habit tabular, parallel to _c_; cleavage perfect, ∥ _c_.
Double refraction negative; optic axis plane _a_ (100).
Usually amorphous, being probably an alteration product of a mineral
rich in lanthanum. Hardness 2; sp. gr. 2·6-2·7.
Colour white to yellowish-white, usually opaque; infusible before the
blowpipe (being converted to the oxide), readily soluble in acids.
Lanthanite occurs with cerite at Bastnäs, and at Bethlehem,
Pennsylvania.
Morton[94] states that he prepared a crystalline didymium carbonate in
the laboratory, of the formula Di₂(CO₃)₃,8H₂O, which was isomorphous
with lanthanite; he concluded that the latter had only eight instead of
nine molecules of water.
[94] See abstract in _Zeitsch. Kryst. Min._ 1886-87, ~12~, 518.
~Parisite~ (~Synchisite~), and ~Cordylite~.--_Parisite_ is a
fluocarbonate of calcium and cerium metals; _Cordylite_ is an analogous
compound in which barium replaces calcium, and is isomorphous with
Parisite. The formula of Parisite is CaR₂F₂(CO₃)₃, where R = cerium
metals. Groth formulates this as (CaF)(RF)R(CO₃)₃, Penfield and Warren
as (RF)₂Ca(CO₃)₃, whilst Schilling gives Ce₂(CO₃)₃,CaF₂. Analogous
formulæ may be proposed for Cordylite, BaR₂F₂(CO₃)₃. Since the two
minerals are very similar in crystallographic properties, one
description will be sufficient for both. The following are Dana’s data
for Parisite:
Hexagonal, _c_ = 3·2891. (0001) ∧ (101̅1) = 75° 15´.
Forms are extremely numerous, and have remarkably high indices. Among
the simplest are the base _c_ {0001}, the prism _m_ {101̅0}, pyramids
_q_ {101̅2}, and _h_ {112̅2}; the other forms are chiefly rhombohedra
and pyramids. The usual habit is that of an acute double hexagonal
pyramid, with form _o_ {202̅1}, terminated by _c_. Cleavage ∥ _c_,
perfect.
It is brownish-yellow to red. Hardness 4¹⁄₂; sp. gr. 4·36.
The double refraction is strong, positive. Soluble in hydrochloric
acid with effervescence.
Both minerals are characteristic pneumatolytic species of the
riebeckite-ægirine rocks. Parisite was discovered by Paris in the
emerald mines of the Muso valley, Colombia, in 1835, and first correctly
analysed by Bunsen in 1845. Before the blowpipe it glows, remaining
infusible (the glow does not appear to have been investigated in this
case).
_Cordylite_ was discovered by Flink in 1900, in Greenland.
It is yellow to brownish-yellow and colourless. Hardness 4¹⁄₂; sp. gr.
4·31. Before the blowpipe it decrepitates, and is infusible; moistened
with hydrochloric acid, it gives the characteristic barium flame.
The so-called Synchisite was discovered by Nordenskiöld who correctly
described it as Parisite. Flink found it in Greenland, and announced it
as a new species, with the formula R₂F₂Ca₂(CO₃)₄, _i.e._ the formula for
parisite plus one molecule of calcium carbonate, CaCO₃. From its
extraordinary resemblance to parisite in physical and crystallographic
properties, Palache and Warren[95] believe that the specimens selected
by Flink for analysis must have consisted, in reality, of parisite with
admixed calcium carbonate. This conclusion has now been confirmed by
Quercigh, by a careful comparison of the optical properties.[96] The
minerals are usually found together, the chief localities being S.
Norway, the gold districts of the Urals, Narsarsuk in S. Greenland, and
Montana, U.S.A.
[95] _Amer. J. Sci._ 1911, [iv.], ~31~, 533.
[96] _Abstr. Chem. Soc._ 1912, ~102~, ii. 773.
* * * * *
The following rare earth carbonates are described in the alphabetical
list:
_Ancylite_, a basic hydrated carbonate.
_Tengerite_, a hydrated carbonate formed by the weathering of
gadolinite.
_Kischtimite_, a fluo-carbonate related to parisite.
_Bastnäsite_ (Harmatite) and _Weibyite_, hydrated fluocarbonates of the
cerium elements.
CHAPTER VI
THE PHOSPHATES AND HALIDES
(_a_) THE PHOSPHATES
~Monazite~, Phosphocerite.--Monazite, by far the most important,
commercially, of all the rare earth minerals, is essentially an
orthophosphate of the ceria earths, of the formula R´´´PO₄.[97] The
yttria earths are usually present in small quantities. Silica and
thoria, in quantities varying from traces up to 6 per cent. of the
former and from 1 to 20 per cent. of the latter, are invariable
constituents; it is almost entirely to the percentage of thoria that the
mineral owes its commercial value. The following also are common
constituents, though usually in very small quantities only--stannic,
ferric and manganous oxides, alumina, lime, magnesia, zirconia and
water. Helium was observed in it by Tilden, and by Ramsay, Collie and
Travers.[98] Boltwood[99] and Zerban[100] found uranium in it; the
latter attributed this to impurities, the former regarded it as an
essential constituent. Strutt[101] found uranium in a pure monazite.
Haitinger and Peters[102] detected radium, their result being confirmed
by Boltwood and Strutt.
[97] For the composition of the earths in monazite, see James, _J.
Amer. Chem. Soc._ 1913, ~35~, 235.
[98] _Trans. Chem. Soc._ 1895, ~67~, 684.
[99] _Phil. Mag._ 1905, [vi.], ~9~, 599.
[100] _Ber._ 1905, ~38~, 557.
[101] _Proc. Roy. Soc._ 1905, A, ~76~, 88 and 312.
[102] _Sitzungsb. kaiserl. Akad. Wiss. Wien_, May, 1904.
Monazite occurs in small crystals belonging to the monoclinic system.
_a_ : _b_ : _c_ = 0·9693 : 1 : 0·9256, β = 76° 20´. These values vary
slightly with different specimens. Common forms--Ortho- and
clino-pinakoids _a_ {100}, _b_ {010}, hemi-prisms _m_ {110}, and _n_
{120}, hemi-ortho-prisms _w_ {101} and _x_ {1̅01}, hemi-clino-prism
_e_ {011}, hemi-pyramid _v_ {1̅11}, etc.; the basal pinakoid _c_ {001}
is rare.
Angles--a ∧ _m_ = 43° 17´, _c_ ∧ _w_ = 37° 8´, _c_ ∧ _e_ = 41° 58´.
Habit tabular, parallel to _a_, needle-shaped by elongation parallel
to _b_ axis, or prismatic by good development of _v_.
Cleavage ∥ _c_, perfect, ∥ _a_, distinct, ∥ _b_, difficult.
Twin plane _a_ (100). Birefringence moderate, positive; plane of optic
axes perpendicular to _b_, nearly parallel to _a_. Acute bisectrix
inclined to _c_ at angle of 1°-4°. Dispersion feeble, ρ < υ. Brittle.
Hardness 5-5¹⁄₂; sp. gr. 4·9-5·3; conchoidal fracture. Lustre
resinous. Colour, red to brown, yellow, yellowish- and greenish-brown.
Transparent when pure; more often translucent to opaque.
Monazite is with difficulty soluble in acids; before the blowpipe it is
infusible; when moistened with sulphuric acid it colours the flame
greenish-blue.
The mineral often occurs massive, yielding angular fragments, but is
most common in rolled grains. It occurs in the gneiss of the Carolinas
and Georgia, and in sands derived from the gneiss, in Idaho and many of
the Pacific States; in Brazil, at various localities in the provinces of
Minas Geraes, Bahia, Espirito Santo; in Queensland, Australia; in
Madagascar; in Ceylon; near Travancore in India; in the Urals; in
Scandinavia, etc. The deposits of commercial value will be treated more
fully in the next chapter. It is of wide distribution as an accessory
constituent of granites, diorites, and gneisses.
Monazite was first described, under the name Turnerite, by Lévy,[103] in
1823; the specimen was from the collection of the English chemist
Turner, who thought it a variety of sphene (titanite), and was named
after him at the suggestion of the mineralogist Heuland. The specimen
was stated to have been found in Dauphiné, but in spite of considerable
examination of the question, the precise locality is still unknown. The
resemblance between Turnerite and the mineral later described as
monazite (μοναζειν = to be solitary) was pointed out by Dana in 1866,
and confirmed by Pisani, 1877. The name Monazite was first used by
Breithaupt[104] in describing a mineral found by Menge (1826)
accompanying zircon in a granite from Miask in the Urals. Breithaupt
concluded, from the high specific gravity, that the mineral contained a
heavy metallic oxide. It was again described as Mengite by Brooke[105]
in 1831. It was re-discovered by Shephard[106] in South Carolina in
1837, and described by him under the name Edwardsite, a variety from
Connecticut being called Eremite. To Shephard belongs the honour of
having discovered its true nature; after analysis he described it as a
‘Basic Sesquiphosphate of the Protoxide of Cerium,’ giving the formula
(modern notation) 3CeO,2P₂O₅, and finding also zirconia, alumina, and
silica in it (his specimen was probably very impure). Gustav Rose[107]
showed this to be identical with monazite in 1840. In 1846 Wöhler
described, under the name Cryptolite, a variety of tetragonal habit
closely resembling zircon. This occurs at Arendal in Norway, enclosed by
apatite, in the granite; it may be obtained by treatment with dilute
nitric acid, which dissolves the apatite.
[103] _Annals of Philosophy_, 1823, ~21~, 241.
[104] _Schweigg. J._ 1829, ~55~, 30.
[105] _Phil Mag._ 1831, [ii.], ~10~, 139.
[106] _Amer. J. Sci._ 1837, ~32~, 162.
[107] _Pogg. Ann._ 1840, ~49~, 223.
The question of the manner in which the thorium is combined in monazite
is of considerable importance, in view of the fact that it is to this
element that the mineral owes its commercial value. The amount present
varies from traces up to over 20 per cent., but the usual value is
between 5 and 7 per cent. The first explanation of its presence was
advanced by Dunnington[108] who suggested, on the result of only one
analysis, that orangite (ThSiO₄) was present mechanically mixed with the
monazite. Penfield[109] supported this suggestion, and stated that in
three analyses of pure material he found the ratio of rare earths to
phosphorus pentoxide and that of thoria to silica exactly equal to
unity, though the actual amounts of thoria varied considerably. He also
quotes an analysis made by Rammelsberg in 1877, in which no thoria was
found, to show that it is not an essential constituent. In a microscopic
examination he found dark resinous particles scattered throughout the
section; after moistening with hydrochloric acid, warming, and washing,
these dark spots became white, and could be stained with fuchsine, the
monazite remaining unaffected throughout. He concluded that these
particles were thorite or orangite.
[108] _Amer. Chem. J._ 1882, ~4~, 138.
[109] _Amer. J. Sci._ 1882, [iii.], ~24~, 250; 1888, ~36~, 322.
Blomstrand[110] disputed Penfield’s conclusions. In twelve analyses of
monazite from various parts of Scandinavia he never once found either
thoria or silica absent. Of these twelve analyses, two give the ratio of
thoria to silica, ThO₂ : SiO₂, exactly unity, in seven cases the ratio
is not greater than 1·25, in five cases it varies considerably. He
summed up his results in three statements:
(_a_) Silica is never absent; its amount depends not on the amount of
thoria, but on the amount of phosphorus pentoxide present.
(_b_) The thoria which is always present is combined partly with
silica, partly with phosphorus pentoxide.
(_c_) In most cases, the rare earths alone are insufficient to satisfy
the ratio R₂O₃ : P₂O₅ = 1.
[110] _J. pr. Chem._ 1890, ~41~, 266.
An exhaustive examination of the question has been made more recently by
Kress and Metzger.[111] They made in all over fifty analyses, using
thirty different specimens of monazite; they estimated silica both as
quartz and as silicate silica, and determined thorium by the fumarate
method--the other investigators had used the thiosulphate method of
Hermann (_vide_ p. 286). Their results may be summarised as follows:
(i.) Silica is always present.
(ii.) The amount of silica usually increases with the thoria, but not
regularly.
(iii.) By far the majority of cases showed insufficient total silica
to combine with the thoria present.
(iv.) In about 9 per cent. of the cases, the thoria present was
insufficient to combine with the silicate silica, from which it
follows that some foreign silicate must be at least occasionally
present.
(v.) A careful microscopic examination showed conclusively that no
thorite (ThSiO₄) was present, the silicate being biaxial; quartz is
present as such.
[111] _J. Amer. Chem. Soc._ 1909, ~31~, 640.
They conclude that thorium is present as phosphate, and is an essential
constituent, but that there is always some admixed silicate, most
probably a felspar.
~Xenotime.~--Chemically this mineral is closely allied to monazite,
being an orthophosphate of rare earths, containing silica and thoria;
whereas, however, in monazite the content of yttria earths does not rise
above 4 per cent., in xenotime these constitute by far the greater part
of the bases, the content of ceria earths ranging from 8·2 to 11 per
cent. The yttria earths, chiefly oxides of yttrium and the erbium group,
vary from 54·1 to 64·7 per cent. There are traces of zirconia; Ramsay,
Collie and Travers detected helium, whilst Boltwood, and also Strutt,
found uranium and radium. It also appears to contain traces of sulphuric
anhydride.
The crystals are tetragonal, holosymmetric. _c_ = 0·6187; (001) ∧
(101) = 31° 45´.
Common forms are the prisms _a_ {100} and _m_ {110}, the basal
pinakoid _c_ {001}, the pyramids _e_ {101}, _f_ {201}, _z_ {111}, etc.
Cleavage ∥ _m_, perfect. Uniaxial, double refraction strong, positive.
Transparent to opaque. Colour, brown to reddish-brown and yellow.
Hardness 4-5; sp. gr. 4·45-4·56.
It is insoluble in acids, and infusible before the blowpipe; when
moistened with sulphuric acid, however, it turns the flame bluish-green,
like most mineral phosphates (_vide_ monazite).
It is not so widely distributed as monazite, but is not uncommon. It
often occurs with zircon--to which it is very closely allied in crystal
form, if the two are not actually isomorphous--in parallel growth, in
granitic rocks. The diamond sands of Diamantina, Brazil, form the
richest source of the mineral, but it is also found in Scandinavia, at
Hitterö, Åro, etc.
The mineral is of considerable importance, chemically, on account of the
high percentage of erbia earths.
In the works of Bauer, Rosenbusch, Weinschenk, Schilling and Iddings
will be found accounts of a mineral named ‘Hussakite.’ These accounts
rested on the work of Kraus and Reitinger,[112] who in 1901 announced
the discovery of a new species. The crystals were obtained as a specimen
of xenotime by Prof. Muthmann from Dr. E. Hussak, in São Paulo, and had
the crystallographic properties of that mineral. On analysis, the amount
of sulphur trioxide present was found to be remarkably high (6·3 per
cent.), and Kraus and Reitinger concluded that the substance was
distinct from xenotime. They announced it as a new mineral, with the
name Hussakite, and the formula 3R₂O₃,3P₂O₅,SO₃ or 6RPO₄,SO₃, and stated
that by the action of dilute alkalies the sulphur trioxide could be
easily and completely removed. They therefore regarded xenotime as a
pseudomorph[113] after hussakite, the sulphur trioxide having been
removed from the latter by the action of the alkaline waters of the
earth’s crust. In support of this view, they gave analyses of opaque
crystals from a Bahia sand represented as containing 2·6 to 2·7 per
cent. of sulphur trioxide, and so as being intermediate forms produced
during the change.
[112] _Zeitsch. Kryst. Min._ 1901, ~34~, 268.
[113] One mineral is said to be pseudomorphous after another when the
first is produced from the second by a chemical change which proceeds
so slowly that the original structure and crystalline form are
unaltered (_i.e._ a change proceeding molecule by molecule). The
pseudomorph is usually opaque and shows clear signs of the alteration.
The latter conclusion was quickly challenged by Brögger, who found no
sulphur trioxide in a perfectly fresh and transparent xenotime from Åro
in Scandinavia. Brögger concluded that the Hussakite of Kraus and
Reitinger was an independent species of the formula 5YPO₄,(YSO₄)PO₃, and
that xenotime was not derived from it.
Basing his work on the barium chloride test given by Kraus and Reitinger
(see below) Rösler[114] declared that ‘Hussakite’ was a common accessory
constituent of igneous rocks, having been previously mistaken for
zircon, which it resembles in appearance and optical properties.
[114] _Zeitsch. Kryst. Min._ 1902, ~36~, 258.
In 1907 Hussak[115] published a paper in which he showed that the
mineral named after him was not a new species at all, but a xenotime of
prismatic habit. Analyses made at his request by Florence in Brazil, G.
T. Prior in London, and Tschernik in St. Petersburg, confirmed the
original values given by Gorceix (sulphur trioxide up to 0·25 per
cent.). He mentions Brögger’s analysis of the Norwegian specimen in
which Kraus and Reitinger had found 2-3 per cent. of sulphur trioxide,
but in which Brögger found none. He explains the results of Kraus and
Reitinger as due to the addition of barium chloride to the acidified
solution of the carbonate fusion of the mineral, by which barium
phosphate was precipitated; this was dried and weighed as barium
sulphate. Rösler’s tests are declared doubtful; xenotime is not a widely
spread rock constituent, the mineral in question being really zircon.
[115] _Centr. Min._ 1907, 533.
In face of these results, there can be little doubt that the name
‘hussakite’ is unnecessary and undesirable, since the mineral to which
it was applied is proved to be xenotime.
* * * * *
In the alphabetical list, particulars of the following rare earth
phosphates will be found:
_Castelnaudite_, a variety of xenotime containing zirconia.
_Churchite_ and _Rhabdophane_ (Scovillite), hydrated phosphates.
_Gorceixite_, an alumino-phosphate of alkaline and ceria earths.
_Retzian_, an hydrated arsenate of manganese, calcium and rare earth
metals.
(_b_) THE HALIDES
~Yttrocerite.~--This mineral is a fluoride of calcium and rare earth
metals, with water. A recent analysis by Tschernik[116] gives the
formula Ce₂F₆,2Y₂F₆,9CaF₂,2H₂O. Putting the rare earth metals together,
this gives 6RF₃,9CaF₂,2H₂O, or R₂Ca₃F₁₂,²⁄₃H₂O. Yttrocerite is of
interest since it was probably in the analysis of this mineral by the
discoverers, Berzelius and Gahn, that the double sulphate method of
separating the yttria from the ceria earths was first employed[117]
(_vide_ p. 156).
[116] _Abstr. Chem. Soc._ 1907, ~92~, ii. 362.
[117] _Schweigg. J._ 1816, ~16~, 244.
It is found only massive or granular. Colour usually white to
violet-blue, sometimes reddish-brown. Hardness 4¹⁄₂; sp. gr. 3·45.
Infusible, but loses colour before the blowpipe. When powdered, it
dissolves completely in boiling hydrochloric acid, and readily in
sulphuric acid with evolution of heat. It has been found at various
localities in Scandinavia.
~Yttrofluorite.~[118]--This is a fluoride of varying composition, very
similar to yttrocerite, but characterised by the absence of water, and
the very small ceria content (1·7 per cent.). It is thus a fluoride of
calcium and the yttrium metals.
[118] T. Vogt, _Centr. Min._ 1911, 373.
Cubic, with poor octahedral cleavage. Colour, yellow to brown and
yellowish-green; transparent to translucent, bleached by weathering.
Very brittle. Hardness 4¹⁄₂; sp. gr. 3·54-3·56.
It is very similar to fluorspar (except that the octahedral cleavage of
the latter is very good), and is regarded by Vogt as an isomorphous
mixture of the latter with yttrium fluoride (or with a double yttrium
calcium fluoride, which is less probable). This view would account for
the variations in composition, and also for the remarkable frequency
with which traces of rare earths are found in fluorspar (_vide_ p. 2).
Yttrocerite is regarded as a similar isomorphous mixture, but containing
cerium metals in addition to the yttrium group.
Yttrofluorite occurs in pegmatite veins in granite in Northern Norway,
with gadolinite, fergusonite, allanite, fluorspar, and the usual vein
minerals.
* * * * *
The other members of this family (see list) are:
_Fluocerite_, a basic fluoride of yttrium and cerium metals.
_Tysonite_, a hydrated fluoride containing carbonates.
It is to be noticed that fluorine is the only member of the halogen
family which occurs in nature in combination with rare earth elements.
This fact is possibly connected with the great age of the rare earth
minerals, and their formation during pneumatolytic metamorphism of
plutonic rocks (_vide_ Chapter I).
CHAPTER VII
THE MONAZITE SANDS
It has been stated that monazite is a not uncommon accessory constituent
of many rocks, particularly of granites, gneisses, diorites, etc. The
crystalline material, of which an account has been given, is found
sometimes in veins in these rocks, more often in tiny crystals
disseminated throughout the mass. Most of these monazite-bearing rocks
are extremely old, belonging to the Archæan or pre-Cambrian age, and
probably none are of secondary (Mesozoic) or later age. It follows,
then, that they have been subjected to erosion during practically the
whole immense period of which geology can give us any detailed
knowledge. Heat, frost, wind, the action of vegetation and of
percolating water, the innumerable weathering agents known to the
geologist, have been at work on them during countless ages, breaking,
crushing, dissolving; rains, brooks, rivers, even ocean-waves have
dissolved or washed away the fragments, sorted them out unerringly
according to density, and re-deposited them, now in a river-bed, now at
the base of some sea cliff, now in a wide alluvial plain from which the
water has long since retired. It is in deposits of this nature that the
monazite has been concentrated. Its relatively high specific gravity
(about 5·0) has secured its separation from the lighter mica, quartz,
and felspar of the parent-rock; but the heavier vein or accessory
minerals have, of course, been concentrated with it. Zircon is an
invariable constituent of these ‘monazite sands,’ as such deposits are
called; and others almost as frequently found are rutile, ilmenite,
sphene (titanite), and apatite. Common, too, are the characteristic
minerals of the metamorphic rocks, garnet, epidote, sillimanite,
tourmaline, etc. Rare earth minerals found in the monazite sands include
xenotime, fergusonite, samarskite, gadolinite, and allanite. The
remaining minerals are oxides of iron and tin, with, of course, a
considerable amount of quartz.
It is apparent, from what has been said above, that monazite will be
concentrated with the heaviest constituents of the rocks from which it
is derived. Very often, indeed usually, these rocks are precisely those
in which gold occurs, disseminated sometimes in tiny particles,
sometimes collected into nuggets in veins of quartz and pegmatitic
minerals. The erosion of these rocks concentrates the gold with the
heaviest minerals; and hence it happens that monazite is an almost
universal constituent of the gold- and gem-bearing sands and gravels. In
the Carolinas and in Brazil, monazite is found in the gold washings; and
though in the past the two have always been extracted separately, the
gold first and the monazite from the washings or tailings, there appears
to be no reason why a system calculated to extract both--where, of
course, the content is high enough--should not be put into operation in
the future.
A chemical test affords the only reliable method of detecting monazite
in a sand. A little of the sand is washed with water to remove the
lighter minerals and warmed with concentrated sulphuric acid. A few
drops of the liquid are poured off, evaporated to small bulk, and one
drop placed on a glass plate. This is placed under a microscope and one
drop of a concentrated solution of sodium acetate is added. If monazite
is present in the sand, tiny pointed oval crystals of sodium cerium
sulphate will separate.
On the commercial scale, monazite is extracted from the sands only, in
the manner described below. An effort was made in North Carolina in 1906
by the British Monazite Company, representing the South Metropolitan Gas
Light Company of London, to extract monazite from the rock in which it
occurs disseminated. The rock was crushed and powdered, and the monazite
separated by washing off the lighter particles on concentration tables
(see below). In the same year, however, the price of thorium nitrate was
suddenly lowered 50 per cent. by the German Thorium Syndicate, which
largely controls the Brazilian output of monazite, and the British
company stopped operations in 1907. At present it may be said that only
the sands are available for profitable extraction.
Up to 1895, the Carolina deposits, which were worked chiefly by the
Welsbach Light Company of New York, either directly or indirectly,
supplied all the demand, but in that year the Brazilian sands were first
worked, and a keen struggle commenced for the market. The American
companies, after keeping up a considerable output for some years, were
forced to suspend operations in May 1910. The Brazil deposits, worked by
the German Thorium Syndicate and the Austrian Welsbach Company, which
have an agreement, now meet practically the whole demand. The Brazilian
sand occurs chiefly along the shores of the southern provinces, having
been concentrated by the action of the tides from the products of
erosion of the cliffs; it is very uniform and considerably richer than
the Carolina sand, and owing to its occurrence on the sea-shore, the
cost of transporting it is very low. It is exported chiefly to Germany,
recently also to the United States, and to a small extent lately to
England. The method of working it is similar to that employed in
Carolina--namely, concentration by washing and magnetic separation.
THE NORTH AMERICAN DEPOSITS[119]
[119] See Nitze, _Bull._ No. 9, _N. Carolina Geol. Survey_, 1895; also
Test, _Colorado School of Mines Bull._ Vol. IV. No. 2, p. 125, Jan.
1908.
There are two important regions in North America within which monazite
sands occur; one extends over the Carolinas, and the north-western part
of Georgia, the other over the Idaho basin and neighbouring counties of
the Pacific Slope. It will be best to treat these separately, as the
deposits are somewhat different in character.
(_a_) ~The Carolina Deposits~, including the unimportant Georgia
deposits, which belong to the same field, occur over an area approaching
4000 square miles. The area is occupied chiefly by the Piedmont plateau,
which is drained by a number of streams rising in the South Mountains,
an eastern outlier of the Blue Ridge; it is in the basins and valleys of
these streams, particularly at the head-waters, that the monazite is
chiefly found. The geology of the district is very complicated,[120] the
rocks being very highly altered granites. The chief bed is known as the
Carolina gneiss, and includes several types of gneiss, usually very much
weathered. The sands, which average about 1 per cent. of monazite, are
worked in and near the stream beds; they occur in the beds, and in
layers 1 to 2 feet in thickness a few feet below the surface of the
surrounding soil.
[120] See Sterret, _U.S. Geol. Survey_ (_Minerals_), 1906, p. 1195.
Concentration was formerly effected chiefly by a crude process of
washing. In this process the sand is thrown on to a sort of sieve, fixed
over the upper end of a long wooden trough, by one workman; a jet of
water is directed on to the sieve, washing the sand through it. The
heavier particles fall to the bottom of the trough, whilst the lighter
are washed right through. A second workman continually turns over the
sand left in the box and on the sieve; at the end of a day’s work the
‘concentrate’ is collected. This averages from 15 to 70 per cent. of
monazite, according to the nature and amount of the heavy minerals
accompanying it in the sand. The concentrate is dried either on rubber
or oiled cloths in the sun, or on an iron plate covering a trough in
which a fire is lighted. The iron minerals are then picked out by means
of a magnet, and the sand filled into sacks for transport.
Before treatment for thorium nitrate, the sand is at the present day
further concentrated by powerful magnetic separators. In a few cases the
older method of concentration by hand-washing has been abandoned for
machine concentration, the Wilfley table being sometimes employed. The
principle here is exactly the same, the sand being fed into a hopper by
means of a moving belt and thence on to a machine-shaken table from
which running water constantly removes the particles, sorting them
according to their specific gravity.
Further separation of the dried concentrate has been effected by three
kinds of separators.[121] The first was of the Edison, or
fall-and-deflection type; in this the sand is allowed to flow in a thin
vertical stream past a horizontal magnet, which deflects the minerals
containing iron; these fall on one side of a partition, the part richer
in monazite on the other. The second was an electrostatic machine; the
heated sand is borne on a moving belt underneath a rotating vulcanite
cylinder, excited by felt-covered rubbers; the lighter particles are
attracted to the cylinder, and dropped on one side, the heavier passing
on. Neither of these machines is of much value in effecting
concentration, and neither is in general use.
[121] See Pratt and Sterrett, _Trans. Amer. Inst. Min. Eng._ 1909,
~40~, 313.
The third, and by far the most efficient and most widely-used machine,
is known as the Wetherill electro-magnetic separator. It depends on the
principle, first applied by the American engineer Wetherill, that not
only the iron minerals, but a large number of other minerals may be
attracted if the magnetic field be sufficiently strong. In all types of
this machine used in cleaning monazite concentrates, four magnetic
fields of increasing intensity are traversed by the sand; the first
removes magnetite, ilmenite, and the larger fragments of garnet; the
second removes all the remaining garnet and ilmenite; the third removes
the coarser, and the fourth the finer monazite, tailings of zircon,
rutile, and silica passing on. Careful adjustment of the magnetic fields
will readily give a 97-99 per cent. monazite.
Two types of this machine are in common use. In the first the magnetic
fields are obtained by four successive electro-magnets, arranged so that
a broad horizontally-moving belt passes between the poles of each in
succession. The upper poles are ground down to a fine edge perpendicular
to the direction of the belt, to secure a more powerful field. Just
beneath these edges, and just above the broad belt are four rapidly
driven horizontal belts moving at right angles to the first or main
belt; these carry off and deposit in separate bins the minerals
attracted by their respective magnets. This type is known as the Rowand
separator.
In the second type four horizontal belts are arranged in the form of
descending steps, as shown in the diagram (Fig. 2). The magnets are
placed at the end of each belt, and within it. The attracted mineral is
held to its own belt, whilst the remainder drops on to the next; the
attracted mineral falls into a bin as soon as its belt carries it out of
the magnetic field. The sand to be cleaned is fed on to the first belt
by means of a hopper.
The almost pure monazite so obtained is now treated chemically for its
thorium. The processes proposed and in use are described in Chapter
XVIII.
[Illustration: FIG. 2.]
As already stated, the extraction of monazite in the United States has
practically ceased; but the processes outlined above, which were first
brought into use in the Carolinas, have been adopted for the treatment
of the Brazilian sands.
(_b_) ~The Idaho Deposits.~--Monazite was first observed in placer-gold
deposits in the vicinity of Boise city near the Snake river. This
deposit was a gold-bearing sand derived from granite. Later the
gold-bearing sands of Oregon were also found to contain monazite; these
sands are rich in zircon, and contain platinum and allied metals as well
as gold. The sands of the Pacific slope are the so-called black sands,
derived from hornblende, and augite-granites, usually porphyritic, which
are much weathered at the surface. The soil is loose and is largely
composed of granite fragments; the rain and streams constantly bring it
down to the valleys, and continually renew the deposits. The
concentrates obtained by washing are rich in well-crystallised zircon,
with titanite and garnet.
In 1906 a company was formed to extract monazite from the black sand
residues left after the extraction of gold. By 1909 they had erected
plant and commenced operations at Centerville, and proposed to work the
poorer auriferous sands for gold during the monazite washing. This, it
was expected, could be done by washing the sands in boxes lined with
amalgamated copper plates, which would retain the gold. Considerable
amounts of monazite had already been extracted from the tailings when a
disastrous fire put a stop to the operations in 1910.
Since then the production of monazite in the United States has
practically ceased.
THE BRAZILIAN DEPOSITS
Brazil first became a serious competitor in the world’s market with the
United States, for the supply of monazite, in 1895. The greater
percentage of thorium, the more even quality of the sands, and above all
the occurrence on the sea-coast, rendered the Brazilian monazite cheaper
from the beginning, so that it soon ousted the Carolina sand, and since
1910 has supplied the whole demand. The deposits at present worked lie
along the coasts of Bahia, Minas Geraes and Espirito Santo, and whilst
they are very rich in monazite, there is the disadvantage that their
position and extent, and so also the possibility of working them, depend
very largely on the variations in the tides, etc. The largest of these
deposits is on the shores of a bay near the island of Alcobaca, on the
southern coast of Bahia.
Monazite also occurs to a considerable extent in the diamond sands and
gold-bearing sands of many of the interior provinces. In Minas Geraes it
has long been known to occur at the celebrated mining centres of
Diamantina and Ouro Preto, where xenotime and other rare earth minerals
are also found; it is also known at various localities in the
surrounding mining provinces of São Paulo, Goyaz, and Matto Grosso. More
recently, extensive inland deposits have been found by Freise, in the
province of Espirito Santo.[122] In the plateau-basin of the Muriahé and
Pomba rivers he found a sand known locally as ‘catalco’ which carries an
average of 2·1 per cent. of monazite and a gold-content of 1·75 grams
per ton. In the Aymoré’s mountains he found monazite, both massive and
granular, in pegmatite veins in granite; analysis showed a thoria
content of 9·23 per cent., which is very high. These deposits would form
a very valuable and extensive source of thoria, if the difficulties of
transport could be overcome.
[122] _Zeitsch. pr. Geol._ 1909, ~17~, 514; _ibid._, 1910, ~18~, 143.
At present, as stated above, only the beach deposits are worked. The
Brazilian Government has laid a very heavy tax on all monazite exported;
it is stated[123] that the German Thorium Syndicate pays 50 per cent. of
its profits in royalties to the Government. In spite of this, the high
quality of the sand and the low cost of transport have enabled this
combine to lower the price of thorium nitrate to a point at which the
Carolina sands cannot be worked, and it appears probable that the
world’s markets will be supplied for some time, at least, entirely from
Brazil. The methods employed in working the sand are similar to those
already described.
[123] _U.S. Geol. Survey_ (_Minerals_), 1906, p. 1195.
In the last few years monazite deposits have been found in various
places, notably in Australia, India, and Ceylon. In the latter locality
it occurs sparingly in the gem-gravels, in association with the much
more valuable thorianite and thorite, but the supply is uncertain, and
the minerals cannot be worked regularly. In Australia it occurs in
Victoria and in Queensland. In Victoria the deposits are poor in
monazite--about 0·025 per cent.--so that working is not profitable. In
Queensland it occurs in beach sands on the southern coast, with gold,
platinum, and cassiterite; there seems to be no reason why these
deposits should not be profitably worked when sufficient labour is
forthcoming. It also occurs in North Queensland, on the Walsh and
Tinaroo mineral fields; here it is found massive and granular in veins
in granite, associated with wolframite, molybdenite, and cassiterite.
Quite recently, deposits of considerable extent have been found near
Travancore, India.[124] These sands contain about 46 per cent. of the
mineral, which is itself very rich in thoria, containing about 10 per
cent. of the oxide; the unconcentrated sand is therefore as valuable as
a source of thorium nitrate as the ordinary Brazilian concentrates,
which average 4 per cent. or less of the oxide.
[124] _Bull. Imp. Inst._ 1911, vol. ix., No. 2, p. 103.
Monazite has also been observed in the tin-bearing sands of Embabaan,
Swaziland, South Africa, and in the province of Ottawa, Canada.
CHAPTER VIII
RADIOACTIVITY OF THE MINERALS
In the present chapter no attempt will be made to give a complete
account of all the phenomena of radioactivity which have been observed
in the mineral world. There are, however, a few problems of the highest
scientific interest which centre about the rare earth minerals, and
mention of these can hardly be avoided in a work which professes to give
a general account of the rare earth group. It is obvious that a detailed
treatment cannot be given without entering into phenomena which would be
quite beyond the range of the present work, and an excuse is hardly
needed, therefore, for the fragmentary and abbreviated account which
follows. The reader’s acquaintance with the general phenomena of
radioactivity is of necessity assumed.
Radioactivity (the spontaneous emission of special radiations) was first
observed by Becquerel, in 1896, in the case of potassium uranyl
sulphate, and was soon found to be common to all uranium compounds, and
to the metal itself. Mme. Curie showed that whilst in uranium salts the
degree of activity varies directly with the percentage of uranium, in
minerals containing the element the same rule does not hold. The
observation that pitchblende is considerably more active than the
uranium it contains led to the discovery of polonium[125] and radium in
1898. Exactly analogous phenomena were shown to hold for thorium salts
and thorium-containing minerals by Mme. Curie and Schmidt in 1898, and
in 1905 Hahn separated Radio-thorium from thorianite. In 1899 Debierne
discovered that the rare earths precipitated from the solution obtained
on treatment of pitchblende are associated with another extremely active
body, which he named Actinium; Giesel found that in the separation of
the rare earths this remains with lanthanum. In 1903 Ramsay and Soddy
experimentally confirmed the prediction of Rutherford and Soddy, that
radium would be found to produce helium continuously. The discovery of
these remarkable phenomena has modified many fundamental physical
conceptions, and has opened up a new field of scientific enquiry, which
is being developed with unexampled rapidity.
[125] Polonium, which was named by Mme. Curie in honour of her native
country, has been shown to be identical with Marckwald’s
‘Radio-tellurium,’ which was named by Rutherford ‘Radium F’; it is one
of the degradation products of radium.
It has been mentioned, in the accounts of the rare earth minerals given
above, that almost all these minerals are radioactive, _i.e._ have the
property of emitting specific radiations. Moreover, radioactivity, to
any considerable extent at least, is, with a few important exceptions,
confined to the minerals which have been already described. It has been
shown by many investigators, chief amongst whom are Strutt and Boltwood,
that the activity is usually due to the presence of uranium or thorium,
or both.[126]
[126] Hauser and Wirth (_Ber._ 1910, ~43~, 1807) observed activity in
some zirconium minerals containing neither thorium nor uranium.
After the discovery of helium in Cleveite (a variety of pitchblende,
_vide_ p. 13) in 1895 by Ramsay, a large number of minerals were
examined for this gas, and it was found that almost all the rare earth
minerals contain helium. The fact that these minerals are also for the
most part radioactive, naturally suggested some relation between the
activity and the presence of helium, and led directly to the discovery
that radium is continuously producing helium; and it became apparent
that helium has been accumulating in these minerals since their
formation, by the decay of radioactive elements. The question of the
origin of helium in minerals will be touched on again.
In 1904 Boltwood advanced the theory that radium is produced by the
degradation of uranium, the parent-element having, however, a much
greater half-life period. If uranium continuously produces radium,
whilst the latter decays much more rapidly than the former, it must
follow that in minerals containing uranium a state of equilibrium is
reached between uranium and radium, and the ratio of these two in all
minerals should therefore be constant, and independent of the geological
age. Boltwood examined a number of the minerals of which descriptions
have been given in the preceding chapters, and found the ratio to be
surprisingly constant.[127] Strutt also examined a large number of
minerals,[128] and whilst on the whole his results seemed to support the
theory, his values for the ratio were by no means so constant as those
of Boltwood. Strutt included in his examination the interesting
radium-containing mineral observed by Danne at Issy l’Evêque.[129] This
was a pyromorphite (lead chlorophosphate) containing neither uranium nor
thorium. Danne suggested that the radium was not an original
constituent, but had been introduced by the action of percolating
waters. This view was confirmed by McCoy and Ross,[130] who found that
the activity was entirely confined to the surface layer.
[127] _Amer. J. Sci._ 1904, [iv.], ~18~, 97; _Phil. Mag._ 1905, [iv.],
~9~, 599.
[128] _Proc. Roy. Soc._ 1905, A, ~76~, 88 and 312. _Ibid._ 1907, A,
~80~, 56.
[129] _Compt. rend._ 1905, ~140~, 241.
[130] _J. Amer. Chem. Soc._ 1907, ~29~, 1698.
Mlle. Gleditsch has also examined the question of the uranium-radium
ratio in minerals. Her earlier work[131] gave ratios which, whilst
constant for each mineral species, varied in much the same manner as
Strutt’s for different species, and afforded very little support to
Boltwood’s theory. Her more recent results,[132] however, are much more
closely in accord with the theory, which has been still further
strengthened by the work of Pirret and Soddy[133] and of Marckwald and
Russell.[134] It may now be regarded as firmly established that radium
is in the line of direct descent from uranium.
[131] _Compt. rend._ 1909, ~149~, 267; _Le Radium_, 1909, ~6~, 165.
[132] _Le Radium_, 1911, ~8~, 256.
[133] _Phil. Mag._ 1911, [vi.], ~21~, 652.
[134] _Ber._ 1911, ~44~, 777.
Boltwood had assumed that the helium in radioactive minerals is produced
from the uranium, during its disintegration. Strutt, however, disputed
this; his experiments showed that very little helium is found even in
the richest radium-uranium minerals unless thorium is also present.
Thus pitchblende contains a very high percentage of uranium, but
relatively little helium (there is usually a considerable thorium
percentage here too, so that nothing conclusive can be deduced from
this). Adams[135] found that carnotite, a mineral very rich in uranium,
but containing no thorium, contains no helium at all; he explained its
absence by the very loose texture and permeability of the mineral, which
would allow the gas to escape. Strutt concluded that whilst helium is
undoubtedly produced by disintegration in the uranium series, in
minerals it is produced more by thorium or, as more recent work
indicates, by radio-thorium, than by uranium.
[135] _Amer. J. Sci._ 1905, [iv.], ~20~, 256.
The question of the origin of helium in minerals is, however, not
definitely settled, for several anomalous cases are known. Thus the
yttria silicate, Thalénite (_q.v._), contains quantities of helium, but
no uranium or thorium is given in the analyses. Similarly, Risörite
contains a relatively large quantity of helium, but only traces of
uranium and thorium. In the last mineral, the active constituent is
precipitated with the lead, so that no radio-thorium appears to be
present. Further, Thomsen analysed a fluorspar from Ivitgut in Greenland
which he found to contain 27 c.c. of helium per kilogram. This specimen
contains no uranium, but gives off the thorium emanation in quantities
which suggest the presence of radio-thorium; moderate quantities of
thorium are also present. Since the α particle has been definitely
identified as a positively charged helium atom, it appears certain that
disintegration in all three series (uranium, actinium, and thorium
series) produces helium, and a mineral containing a member of any of
these series (which gives α rays or α ray-giving products) would also
contain helium.
Even so, there is a case in which the helium content is anomalous, if
not altogether beyond explanation at the present stage. In examining a
large number of minerals for helium, Strutt[136] found that some samples
of beryl, a beryllium aluminium silicate, contain a relatively very
large amount of helium, but only traces of thorium, and was altogether
inactive. The absence of any active constituent renders untenable the
ordinary explanations of the presence of such a surprising quantity of
helium. Boltwood has put forward a suggestion which in the present state
of our knowledge must be regarded as a provisional explanation. He
conceives that in the concentration of beryllium from the parent magma,
it may have become associated with some short-lived intermediate
radioactive element, which had been altogether separated from its
long-lived parent element in the process of concentration; this
intermediate element, having collected in the crystallised beryl,
decayed completely in the course of the great period which must have
elapsed, leaving the helium to which it had given rise during its
disintegration enclosed in the mineral. It is difficult to see how two
substances which must be so intimately connected as a parent-element and
its product could be completely separated in the process of cooling of a
magma; but since so little is known of the process of crystallisation of
minerals, the suggestion can hardly be rejected on geological grounds.
In any case, we have here only one strongly marked exception to the very
definite rule that in all cases in which helium occurs in minerals, it
is accompanied by and undoubtedly produced from, a radioactive element
or elements; and in the majority of cases, the helium in minerals is
produced by disintegration of uranium or thorium and their products.
[136] _Proc. Roy. Soc._ 1908, A, ~80~, 572.
Strutt found that traces of helium are universal in the mineral world.
His method of determining helium was approximate only. He obtained the
gas content by heating the powdered mineral--a method which, as Wood has
shown,[137] will only give all the gas when very high temperatures (up
to 1000°C.) are employed. The gases were freed from oxygen and hydrogen
by passing over a heated, partially oxidised, copper spiral, and from
carbon dioxide by means of potash. Nitrogen was removed by sparking with
excess of oxygen and shaking over potash; the excess of oxygen was
removed by melted phosphorus. The inert gases so obtained were freed
from all impurities by the use of the liquid alloy of sodium and
potassium for the electrodes of the spectrum tube in which the gases
were examined spectroscopically.[138] Argon, if present--it seems to be
a universal constituent of igneous rocks, into which it may have been
absorbed from the air--was removed by charcoal at a temperature of
-80°C. The helium so left was examined spectroscopically, and measured
in a MacLeod gauge.
[137] _Proc. Roy. Soc._ 1910, A, ~84~, 70.
[138] As soon as the discharge is started in such a tube, all the
gases present other than those of the helium family are absorbed by
these electrodes.
As stated, helium was found in traces in nearly all minerals, and its
presence is to be attributed to traces of radium, which also appears
universal. In minerals containing uranium or thorium, or rare earths
(the latter are almost always accompanied by uranium and thorium),
helium is found to a much greater extent, and Ramsay considers it
possible that some fraction of the helium content may arise from the
rare earth metals. There is, however, no positive evidence to support
the conjecture. He found that the helium ratio, _i.e._ the volume of
helium per gram of uranous oxide, UO₂, varies with the amount of thoria
present; but where the latter is absent the variations are much less
marked. If helium were produced in a mineral from uranium alone, and
none escaped, it is obvious that the helium ratio would depend only on
the age of the mineral. For minerals of about the same age, and
containing no thorium, the helium ratio would be roughly constant, if no
disturbing factor required consideration.
In 1905 Strutt pointed out that in all the minerals he had examined,
thorium was never present unless accompanied by uranium and radium,
whilst uranium and radium often occurred without thorium. He suggested
that the present atomic weight of thorium, 232·5, was too low, and that
it was really the parent of uranium (at. weight 238·5); he further
supposed that the next permanent member in the line of descent was one
of the cerium metals. These suggestions have been negatived by later
work of Boltwood and Holmes. The former pointed out[139] that it was far
more likely that thorium is a disintegration product of uranium of
considerably longer life. On the whole, however, there is very little
positive evidence to connect thorium with uranium.
[139] Boltwood, _Amer. J. Sci._ 1905, [iv.], ~20~, 256.
In the same year Boltwood (_loc. cit._) drew attention to the persistent
appearance of traces of lead, bismuth, barium, etc., in the radioactive
minerals, and also pointed out that the variations of the ratio of
helium to uranium in pitchblende might be used to determine the age of
the mineral. In 1907 he suggested[140] that lead was the final product
of the degradation of uranium, from which it follows that the ratio of
uranium to lead should be constant for minerals of the same age (since,
lead decays, if at all, at an infinitely slower rate than uranium). He
collected all the available analyses, and classified the minerals dealt
with into six groups according to the value of the ratio. The order
given by the ratio was declared to be in accordance with the order of
age as given by geological evidence.
[140] _Amer. J. Sci._ 1907, [iv.], ~23~, 77.
Holmes[141] has further extended this work. He examined a number of rare
earth and allied minerals from the Christiania district, which Brögger
considers to be of approximately Lower Devonian age, and found the ratio
of lead to uranium to approximate quite closely, for almost all the
minerals examined, to 0·045. Representing the change in the usual way as
U → 8He + Pb
238·5 → 31·92 + 207·1
and using the data calculated by Rutherford and others for the rates of
decay, he gives the age of Lower Devonian strata as about 370 million
years. This figure is about twice as great as that deduced by
palæontologists from the flora and fauna, and greater still than the
times based on physical data, _e.g._ rates of cooling, precession and
nutation, etc. His figures for pre-Cambrian rocks, based on the same
ratio, range between 1000 and 1640 million years, the later being
deduced from a thorianite from the Archæan rocks of Ceylon. Strutt’s
figure for Archæan rocks is about 700 million years; this was derived
from work on the helium ratio, which must now be considered.[142]
[141] _Proc. Roy. Soc._ 1911, A, ~85~, 248.
[142] See Strutt, _Proc. Roy. Soc._ 1908, A, ~82~, 166; 1909, ~83~,
96; 1909, ~83~, 298; 1910, ~84~, 194.
In 1898 Travers[143] had examined the effect of heat on cleveite and
fergusonite, and found that about half the total helium, together with
hydrogen, is given off at a bright red heat. He considered it likely
that the helium was combined with a metal (though he recognised no
distinction between occlusion and combination) and remarked: ‘The
results of such experiments cannot therefore serve as a basis for
speculation as to the origin or history of the substances in question.’
The chemical inactivity of helium, however, as well as the experiments
of Moss and Gray, who showed that helium was evolved on grinding the
materials,[144] indicate that the gas is mechanically bound only. This,
however, introduces the difficulty, if an attempt be made to use the
helium-uranium ratio to calculate the age of minerals, that the gas
would be expected to escape from a porous material, so that its amount
is never so great as it should be. Strutt himself found that helium
escapes rapidly from powdered monazite, whilst even the solid mineral
was found to evolve helium at a rate much in excess of the probable rate
of production by radioactive changes. Similar results were found with
thorianite, and the only conclusion, since helium is found in the
minerals, is that under the conditions under which these minerals exist
in the earth’s crust, this escape is checked or altogether prevented. It
follows, however, that any age determined from the helium ratio must be
a minimum age, since there is always the chance of loss; this of course
is not the case--except where the minerals have suffered chemical
changes--with the lead ratio, and may account for the discrepancies
observed.
[143] _Proc. Roy. Soc._ 1898-99, ~64~, 140.
[144] _Vide_ Gray, _Proc. Roy. Soc._ 1908, A, ~82~, 306.
Strutt’s earlier work on the helium ratio was made with phosphate
minerals (coprolites and fossil bones) of known ages. The ratios found
were not in order of age, the minerals being very permeable, so that
helium had probably been lost. He next turned his attention to igneous
rocks, and selected zircon for the work. Here he obtained some sort of
regularity in the order of age and the order given by the ratio, and
assumed that if helium were lost at all, it must be lost in roughly
proportional amounts by reason of the similarity in conditions.
Geological criticism tends to lessen the trustworthiness of the
conclusions; it is pointed out that the age of a specimen of zircon is
not necessarily that of the rock in which it occurs, for zircon is an
extremely stable mineral, and might survive unchanged several fusions
and re-crystallisations of the magma. Strutt replies to this that at the
temperature of fusion of a rock, zircon would certainly give up its
accumulated helium, so that the age determined from the helium content
would be that of the last fusion, _i.e._ the age as given by geological
data. On the other hand, our ignorance of the real mechanism of the
crystallisation of a magma, and especially of the amount and effect of
the pressures obtaining, robs this reply of its force, and the objection
must be counted valid.
In still later work Strutt used sphene and thorianite, and his results
agree as well as can be expected. The sphenes used were all from Archæan
rocks, except one, which was from a Tertiary volcanic deposit of the
Laacher See, near Coblenz (the lake is in the crater of an extinct
volcano). In this case the helium ratio was very much smaller (about
¹⁄₄₀₀₀ of the values for Archæan rocks) indicating the (comparatively)
extremely recent formation of the deposit.
The most recent results in the study of radioactivity point to the
conclusion that elements which differ in atomic weight and radioactive
properties may be chemically identical, or at least chemically
inseparable; such elements have been termed isotopes. The end product of
the thorium series of radio-elements should have an atomic weight of
about 208·4, and it has been suggested that the element actually
produced in this series of changes may be bismuth. The latest results,
however, rather point to the conclusion that disintegration in the
thorium series gives rise to an isotope of lead. If this hypothesis be
true, the lead derived from a mineral rich in thorium and poor in
uranium should have an atomic weight appreciably higher than that of
ordinary lead. Experiments to test this conclusion have recently been
carried out by Soddy and Hyman.[145]
[145] _Proc. Chem. Soc._ 1914, ~30~, 134.
These authors have made analyses of Ceylon thorite, which they find to
contain 0·35 per cent. of lead; from the ratio of thorium to uranium in
the mineral, they calculate that the lead should have an atomic weight
of 208·2, that of ordinary lead being 207·1. Preliminary comparative
experiments on 1 gram of pure lead chloride extracted from the mineral
point to an atomic weight for the thorite lead of 208·4, a result
surprisingly in accord with theory. More extended experiments on this
most interesting question are in progress.
The present chapter would be incomplete without a reference to the
interesting work of Goldsmidt on radioactivity as an aid in identifying
mineral species.[146] He describes a simple method by which the activity
of a mineral may be rapidly and easily measured to a sufficient degree
of approximation, and shows how the determination enables a line to be
drawn on a diagram already mapped out; this line will intersect an area
on the diagram which corresponds to the particular mineral. Owing to
lack of analytical data, and to the great difficulty of determining with
accuracy small quantities of uranium and thorium, the method is at
present of scientific interest only; but it is capable of development,
and its development would be of undoubted value in the further study of
this branch of radioactivity.
[146] _Zeitsch. Kryst. Min._ 1907-8, ~44~, 545; _ibid._ 1908, ~45~,
490.
In order to make this part of the subject as clear as possible, the
chief points in this chapter are summarised as follows:
1. Radioactivity is only observed to an appreciable extent in some
rather rare minerals. These minerals as a rule contain radium, uranium,
thorium, rare earths, and helium.
2. The helium has been produced during geological time by the
degradation of one or more members of the three series of active
elements (the Uranium, Actinium, and Thorium series).
3. Radium is a degradation product of uranium, and itself is degraded
continuously; the final product of degradation is probably lead.
4. The age of minerals has been calculated from the ratio of lead to
uranium; the figures obtained are much greater than those put forward by
geologists and physicists.
5. The helium ratio has also been used, but appears less trustworthy,
owing to escape of helium, and uncertainty as to geological age of the
minerals employed.
6. Some connection between radioactivity and the presence of the yttrium
or cerium metals appears highly probable, but no satisfactory theories
have been advanced on this point; it has been shown that actinium is
very closely allied to lanthanum.
PART II
THE CHEMISTRY OF THE ELEMENTS
CHAPTER IX
GENERAL PROPERTIES OF THE CERIUM AND YTTRIUM GROUPS
The chemistry of the rare earth elements begins in the year 1794, with
Gadolin’s discovery of the new oxide ‘Ytterbia,’ for which the name
Yttria was subsequently proposed by Ekeberg, and generally adopted (see
Chapter I, and under Gadolinite, p. 35). The discovery of Ceria followed
in 1804 (see under Cerite, p. 32). The classical work of Mosander,
carried out between 1838 and 1842, showed the complex nature of the new
oxides. From ceria he separated three new earths, Ceria proper,
Lanthana, and Didymia. Yttria was shown to be a mixture of at least
three oxides, for which the names Yttria, Erbia, and Terbia were
proposed. These oxides were believed to have the general formula RO, by
analogy with the alkaline earths, which they were found to resemble in
many respects, notably in their strongly basic character.
The properties of the new oxides were examined during the next twenty
years by many chemists, the chief workers being Marignac, Rammelsberg,
and Hermann, but the next important advance was the investigation of the
absorption spectra of solutions of the rare earth salts, first suggested
by Gladstone in 1856, and developed more fully by Bunsen and Kirchhoff
in 1860 and the following few years. The introduction of the methods of
spectrum analysis furnished a very delicate and valuable method of
examining and identifying the various oxides, and so greatly assisted
the laborious processes of separation.
Sixteen elements (excluding thorium and zirconium) are at the present
time recognised as belonging to the rare earth group. With one or two
exceptions, these show the closest resemblance to one another, both in
chemical behaviour and in the properties of their compounds, so that the
difficulties of separating and purifying them are very great. They may
be said to form a series, in which the properties vary continuously but
gradually from member to member, so that no sharp differences are
anywhere perceptible. The method of division into groups is, therefore,
almost entirely one of convenience, and has arisen from the course which
the separations have followed.
The elements are divided into two chief families or groups, that of the
cerium metals and that of the yttrium metals respectively. The cerium
elements are separated by a process depending on the relative
insolubility of their alkali double sulphates; in this group are
included cerium, lanthanum, praseodymium, neodymium, and samarium. The
yttrium family is further divided into four sub-groups: the first
consists of scandium and yttrium; the second or terbium group of
europium, gadolinium, and terbium; the third or erbium group of
dysprosium, holmium, erbium, and thulium; and the fourth or ytterbium
group of ytterbium and lutecium--the element celtium, recently
discovered by Urbain, will also fall into this sub-group, but the
discovery awaits confirmation. Whilst scandium and yttrium fall into
somewhat abnormal positions, corresponding to their low atomic weights,
the terbium elements occupy an intermediate position between the cerium
elements and the remaining yttrium elements, or yttrium group proper,
and so are frequently classified as a third or intermediate group.
This list does not include all the names which have been put forward to
designate what have been claimed from time to time as new elements;
whilst the individuality of some of those included is not yet fully
established, and the homogeneity of others has been called in question.
The uncertainty is more pronounced among the yttrium elements than among
the cerium elements; owing to the opportunities for investigation
furnished by the commercial treatment of monazite, the chemistry of the
cerium group may be regarded as complete.
In the following table the elements are arranged in order of increasing
atomic weight, and it can be seen at once how closely the division into
groups follows this order:
ELEMENT ATOMIC WT. COLOUR OF SALTS
{Scandium, Sc 44·1 Colourless
{Yttrium, Yt 89·0 Colourless
{Lanthanum, La 139·0 Colourless
{Cerium, Ce 140·25 Cerous, colourless; ceric,
Cerium { orange to red
Group. {Praseodymium, Pr 140·6 Green
{Neodymium, Nd 144·3 Red to reddish-violet
{Samarium, Sa 150·4 Topaz yellow
Terbium {Europium, Eu 152·0 Faint rose
Group. {Gadolinium, Gd 157·3 Colourless
{Terbium, Tb 159·2 Colourless
{Dysprosium, Dy 162·5 Bright green
Erbium {Holmium, Ho 163·5 Yellow to orange
Group. {Erbium, Er 167·7 Deep rose
{Thulium, Tm 168·5 Bluish-green
Ytterbium {Ytterbium, Yb 172·0 Colourless
Group. {Lutecium, Lu 174·0 Colourless
In their chemical relations, the rare earth elements may be placed
between the metals of the alkaline earths, and the trivalent metals
iron, aluminium, and chromium. With the exceptions of cerium in the
ceric salts, and of samarium and europium in the recently discovered
dichlorides, they are uniformly trivalent, but the oxides are very
strong bases, and the salts very slightly hydrolysed in dilute
solutions; generally, therefore, they resemble the calcium family rather
than the aluminium group. Among the common salts, the oxalates,
phosphates, chromates, iodates, fluorides, carbonates, tartrates, and
borates are almost insoluble; the sulphates are only sparingly soluble
at ordinary temperatures. Among the double salts, the alkali double
sulphates are of great importance from their employment for separations;
the tendency to the formation of complex salts is greater among the
yttrium than among the cerium elements, increasing with the atomic
weight, and with the decrease in basic strength of the oxides.
The great similarity in chemical behaviour of the rare earth elements is
apparent not only in the similarity in composition, solubility and
chemical properties of the salts--which is so great that the general
account of the compounds which follows applies almost in its entirety to
each member of the group--but also in the crystallographic relations
between corresponding compounds. Many of the salt hydrates form
isomorphous series; the sulphate octohydrates, for example, appear to be
isomorphous throughout the whole group, and probably the relation would
be found to apply even more completely than is generally accepted, if
the necessary data were forthcoming. Of great interest and practical
importance is the isomorphism between the nitrates and double nitrates
of the cerium elements and bismuth, which has been utilised with such
valuable results in the processes of fractional crystallisation.
~The Metals.~--The earlier attempts to reduce compounds of the rare
earth elements to the metallic condition, by means of metallic sodium or
potassium, did not yield pure products; nor did the use of aluminium or
magnesium lead to results of practical importance. The metals were first
obtained in a coherent physical condition by Hillebrand and Norton,[147]
by electrolysis of the fused chlorides. These investigators obtained
cerium, lanthanum, and the so-called didymium, and measured their
specific heats; their results confirmed the atomic weights assigned to
the elements by Mendelejeff, except in the case of lanthanum. Their
method has since been elaborated by Muthmann, Hofer and Weiss,[148] who
have prepared large quantities of the cerium elements in the pure
state. More recently, Hirsch has prepared metallic cerium in large
quantities,[149] and has studied its properties.
[147] _Pogg. Ann._ 1875, ~155~, 631; ~156~, 466.
[148] _Annalen_, 1902, ~320~, 231; see also Muthmann and Weiss,
_ibid._ 1904, 331, 1.
[149] _Met. Chem. Eng._ 1911, ~9~, 543.
By electrolytic reduction of the mixed chlorides of the cerium elements,
a mixture known as ‘Misch metal’ is obtained; this has powerful reducing
properties, and, like aluminium, reduces the oxides of iron, chromium,
etc., with great development of heat.[150] The yttrium metals have not
yet been obtained in the pure state, the electrolytic method giving
unsatisfactory results on account of the high melting-points of the
metals, and the volatile nature of their chlorides.
[150] A full account of the properties and preparation of the cerium
metals and their alloys will be found in the monograph of Kellermann,
‘_Die Ceritmetalle und ihre pyrophoren Legierungen_, Wilhelm Knapp,
Halle, 1912.
The cerium metals are white or slightly yellowish in colour, and are
moderately stable in dry air. In moist air they tarnish slowly,
lanthanum, as the most positive, being most readily oxidised. The
melting-points and specific gravities are as follows:
Element Melting-point Specific Gravity
Cerium 623° 7·0242
Lanthanum 810° 6·1545
Praseodymium 940° 6·4754
Neodymium 840° 6·9563
Samarium 1300°-1400° 7·7-7·8
The metals decompose water slowly in the cold, but rapidly at the
boiling-point, with evolution of hydrogen. They have a great affinity
for oxygen, the heats of formation of the oxides being of the order of
those of alumina and magnesia:
Heat of Formation per Equivalent
Weight of Oxide[151]
¹⁄₃La₂O₃ 74·1 K
¹⁄₃Nd₂O₃ 72·5 „
¹⁄₃Pr₂O₃ 68·7 „
¹⁄₄CeO₂ 56·1 „
¹⁄₃Al₂O₃ 64·3 „
¹⁄₂MgO 71·9 „
[151] Muthmann and Weiss, _loc. cit._; K = 1 kilogram-calorie, or 1000
cal.
In consequence of the high values of the heats of combustion, the metals
have powerful reducing properties.
The cerium metals form alloys with magnesium, zinc, aluminium, and iron,
and combine with boron and silicon. The alloys of cerium, and the metal
itself, are remarkable for their property of emitting brilliant sparks
when scratched (see Chapter XXI). Cerium also forms an amalgam with
mercury.
The metals burn brilliantly when heated in oxygen, and dissolve readily
in dilute mineral acids. When heated to a temperature of 200°-300° in a
current of hydrogen, they absorb the gas very readily, forming the
_hydrides_. These compounds are also obtained by heating the oxides with
magnesium in a current of hydrogen. They were first prepared by
Winkler,[152] who deduced from his analyses the general formula RH₂; the
more recent work of Muthmann and Beck,[153] however, points to the
formula RH₃.
[152] _Ber._ 1890, ~23~, 2642; 1891, ~24~, 873.
[153] _Annalen_, 1904, ~331~, 58.
If nitrogen be substituted for hydrogen in either of the above methods
of preparation, _nitrides_ of the general formula RN are obtained;
cerium nitride, however, cannot be obtained by heating the element in
the gas.[154] These compounds are also obtained when the carbides are
heated in ammonia. They are amorphous solids, which yield ammonia when
acted upon by water.
[154] Dafert and Miklanz, _Monats._ 1912, ~33~, 911.
~Hydroxides.~--The hydroxides are thrown down as gelatinous precipitates
on the addition of alkalies to hot dilute solutions of the salts;
precipitation in the cold, or in strong solution, usually gives a basic
salt, or an hydroxide mixed with a large quantity of basic salt. The
hydroxides are insoluble in excess of precipitant, but the precipitation
is inhibited by the presence of some organic hydroxy-acids.[155]
[155] For effect of tartaric acid, see p. 133.
The hydroxides are insoluble in water, but dissolve very readily in
acids. The most basic of them absorb carbon dioxide from the air;
lanthanum hydroxide is exceptional in that it colours litmus blue.
Whilst hydrogen peroxide in neutral solution does not react with rare
earth salts,[156] alkalies in presence of this reagent precipitate
gelatinous hydrated peroxides, which are very unstable, decomposing on
standing, or on treatment with acids, with evolution of oxygen. The
general formula R₄O₉ + _x_H₂O was proposed for these compounds by Cleve,
but more recently the formula R(OOH)(OH)₂ has been advanced.[157]
[156] Compare behaviour of thorium and zirconium, Ch. XVI.
[157] Melikoff and Pissarjewski, _Zeitsch. anorg. Chem._ 1899, ~21~,
70; Melikoff and Klimento, _Chem. Zentr._ 1902, ~1~, 172.
~Oxides.~--In their most stable state of oxidation, the rare earth
elements are generally trivalent. In the case of cerium, the dioxide,
CeO₂, is more stable than the sesquioxide Ce₂O₃, but the ceric salts are
unstable, and are very readily reduced to cerous compounds,
corresponding to the oxide Ce₂O₃. Higher oxides are known with certainty
among the other elements only in the cases of praseodymium and terbium,
but these do not give rise to salts.
The oxides R₂O₃ are fairly strong bases, being comparable in strength to
the alkaline earths, and far more strongly basic than alumina and oxides
of other trivalent elements; thus they liberate ammonia from ammonium
compounds, whilst the salts they form with strong acids are not easily
hydrolysed. Their relative strengths as bases are expressed in the
following series, in which the elements are placed in order of
diminishing electropositive character:[158]
La, Ce´´, Pr, Nd, Yt, Eu, Gd, Sa, Tb, Dy, Ho, Er, Tm, Yb, Lu, Sc,
Ce^{iv}.
[158] The position of yttrium in this series is not known with
certainty; it is probably as positive as neodymium. It is usually
stated (see Meyer and Hauser, pp. 32-33) that the terbia oxides are
intermediate in basic strength between the ceria and yttria earths,
though the arrangement into two series, consisting of the cerium and
yttrium groups respectively, is generally adopted; the electropositive
character of the elements in each series then weakens as the atomic
weight rises, scandium being of course exceptional.
It will be seen that, with the exception of scandium and yttrium, the
metals of the cerium and yttrium groups become less electropositive as
the atomic weight increases.
This arrangement is obtained by ascertaining the order in which the
various hydroxides are precipitated from a solution by gradual addition
of a dilute solution of a strong base. The weakest base is precipitated
first, and the strongest last; those intermediate in strength are thrown
down in ascending order of strength. Similar results may be obtained by
the fractional decomposition of the nitrates by heat; in this case the
nitrate of the weakest base is decomposed at the lowest temperature.
This order is also confirmed, as far as the data are available, by
measurements of the equivalent conductivities of solutions of the salts
(see, for example, p. 122).
Quite recently, a very different order has been obtained from a
consideration of the dissociation tensions, and of the heats of
dissociation of the anhydrous sulphates.[159] In the following table the
elements are arranged in the order of the increase of the dissociation
tension (T) measured at 900°, which is the same as the order of decrease
of the heats of dissociation (Q):
Element At. Wt. T. (Mm. Hg.) Q.
La 139·0 2 59·8
Yt 89·0 3 58·9
Lu 174·0 3·5 58·5
Yb 172·0 4 58·2
Er 167·7 5 57·6
Pr 140·6 5·5 57·4
Nd 144·3 6 57·2
Gd 157·3 7 56·9
Sa 150·4 8 56·5
Sc 44·1 11 54·5
Ce 140·25 52·4
[159] Wöhler and Grünzweig, _Ber._ 1913, ~46~, 1726.
It will be observed that the order is very different from the order of
increase of atomic weight, the positions of lutecium and ytterbium being
especially surprising; these elements are generally considered to be
among the least electropositive of the whole series. The anomalous
position of cerium is probably due to the fact that the sulphate on
decomposition leaves the dioxide, and not the sesquioxide, as with the
other elements; this would undoubtedly affect the values. The heats of
dissociation are the greatest yet observed for the sulphates of
trivalent metals, a further evidence of the strongly basic nature of the
oxides.
Ignited lanthana resembles quicklime in that it readily absorbs carbon
dioxide from the air, and hisses when slaked with water; as the basicity
becomes weaker, the affinity for water and carbon dioxide becomes less
marked. All the oxides are soluble in dilute acids, even after prolonged
ignition; but the ease with which solution occurs is naturally much
influenced by the treatment to which the oxide has been subjected, as
well as by its strength as a base.
The rare earth oxides are capable of existing in more than one
modification, the compounds obtained by ignition of the hydroxides
differing in appearance and reactivity from those prepared by ignition
of the oxalates or nitrates, and so on; they are probably highly
polymerised. Cerium dioxide, CeO₂, is remarkable for its power of
combining with the other oxides, R₂O₃, of the rare earth metals. The
pure dioxide is insoluble in nitric acid, but mixtures of earths
containing up to 50 per cent. of the dioxide dissolve readily. The
various colours of mixtures of the ceria earths may sometimes be
attributed to a similar combination,[160] and there can be little doubt
that the dioxide sometimes functions as an acid in the rare earth
minerals.
[160] The brown colour of a mixture of ceria oxides containing
praseodymium is generally attributed to the presence of the strongly
coloured peroxide of that element.
~Sulphides.~--These compounds cannot be prepared in the wet way, that
is, by the action of hydrogen sulphide or ammonium sulphide on the salts
in solution; the former reagent gives no precipitate, the latter throws
down the hydroxides. In this behaviour, the rare earth elements resemble
aluminium and chromium.
The normal sulphides, R₂S₃, are obtained by reduction of the anhydrous
sulphates, or from the oxides at high temperatures, by treatment with
hydrogen sulphide. They are strongly coloured compounds, fairly stable
towards cold water, but readily hydrolysed on boiling.
Disulphides, RS₂, are known in the cases of cerium, lanthanum, and
praseodymium; these are to be regarded as polysulphides, since on
treatment with dilute acids they yield hydrogen persulphide, H₂S₂.
~Carbides.~--By reduction of the oxides with carbon in the electric
furnace, Moissan obtained the carbides in the form of microscopic yellow
crystals. They have the general formula RC₂, and are attacked by water
and dilute acids, with evolution of very complex mixtures of gases.[161]
The principal product is acetylene, with various higher homologues, and
in smaller quantities ethylene and ethane and their homologues. No
methane is formed,[162] but hydrogen is always present, the olefines and
paraffins probably arising from its action on the acetylenic
hydrocarbons. The relation of the rare earth elements to the calcium
group is here very close; calcium carbide when attacked by water yields
pure acetylene, whereas aluminium carbide gives pure methane.
[161] Damiens, _Compt. rend._ 1913, ~157~, 214.
[162] Moissan stated that 24-30 per cent. of methane was formed in
this action; compare _Compt. rend._ 1900, ~131~, 595.
~Halogen Salts.~--The halides of the rare earth elements show a close
analogy with the corresponding compounds of the alkaline earth elements.
The _fluorides_ are insoluble in water and dilute mineral acids, and are
obtained as gelatinous precipitates by the addition of hydrofluoric
acid, or a soluble fluoride, to solutions of the salts. They may be
prepared in the crystalline condition by heating the carbides in a
stream of fluorine, or by the action of hydrofluoric acid upon the
hydroxides in aqueous suspension. The rare earth elements, as well as
thorium, may be separated from zirconium by taking advantage of the
insolubility of their fluorides in excess of hydrofluoric acid or alkali
fluorides, since zirconium fluoride is readily soluble in excess of the
precipitant. The solubility of the fluorides in a large excess of
concentrated acid increases with the electropositive character of the
metal, the fluorides of the more negative elements being the least
soluble. Thorium and scandium may, therefore, be concentrated to a large
extent by repeated precipitation with hydrofluoric acid in acid
solution.
The _silicofluorides_ of the rare earth elements have been used by R. J.
Meyer in the extraction of scandium from wolframite (see Chapter I and
under Scandium, p. 215). They are thrown down as gelatinous precipitates
on addition of potassium or sodium silicofluoride to boiling, neutral
solutions of rare earth salts. In presence of mineral acids, however,
they are not thrown down in the cold; on boiling, the cerium metals are
precipitated as fluorides, by hydrolysis of the silicofluorides--the
yttrium elements, with the exception of scandium, being held in solution
by the mineral acid.
With the exception of the fluorides, the halogen salts of the rare earth
metals are readily soluble in water, and crystallise from the
concentrated solutions in the hydrated form. The bromides and iodides
have not been so fully studied as the chlorides; they are hygroscopic
salts, and decompose rather easily. The iodides have been obtained by
Moissan in the anhydrous state, by the action of iodine vapour on the
carbides at high temperature.
The anhydrous _chlorides_ may be obtained by the application of any of
the ordinary methods, _e.g._ by heating the oxides with carbon in a
stream of chlorine, by heating the carbides in the same gas, by heating
the sulphides or hydrated chlorides in hydrogen chloride, or by
evaporating the solutions of the hydrated salts to dryness in presence
of ammonium chloride, and then igniting till the latter has all been
removed. As obtained by any of these methods, they are fusible at a red
heat, but only slightly volatile; they are easily soluble in water or
alcohol, with disengagement of heat. They are insoluble in most organic
solvents, but dissolve to some extent in some bases; the chlorides of
the yttrium elements, for example, are readily soluble in pyridine. With
such solvents, the chlorides form compounds which may be considered as
derived from the hydrated forms, by replacement of the so-called water
of crystallisation by the organic base.
Conductivity measurements show that the salts are not perceptibly
hydrolysed in moderately dilute aqueous solutions, though the values for
the equivalent conductivities vary somewhat with the variations in the
electropositive character of the elements. In the following table, the
equivalent conductivities of the chlorides in solutions of dilution 32
and 1024 at 25°C. are given. It will be seen that the value (λ₁₀₂₄ -
λ₃₂) ÷ 10 is in all cases (except for the highly hydrolysed scandium
salt) very close to 3, an experimental proof of the trivalent nature of
the elements. The values for the chlorides of iron, aluminium and
chromium are included; it will be seen that these elements are
considerably less positive than the rare earth metals (with the
exception, of course, of scandium).
Salt λ₃₂ λ₁₀₂₄ λ₁₀₂₄ - λ₃₂
LaCl₃ 105·8 131·5 25·7
CeCl₃ 107·8 135·2 27·6
PrCl₃ 105·5 135·9 30·4
NdCl₃ 103·8 134·3 30·5
YtCl₃ 98·8 123·4 24·6
YbCl₃ 107·4 140·4 33·0
ScCl₃ 116·9 257·9 141·0
AlCl₃ 99·9 138·0 38·1
CrCl₃ 98·4 152·6 54·2
FeCl₃ 117·2 200·7 83·5
From aqueous solutions the chlorides crystallise with six molecules of
water, except praseodymium chloride, which has seven. The hydrated
salts, when heated to 120° in the air, form insoluble oxychlorides of
the general formula ROCl.
The chlorides do not show a great tendency to form double salts with
other metallic chlorides; on the other hand, they readily form complex
compounds with the chlorides of the less electropositive metals, e.g.
tin, bismuth, gold, and platinum.
Subchlorides of samarium and europium have recently been obtained; in
these compounds, for the first time, rare earth metals have been shown
to be capable of functioning as divalent elements.
_Cyanides_ of the rare earth elements are not known; addition of
potassium cyanide to solutions of the salts throws down the hydroxides.
The _platinocyanides_ may be obtained by double decomposition of the
sulphates with barium platinocyanide. They are very stable and
characteristic bodies, of the general formula R₂[Pt(CN)₄]₃, with 18 or
21 molecules of water. The compounds of the cerium elements are yellow,
with a strong blue fluorescence; they crystallise in the monoclinic
system. The platinocyanides of the yttrium metals are red or crimson,
with a splendid green fluorescence, and crystallise in the rhombic
system. Scandium platinocyanide is of great interest from the fact that
it exists in two modifications, which show the characteristic appearance
of the two groups of compounds respectively.
Potassium ferrocyanide precipitates _potassium earth ferrocyanides_ of
the general formula KR(FeC₆N₆),3H₂O, from neutral solutions;[163] the
precipitate is somewhat soluble in excess. The ferrocyanides have been
proposed for the purification of yttrium; the method is useful where
rapid concentration of the element is required, yttrium ferrocyanide
being far more soluble than the analogous compounds of the erbium and
ytterbium metals, but the precipitates are gelatinous, and very
difficult to handle.
[163] Compare Astrid Cleve, _Zeitsch. anorg. Chem._ 1902, ~32~, 129.
~Halogen Oxy-salts.~--_Perchlorates_ and _periodates_ of the rare earth
elements, of the general formula R(XO₄)₃,_x_H₂O, have been obtained. The
existence of _chlorates_ has been observed only in the yttrium group;
yttrium chlorate, Yt(ClO₃)₃,8H₂O, has been prepared by double
decomposition of the sulphate with barium chlorate. The _bromates_ are
also prepared in this way. They are readily soluble compounds, of which
several hydrated forms are known. They are of considerable importance
for purposes of separation in the yttrium group.
The _iodates_ are sparingly soluble bodies, precipitated by addition of
the alkali compound to solutions of the rare earth salts. The rare earth
iodates are soluble in nitric acid, the solubility increasing as the
electropositive character of the element becomes stronger. A method for
the purification of yttrium has recently been based upon this property
of the iodates, whilst the fact that thorium iodate is completely
insoluble in nitric acid allows of the easy separation and estimation of
thorium in minerals or mixtures containing rare earth elements.
~Sulphates.~--The sulphates of the rare earth elements are obtained by
dissolving the oxides or hydroxides in sulphuric acid. From the
solutions so obtained, various hydrated salts separate according to the
temperature of crystallisation. By heating the hydrated salts to a
temperature of 300°-400°, the anhydrous salts are prepared. These are
extremely soluble in water at 0°, having a great tendency, which is
indeed to be observed in the hydrated forms also, to form supersaturated
solutions. When the temperature of such a solution is allowed to rise,
larger or smaller quantities of an hydrated form separate out, the
differences of solubility among the sulphate hydrates of the various
elements being sometimes considerable.
The hydrated sulphates of the cerium elements have been very closely
studied in connection with the purification of thorium. Cerium sulphate
itself forms hydrates with 12, 9, 8, 5, and 4 molecules of water, but
sulphates of the other elements generally form fewer hydrates; the
commonest have 12, 8, or 4 molecules of water, and numerous cases of
isomorphism are known among them. The solubility curve of the cerium
sulphate hydrates is shown in the diagram. Fig. 3. The sulphates of the
yttrium elements have not yet been systematically investigated, and in
most cases only the octohydrates are known. Scandium sulphate is notably
different from the other sulphates, in that it is considerably more
soluble, and crystallises with six molecules of water.
[Illustration: FIG. 3.]
It is an important characteristic of the rare earth elements that the
solubility of the sulphates diminishes rapidly as the temperature
rises. The study of the various equilibrium conditions is greatly
complicated by the tendency to form supersaturated solutions, and the
fact that many hydrates can exist throughout considerable ranges of
temperature in the metastable condition; in consequence of this, also,
the solubilities of many hydrates are known for temperatures far beyond
the transition points. Foreign elements may be separated by taking
advantage of the very great solubility of the anhydrous sulphates at 0°,
and the rapid decrease in solubility with rise of temperature. For this
purpose, a solution of the anhydrous sulphates saturated at 0° is
prepared, and after filtration is slowly allowed to come to room
temperature; the hydrated rare earth sulphates then separate, leaving in
solution the foreign sulphates. This method may indeed be used instead
of the oxalate separation (see p. 147).
In presence of excess of sulphuric acid, _acid sulphates_ of the general
formula R(HSO₄)₃ are formed. These are fairly stable, and must be heated
to a temperature of 400°-500° to decompose them completely to the normal
salts; even at that temperature, traces of acid are tenaciously
retained, a fact which renders the determination of the equivalents by
the sulphate method unreliable, unless special precautions are taken. On
further heating, the normal sulphates pass into _basic salts_, R₂O₃,SO₃,
and finally, at the temperature of the blowpipe flame, into the oxides.
The temperatures at which these decompositions occur vary with the
positive character of the elements; the most basic oxide clings most
tenaciously to sulphuric anhydride, and forms the most stable acid salt.
Lanthanum sulphate, for example, requires to be heated for a
considerable time at a white heat if the pure oxide is required, whilst
the sulphates of the less positive elements are easily decomposed at a
red heat. The order of basic strength of the oxides, as determined by
the ease with which the sulphates are decomposed, seems, however, to be
very different from the order determined by decomposition of the
nitrates (see p. 118).
With the alkali sulphates, the sulphates of the rare earth elements
readily form _double salts_, which are of great importance in
separation, on account of the great differences in solubility. The
double sulphates of the cerium group are almost insoluble in excess of
alkali sulphate, whereas the yttrium double sulphates, with the
exception of those of the terbium metals, which occupy an intermediate
position, are very easily soluble. This method of separating the
elements into the two main groups was first employed by Berzelius, and
though a century has elapsed, it remains to-day the most efficient
method of effecting the separation.
The _ethylsulphates_ have been employed by Urbain and others in
effecting separations, especially in the erbium and terbium groups. The
solubilities of these salts are in the same general order as those of
the alkali double sulphates, and they are especially convenient for
separating the metals into the three groups of the cerium, terbium, and
yttrium elements respectively. They may be prepared by double
decomposition of the rare earth sulphates with barium ethylsulphate, but
on account of the ease with which the alkylsulphates are hydrolysed by
acids, it is essential that the solutions should be quite neutral. A
more convenient method, according to James, is the treatment of the
anhydrous chlorides in alcohol solution with sodium ethylsulphate
dissolved in the same medium; sodium chloride is precipitated, whilst
the ethylsulphates of the rare earth elements remain in solution.
The _sulphites_ of the rare earth elements are sparingly soluble
crystalline salts, of the general formula R₂(SO₃)₃,_x_H₂O. They are
obtained by passing sulphur dioxide into a suspension of the hydroxides
in water, or by double decomposition of soluble salts with alkali
sulphite. They dissolve in excess of sulphurous acid, and on evaporation
of the solution are deposited unchanged. They are distinguished from
thorium sulphite by the fact that they form no alkali double salts. The
strongly electropositive character of the rare earth metals is shown by
the fact that they form normal and not basic sulphites.
The _thiosulphates_ are readily soluble, crystalline bodies. With the
exception of the ceric and scandium salts, they are not hydrolysed in
boiling solution, a fact which allows of a complete separation from the
readily hydrolysed thiosulphates of zirconium and thorium.
_Dithionates_ of the commoner rare earth elements, of the general
formula R₂(S₂O₆)₃,_x_H₂O, have been prepared by double decomposition of
the sulphates with barium dithionate. They are readily soluble,
crystalline salts.
The _selenates_ are soluble, crystalline salts, which separate from
aqueous solutions in various hydrated forms. They resemble the sulphates
in being less soluble in hot than in cold water, and numerous cases of
isomorphism have been observed among the corresponding sulphate and
selenate hydrates. Several alkali double selenates have been described;
they show a close resemblance to the analogous double sulphates.
The _selenites_ are amorphous, insoluble compounds, obtained by the
action of selenious acid on the carbonates, or on solutions of neutral
salts. Basic and acid selenites are also known.
~Nitrates.~--The nitrates are crystalline, deliquescent compounds,
readily soluble in water and alcohol, but less easily in nitric acid, a
fact which has been of considerable importance for purposes of
separation. The solubility is greatest in the case of lanthanum nitrate,
diminishing through the cerium group to a minimum in gadolinium nitrate,
and then increasing again. They separate from aqueous solution in the
form of crystalline hydrates; in the cerium group, these have commonly
the formula R(NO₃)₃,6H₂O, whilst the nitrates of the yttrium elements
usually crystallise with 3 or 5 molecules of water. By carefully heating
the hydrated salts, basic nitrates may be obtained, which in the yttrium
group are soluble in water, and may be obtained crystalline; in the
cerium group, the basic nitrates are insoluble. By further heating,
insoluble ‘superbasic salts,’ and finally the oxides, are obtained in
all cases. The temperatures at which these basic and superbasic
compounds are formed vary with the electropositive character of the
element; this fact affords a method of separation which has been very
frequently employed.
An interesting series of addition compounds of the rare earth nitrates
with antipyrine (dimethylphenylpyrazolone, C₁₁H₁₂ON₂) has been described
recently by Kolbe.[164] Those of the cerium metals have the general
formula R(NO₃)₃,3C₁₁H₁₂ON₂; the yttrium nitrates appear to combine with
four molecules of the base.
[164] _Zeitsch. anorg. Chem._ 1913, ~83~, 143
The tendency to form double nitrates with nitrates of the metals of
Group IA and Group IIA also varies with the basic strength of the
hydroxides. In the most positive elements of the cerium group, the
tendency is very pronounced, and there are a large number of stable,
crystalline double salts; but the stability decreases rapidly as the
atomic weight of the element rises, and in the terbium and yttrium
groups crystallised double nitrates cannot be obtained. The solubility
of these double salts increases rapidly in the same direction, the
lanthanum double nitrates being the least soluble. For this reason,
these compounds are of great importance for the purpose of separation,
especially in the cerium group. Bismuth nitrate and the various bismuth
double nitrates are isomorphous with the corresponding compounds of the
cerium group, and the double bismuth ammonium and bismuth magnesium
salts have been largely used by Urbain in the separation of samarium and
the elements of the terbium group.
~Phosphates.~--Addition of phosphoric acid, or an alkali phosphate to
solutions of rare earth salts throws down the phosphates as gelatinous
precipitates, which slowly become crystalline on standing. The
precipitate is soluble in excess of phosphoric acid, and in other
mineral acids, a fact of great importance in the commercial treatment of
monazite. The composition of the precipitate is not known with
certainty; both neutral and acid phosphates can probably be obtained
according to the conditions. Double salts with the alkali phosphates can
be prepared by fusion methods. The naturally occurring phosphates,
monazite and xenotime, are mixtures of the orthophosphates of the cerium
and yttrium elements respectively.
_Phosphites_ are known in a few cases only; _arsenates_ and _arsenites_
of lanthanum have been prepared. _Vanadates_ of some of the rare earth
elements have been described.
~Chromates.~--The rare earth chromates are, as a rule, sparingly soluble
in water, and show considerable differences of solubility amongst
themselves; for this reason, they have been of some use in the
separation of the cerium elements.[165] They are obtained by addition of
potassium chromate to neutral solutions of rare earth salts as
crystalline precipitates, of the general formula R₂(CrO₄)₃,8H₂O; with a
large excess of alkali chromate, double chromates are obtained, which
are more readily formed, and more soluble, in the yttrium series than in
the cerium group. Addition of chromic acid or alkali bichromate to
solutions of the soluble salts gives no precipitate, a fact which allows
of the separation of zirconium and thorium, and of cerium in the
tetravalent state, since the tetravalent elements are precipitated by
both these reagents.
[165] Muthmann and Böhm, _Ber._ 1900, ~33~, 42; Böhm, _Zeitsch. angew.
Chem._ 1904, ~15~, 372 and 1282.
Ammonium molybdate throws down from neutral solution of rare earth salts
gelatinous precipitates of the _molybdates_; the formula La₂2(HMoO₄)₆ is
assigned to the lanthanum compound obtained in this way. No
precipitation occurs if the solution be strongly acid; on this fact a
process has recently been based for the volumetric estimation of
thorium, in presence of rare earth salts, by means of ammonium molybdate
(see p. 289).
Various _silicotungstates_ and _double tungstates_ have been described.
~Carbonates.~--The more pronounced electropositive character of the rare
earth elements, as contrasted with other trivalent metals, is well
illustrated by the fact that they form stable neutral carbonates of the
formula R₂(CO₃)₃,_x_H₂O. These may be obtained by passing a current of
carbon dioxide through an aqueous suspension of the hydroxides, or by
addition of an alkali carbonate to neutral solutions of the salts. Basic
carbonates are known in the case of the less positive yttrium elements
only; both these and the neutral carbonates are insoluble in water.
In presence of a large excess of alkali carbonate, double carbonates are
formed. The stability as well as the solubility of these compounds
increases in passing from the cerium to the yttrium group, _i.e._ as
the electropositive character becomes weaker. The double carbonates of
the cerium elements are sparingly soluble, and are decomposed by water,
especially on warming; they may, however, be recrystallised from alkali
carbonate solution. The sodium and ammonium double salts are less
soluble than the potassium compounds. The latter have the general
formula R₂(CO₃)₃,K₂CO₃,12H₂O, and are of considerable importance in many
processes of separation. The yttrium elements can be separated from the
cerium metals, and the latter from one another, by taking advantage of
the differences of solubility shown by the potassium double carbonates.
If a concentrated solution of the salts in potassium carbonate solution
be fractionally diluted with water, the cerium elements separate in the
order: lanthanum, praseodymium, cerium, neodymium, and samarium; the
more soluble yttrium compounds remain in the solution. Thorium forms
double alkali carbonates which are very readily soluble in excess of
alkali carbonate; this property is of great importance for the technical
separation of the element.
~Oxalates.~--The oxalates of the rare earth elements are of the greatest
importance, on account of the fact that they are not only insoluble in
water, but are also very sparingly soluble in dilute mineral acids, and
in excess of oxalic acid. They can be completely precipitated even from
strongly acid solutions by addition of sufficient excess of oxalic acid,
or alkali oxalate, and thus afford a means of easily and completely
separating the rare earth group from the commoner elements.
They are thrown down by addition of oxalic acid, or alkali oxalate, as
amorphous precipitates, which rapidly become crystalline, especially if
the solution is warmed. From water at normal temperatures they usually
separate as the decahydrates, R₂(C₂O₄)₃,10H₂O, but hydrates with 7, 9,
and 11 molecules of water of crystallisation are also known. From
strongly acid solutions, mixed oxalo-salts of the general formula
R(C₂O₄)X, where X = Cl, NO₃, HSO₄, etc., may be obtained. These mixed
salts may also be prepared by dissolving the oxalates in concentrated
solutions of the chlorides, nitrates, etc., whilst nitro-sulphates,
R(SO₄)NO₃, have been obtained by recrystallising the sulphates from
strong nitric acid. The tendency to form salts with mixed acid radicles
appears to be general.[166]
[166] See Meyer and Marckwald, _Ber._ 1900, ~33~, 1003; also Matignon,
_Ann. Chim. Phys._ 1906, [viii.], ~8~, 243.
The solubilities of the oxalates in mineral acids of various
concentrations have been examined by Hauser and Wirth.[167] Whilst the
solubilities in water are exceedingly slight, and increase with
increasing atomic weight of the elements, _i.e._ from the cerium to the
yttrium group, in mineral acids of concentration 3-4N the solubility
becomes noticeable, and is greatest for the oxalates of the most
positive elements. The solubility is greatly lessened, however, if
considerable excess of oxalic acid be present.
[167] _Zeitsch. anal. Chem._ 1908, ~47~, 389.
Double oxalates with the alkali oxalates can be obtained with the salts
of the yttrium elements only, the oxalates of the cerium elements being
almost insoluble in excess of alkali oxalate in the cold. Of the alkali
double oxalates, the potassium compounds are the most soluble, but the
ammonium compounds show the greatest differences in solubility; von
Welsbach has employed the method of fractional crystallisation of these
salts from a saturated solution of ammonium oxalate for separations in
the yttrium group. The sodium double oxalates are the least soluble of
these double salts.
Since the rare earth elements are almost always separated in the form of
the oxalates, the methods for transforming these into soluble compounds
become important. They may be ignited to oxides, and these dissolved in
nitric acid; if the content of ceria is very high, the oxide mixture may
become insoluble, but this difficulty may be overcome by addition of a
reducing agent--hydrogen peroxide is very convenient for this purpose.
The oxalates may also be dissolved directly in fuming nitric acid, care
being taken to avoid loss; if the mixture contains cerium, the oxidation
is hastened, ceric salts having the property of acting as oxygen
carriers. By boiling for a short time with potash, the oxalates may be
easily transformed into the hydroxides, which can be dissolved in dilute
acids.
~Formates.~--On account of the considerable differences in solubility by
which they are characterised, these salts have been employed for
separations. The formates of the cerium group are considerably less
soluble than those of the yttrium group. They may be partly precipitated
from solutions of rare earth salts by addition of alkali formate--formic
acid itself causes precipitation only with salts of weak acids, _e.g._
the acetates--but are best prepared by dissolving the oxides in formic
acid; on concentration of the solution, the formates of the cerium and
terbium elements successively separate, the salts of the yttrium group
remaining in solution. The separation of the terbium earths by this
method was attempted by Delafontaine; his ‘new’ element, Philippium,
obtained from the mother-liquors, was in reality a mixture of the
terbium and yttrium elements, which cannot be completely separated by
the formate method.[168]
[168] See Urbain, _Ann. Chim. Phys._ 1900, [vii.], ~19~, 184.
The _acetates_ are readily soluble in water, the yttrium salts being
rather less easily soluble than those of the cerium group. They are
therefore obtained by dissolving the oxides in acetic acid; addition of
alkali acetate to a solution of a rare earth salt gives no precipitate,
even on boiling, behaviour which is in marked contrast to the ease with
which the salts of other trivalent metals are hydrolysed under these
conditions. In this respect the rare earth elements differ also from the
tetravalent elements zirconium and thorium (and from cerium in the
tetravalent state); soluble salts of the latter, on boiling with sodium
acetate, give insoluble basic acetates. Even sparingly soluble compounds
of the rare earth elements are as a rule taken into solution by
digestion with ammonium acetate.
_Tartrates._--Addition of ammonium tartrate to a neutral solution of
rare earth salts throws down an amorphous precipitate, which dissolves
easily in acids, and in excess of the precipitant. In the presence of
tartaric acid, precipitation of the earths by addition of sodium
hydroxide is completely inhibited. Potassium hydroxide under these
conditions gives a precipitate in the case of the yttrium elements,
though only on boiling; ammonia gives a crystalline precipitate even in
the cold with this group. These precipitates are alkali double tartrates
of the yttrium metals; the cerium elements give no precipitate at all.
In all cases, therefore, the precipitation of the hydroxides is
inhibited by the presence of tartaric acid.
A very large number of organic salts of the rare earth elements has been
prepared and examined during the past two decades, in the endeavour to
find some class of compounds which will allow of an easy separation of
the group. The _benzoates_, _succinates_, _hippurates_, _citrates_ and
similar relatively simple salts first received attention, but less
common acids, as _e.g._ the hydroxynaphthalenesulphonic acids, have also
been employed.[169] The use of various organic acids for the separation
and estimation of thorium in presence of the rare earths is outlined in
that connection (see p. 288). More recently, the glycollates and
cacodylates have been prepared. The _glycollates_[170] of the cerium
elements have the general formula R(C₂H₃O₃)₃, and crystallise in crusts;
they are more soluble than the yttrium compounds, which have the formula
R(C₂H₃O₃)₃,2H₂O, and crystallise in needles. The _cacodylates_,[171]
R₂[As(CH₃)₂O₂]₆, crystallise with 16 or 18 molecules of water, and have
similar solubility relations.
[169] Erdmann and Wirth, _Annalen_, 1908, ~361~, 190; see also Pratt
and James, _J. Amer. Chem. Soc._ 1911, ~33~, 1330; Baskerville and
Turrentine, _ibid._, 1904, ~26~, 46; James, Hoben and Robinson,
_ibid._, 1912, ~34~, 276, etc.
[170] Jantsch and Grünkraut, _Zeitsch. anorg. Chem._ 1913, ~79~, 305.
[171] Whittlemore and James, J. _Amer. Chem. Soc._ 1913, ~35~, 627.
The _phthalates_ of the yttrium group have been found to be very
valuable for purposes of separation by Meyer and Wuorinen.[172] The
salts are readily obtained in solution by shaking together cold aqueous
suspensions of the rare earth hydroxides, and phthalic acid; the clear
solutions when warmed become cloudy, the organic salts hydrolysing very
easily, with separation of the hydroxides. The most positive elements
naturally remain longest in the solution, the weakly basic oxides
accumulating in the first precipitates.
[172] _Zeitsch. anorg. Chem._ 1913, ~80~, 7.
An organic compound which has proved very useful in the treatment of
the rare earths is acetylacetone, CH₃.CO.CH₂.CO.CH₃.[173] In its enolic
form, this substance forms salts with metals, which in the case of the
rare earth elements are especially characterised by the ease with which
they may be obtained, and their high crystallising power. They may be
prepared by double decomposition of neutral solutions of rare earth
salts with ammonium acetylacetone, and crystallise readily from dilute
alcohol. They have been used by Urbain in the fractionation of the
yttrium group, and for determination of molecular weights by the boiling
point method; Biltz[174] has shown that in solution they generally have
the double formula R₂(C₅H₇O₂)₆.
[173] Urbain, _Bull. Soc. chim._ 1897, [iii.], ~17~, 98; Urbain and
Budischofsky, _Compt. rend._ 1897, ~124~, 618; Biltz and Clinch,
_Zeitsch. anorg. Chem._ 1904, ~40~, 218.
[174] _Annalen_, 1904, ~331~, 334.
THE RARE EARTH ELEMENTS, AND THE PERIODIC CLASSIFICATION
At the time of the introduction of the periodic classification the rare
earth elements were generally believed to be divalent. This belief,
which has persisted until quite recently,[175] was based chiefly on the
electropositive character of the metals, and their general chemical
resemblance to the elements of the alkaline earths; the isomorphism of
the tungstates of calcium and the cerium elements, and of the molybdates
of lead and the cerium elements, also supports this view. The physical
evidence in favour of Mendelejeff’s view, however, is quite
overwhelming; the specific heats of the metals, the equivalent
conductivities of the chlorides, and molecular weight determinations by
means of vapour densities and the boiling point method, prove beyond
doubt that the elements are in fact trivalent.
[175] See Wyrouboff, _Bull. Soc. franc. Min._ 1896, ~19~, 219;
Wyrouboff and Verneuil, _Compt. rend._ 1897, ~124~, 1230 and 1300;
_ibid._, 1899, ~128~, 1573; etc.
In deciding in favour of the trivalent nature of the rare earth metals,
Mendelejeff was influenced chiefly by the fact that there was no room in
the table for divalent elements with the equivalent weights then
assigned to the cerium and yttrium elements. At that time, only the six
oxides obtained by Mosander were known; of these the accepted
equivalents and atomic weights were as follows:
Element. Equivalent. Atomic Weight.
Lanthanum 46 92
Cerium 46 92
Didymium 48 96
Yttrium 31 62
Erbium 56 112
the values for terbium being uncertain. If cerium be considered
trivalent in the cerous salts, its atomic weight becomes 138, that of
barium being 136. Mendelejeff placed cerium in Group IV, series 8, in
the position which it still occupies; he pointed out that the accepted
equivalent must be too low, and suggested that the atomic weight should
be at least 140, almost exactly the value accepted to-day.
This choice left the positions in Group III, series 8, horizontally
before cerium, and in Group IV, series 10, vertically below it (see
figure), to be filled by the two elements, lanthanum and didymium. No
chemical evidence being available to decide the choice, he provisionally
assigned didymium to the first (Group III, series 8), and lanthanum to
the second (Group IV, series 10) position, at the same time expressing
the opinion that didymium was probably a mixture of closely related
elements. Yttrium then fell into place in Group III, series 6, above
didymium, and erbium in Group III, series 10, below it. To the vacant
space above yttrium in Group III, series 4, he assigned the hypothetical
element Eka-boron, with atomic weight 44; this space is now occupied by
scandium, which corresponds almost exactly in properties to the metal
described by the Russian chemist. A part of the table illustrating these
positions is shown in Fig. 4.
The determination of the specific heats of the metals by Hillebrand and
Norton in 1875, whilst confirming the trivalency of the elements,
rendered it necessary to alter the position of lanthanum, which was
placed in Group III, series 8, instead of didymium, which was thus left
without a place. This first indication that all the rare earth elements
could not be fitted into the table without difficulties was soon
followed by the discovery of several other members of the group, for
which places could not easily be found.
[Illustration:
+---------+--------+--------+--------+--------+--------+--------+
| Group | 0 | I | II | III | IV | V |
| | |A B|A B|A B|A B|A |
+---------+--------+--------+--------+--------+--------+--------+
|Series 1| | H | | | | |
+---------+--------+--------+--------+--------+--------+--------+
| „ 2| | Li | Be | B | C | |
+---------+--------+--------+--------+--------+--------+--------+
| „ 3| | Na | Mg | Al | Si | |
+---------+--------+--------+--------+--------+--------+--------+
| „ 4| |K |Ca |_Eka- |Ti | |
| | | | |boron_ | | |
| „ 5| | Cu| Zn| | | |
+---------+--------+--------+--------+--------+--------+--------+
| „ 6| | |Sr |~Yt~ |Zr | |
| | | | | | | |
| „ 7| | Ag| Cd| | Sn| |
+---------+--------+--------+--------+--------+--------+--------+
| „ 8| | |Ba |~Di?~ |~Ce~ | |
| | | | | | | |
| „ 9| | | | | | |
+---------+--------+--------+--------+--------+--------+--------+
| „ 10| | | |~Er~ |~La?~ | |
| | | | | | | |
| „ 11| | Au| Hg| | Pb| |
+---------+--------+--------+--------+--------+--------+--------+
| „ 12| | | | |Th | |
+---------+--------+--------+--------+--------+--------+--------+
FIG. 4.--PART OF THE PERIODIC TABLE, SHOWING THE POSITIONS ORIGINALLY
ASSIGNED TO THE RARE EARTH ELEMENTS BY MENDELEJEFF]
It was first pointed out by Brauner in 1881 that, with the exception of
scandium (44·1) and yttrium (89·0), the rare earth elements form a zone
of increasing atomic weight between barium (137·37) and tantalum
(181·5). In 1902 he proposed[176] to consider the rare earth metals as a
kind of zone or belt among the elements, comparable to the asteroids in
the solar system, extending from cerium in Group IV to tantalum in Group
V in a continuous series. The suggestion seems at first sight contrary
to the whole principle of periodic classification, but it accords very
well with the anomalous position of the rare earth group among the other
elements; it is very well illustrated in the accompanying Fig. 5, which
shows an helical or space representation of the table.
[176] _Zeitsch. anorg. Chem._ 1902, ~32~, 1.
[Illustration: FIG. 5.--HELICAL REPRESENTATION OF THE PERIODIC LAW
Electropositive Elements, above plane of paper, black letters on white
ground. Electronegative Elements, below plane of paper, white letters on
black ground. Intermediate Elements, in plane of paper, black letters on
sectioned ground.]
[Illustration:
+---------+--------+--------+--------+--------+--------+--------+
| Group | III. | IV. | V. | VI. | VII. | VIII. |
| |A B|A B|A B|A B|A B| |
+---------+--------+--------+--------+--------+--------+--------+
|Series 6|~Yt~ |~Zr~ |Cb |Mo | |Ru Rh Pd|
| | | | | | | |
| „ 7| In| Sn| Sb| Te| I| |
+---------+--------+--------+--------+--------+--------+--------+
| „ 8|~La~ |~Ce~ |~Pr~ |~Nd~ |~Sa~ |~Eu~ |
| | | | | | | |
| „ 9| ~Gd~| ~Tb~| ~Dy~| ~Ho~| ~Er~|~Tm~~Yb~|
+---------+--------+--------+--------+--------+--------+--------+
| „ 10|~Lu~ | |Ta |W | |Os Ir Pt|
| | | | | | | |
| „ 11| | | | | | |
+---------+--------+--------+--------+--------+--------+--------+
FIG. 6.--PART OF THE PERIODIC TABLE, SHOWING THE POSITIONS ASSIGNED TO
THE RARE EARTH ELEMENTS BY BRAUNER IN 1908]
Brauner’s conception is also in accord with the physical properties of
the elements and their compounds. These vary continuously throughout the
group, and show nowhere the sudden transitions which are characteristic
of other series in the table. Benedicts[177] has collected all the data
bearing on the atomic volumes, and finds that those also vary
continuously, with rise in the atomic weights, within quite small
limits, all lying between the values for barium and tantalum. In face of
all the evidence furnished by physical and chemical properties, however,
Brauner[178] has recently reverted to an idea which he put forward in
1881, according to which lanthanum and cerium are placed as usual in
Groups III and IV, series 8, whilst the other elements are distributed
in order throughout the remaining groups, as shown in Fig. 6.
[177] _Zeitsch. anorg. Chem._ 1904, ~39~, 41.
[178] _Monats._ 1881, ~3~, 1; _Zeitsch. Elektrochem._ 1908, ~14~, 525.
In support of this arrangement, he quotes the fact that some of the
elements appear to be able to form higher oxides in the presence of
other oxides, which act as oxygen carriers (see pp. 174, 177-8), though
these higher oxides are certainly not salt-forming. He also deduces,
from the rates of hydrolysis of the sulphates, that the elements fall
into two parallel series, according to the strengths of the hydroxides
as bases, on which ground he justifies the distribution throughout
series 8 and 9. There can be no doubt, however, that this disposition is
far less in accordance with the behaviour and properties of the rare
earth elements than is the first arrangement, which places them in a
transition zone between barium and tantalum; it is impossible, for
example, to reconcile the properties of praseodymium with those of
columbium and tantalum, or to find the slightest analogy between
neodymium and molybdenum or tungsten, as the second arrangement
requires.
The analogy of the rare earth group to the elements of Group VIII has
been pointed out by many authors.[179] On the ground that the rare earth
elements cannot be spread over the table in series 8-10, Steele[180]
favours the early classification of Thomsen, according to which the
elements are divided into three groups. The first, corresponding to
Groups I and II of Mendelejeff’s table, consists of two sub-groups, each
containing seven elements[181]; the second, corresponding to the first
two long series of the periodic table, has two sub-groups, each of
seventeen elements, of which the first and last seven are
analogous--these elements fall into the same groups in the periodic
table--whilst the middle three are interperiodic. These interperiodic
elements are those which Mendelejeff places in Group VIII. The third
division consists of one (or two) group(s) of thirty-one elements; here
again, the first and last seven are analogous, whilst the interperiodic
elements, which are seventeen in number, include the rare earth metals.
[179] Compare Biltz, _Ber._ 1902, ~35~, 562.
[180] _Chem. News_, 1901, ~84~, 345.
[181] The inert gases are not included.
Steele’s idea has been extended by Werner,[182] who has drawn up a table
to illustrate it. In this classification, the elements are arranged in
order of atomic weight, but arbitrary gaps are left in such a way that
similar elements may fall into the same vertical columns, as in the
periodic table. The arrangement has the advantage that the interperiodic
elements, consisting of the rare earth elements and the elements placed
in Group VIII of the periodic table, here do fall in the middle of their
respective periods, but it has several drawbacks, and does not represent
the transition of properties from element to element so well as the
helical representation of the periodic table, which brings out most
clearly the true relations between the elements, and the anomalous
position of the rare earth metals.
[182] _Ber._ 1905, ~38~, 914.
Mention must be made at this point of the theory of ‘Meta-elements’ put
forward in 1888 by Sir William Crookes.[183] From his work on the
cathode luminescence of some of the oxides (see next chapter), that
author was led to the conclusion that several of the then-accepted rare
earth elements, notably samarium and yttrium, were in reality
heterogeneous, consisting of large numbers of very closely related
bodies, differing so very slightly in properties that only the most
refined methods could perceive the variations; for these he proposed the
name Meta-elements. Though it has been proved that the differences
observed by Crookes in the luminescence spectra were really due to the
presence of very small quantities of impurities, his paper is of great
interest, in that it contains a theory of evolution of the elements, and
postulates the possibility of their decay. Modern developments in
radioactivity have not only lent a curious force to these speculations,
but even support his contention that a chemical element, in the ordinary
sense of the word, is not necessarily homogeneous.[184] In the field of
the rare earths, also, the homogeneity of elements is even now
continually being called into question (see Thulium, p. 204). In any
case, we have in the rare earth elements a series of bodies in which the
change of properties from one member to another--and the consequent
possibility of easy separation--is so very slight, and so far without
parallel in the whole field of chemistry, that we are at least justified
in asking whether some extension of our ordinary conception of an
element is not required.
[183] _Trans. Chem. Soc._ 1888, ~53~, 487.
[184] See Soddy, _The Chemistry of the Radio-Elements_, Part II.,
Introduction.