MINERALOGY AND CRYSTALLOGRAPHY.
THE MINERALS—CHARACTERISTICS—CRYSTALS AND THEIR FORMS—DESCRIPTIONS OF
MINERALS.
MINERALS are constituent parts of the earth. All parts of minerals are
alike. There are simple minerals and mixed. The former are the true
minerals, and are generally considered under the heading MINERALOGY.
The others constitute a branch of GEOLOGY, as they form aggregate
masses, and as such compose a large portion of the earth. We must learn
to distinguish minerals and crystals as inorganic forms of nature. In
the animal and vegetable kingdoms we have forms which are possessed of
organs of sight, smell, taste, and certain structures indispensable
to their existence and development. But in minerals we have no such
attributes. They are INORGANIC, and have a similar structure; a
fragment will tell us the story as well as a block of the same mineral.
These inorganic substances are possessed of certain attributes or
characteristics. We find they have FORM. They have chemical properties,
and they behave differently when exposed to light and electricity. They
are generally solid. All the elements are found in the mineral kingdom,
and a mineral may be an element itself, or a chemical combination of
elements. These compounds are classed according as the combination is
more or less simple. An alliance of two elements is termed a _binary_
compound, of three a _ternary_ compound, forming a base and an acid.
We have learnt from our chemistry paper that there are between sixty
and seventy elementary bodies in nature. When we speak of “elements,”
we do not mean to apply the popular and erroneous definition of the
word. Earth, air, fire, and water are not elements; they are compounds,
as we have seen. The list of elements has been given; we will now
give the names of the more important minerals. We have no space for a
detailed description, but in the British Museum the cases contain some
hundreds, and the student will find them classified and described with
the greatest care, and according to the arrangement of Berzelius.
PRINCIPAL MINERALS AS ARRANGED BY PROFESSOR ANSTED.
I.
Diamond.
Graphite.
Anthracite.
Coal.
Lignite.
Bitumen.
Amber.
Sulphur.
Quartz.
Amethyst.
Agate.
Chalcedony.
Flint.
Jasper.
Opal.
II.
Sal-ammoniac.
Nitre.
Rock-salt.
Borax.
III.
Witherite.
Spar.
Strontianite.
Celestine.
Calc-spar.
Marble.
Dolomite.
Fluor-spar.
Gypsum.
Apatite.
Magnesite.
Corundum.
Sapphire.
Emery.
Turquoise.
Alum-stone.
IV.
Cyanite.
Christolite.
Clay.
Fullers-earth.
Garnet.
Iolite.
Jade.
Emerald.
Beryl.
Felspar.
Obsidian.
Pumice.
Talc.
Serpentine.
Zircon.
Hornblende.
Asbestos.
Augite.
Diallage.
Topaz.
Tourmaline.
Lapis-lazuli.
Chrysoberyl.
V.
Wolfram.
Molybdenite.
Chromite.
Pitch-blende.
Uranite.
Pyrolusite.
Wad.
Manganese-spar.
Arsenic.
Realgar.
Orpiment.
Antimony (grey).
Bismuth.
Blende.
Calamine.
Spartalite.
Tinstone.
Galena.
Pyromorphite.
Iron-pyrites.
Mispickel.
Magnetic iron ore.
Micaceous iron.
Hematite.
Spathic iron.
Cobalt.
Copper.
Oxides of copper.
Copper pyrites.
Azurite.
Malachite.
Mercury.
Cinnabar.
Silver.
Gold.
Platinum.
Palladium.
The above is the arrangement best suited for beginners.
Professor Nichol prefers the following arrangement:—
ORDER I.—THE OXIDISED STONES.
Quartz.
Felspar.
Scapolite.
Haloid stones.
Leucite.
Zeolite.
Mica.
Serpentine.
Hornblende.
Clays.
Garnet.
Cyanite.
Gems.
Metallic stones.
ORDER II.—SALINE STONES.
Calc-spar.
Fluor-spar.
Heavy-spar.
Gypsum.
Rock-salt.
ORDER III.—SALINE ORES.
Sparry iron ores.
Iron salts.
Copper salts.
Lead salts.
ORDER IV.—OXIDIZED ORES.
Iron ores.
Tinstone.
Manganese ores.
Red copper ores.
White antimony ores.
ORDER V.—THE NATIVE METALS.
ORDER VI.—SULPHURETTED METALS.
Iron pyrites.
Galena.
Grey antimony ore.
Grey copper ore.
Blende.
Ruby-blende.
INFLAMMABLES.
Sulphur.
Diamond.
Coal.
Mineral resins.
Combustible salts.
These are only a portion of the minerals, but it would be scarcely
interesting to give the list at greater length. In the foregoing we
recognize the metals and various combustible and non-combustible
substances familiar to us, existing, as people say sometimes, in
“lumps.” But if any one will take the trouble to examine a “lump,” he
will find the shape is definite and even. These regular forms of the
minerals are called CRYSTALS, from the Greek word krustallos, _ice_.
The term was originally applied to quartz, for in olden times it
was thought that quartz was really congealed water. We can define a
crystal as “an inorganic solid bounded by plane surfaces arranged round
imaginary lines known as _axes_.” It must not be imagined that crystals
are small bodies; they may be of any size. There are crystals of many
hundredweight; and although the usual crystal is comparatively small,
it may be any size.
Crystallization has occurred by cooling, or by other natural means;
and we can form crystals by evaporation from certain salts deposited
in water. So we may conclude also that the evaporation of water in the
early periods deposited many forms of crystals. We have crystals in the
air, such as snowflakes, which are vapours crystallized. Carbon, when
crystallized, is the diamond. Boron is very like it. Oxygen cannot be
crystallized. Alumina makes sapphires and ruby with silica. Alumina
and earth give us spars, tourmaline, and garnets. Limestone also has
beautiful forms, as in Iceland spar. Crystals, therefore, are certain
forms of nature, corresponding in the inorganic kingdom to the animals
and plants of the organic.
Let us look a little more at these. Here we have a group of crystals
of different forms. Earths are metals combined with oxygen, and the
principal earths are alumina, lime, and silica. To these three we are
chiefly indebted for the ground we live on, and from which we dig so
many useful metals and other minerals. Earths are coloured by the
substances mixed with them. We can thus find copper, silver, gold,
lead, etc., by noting the appearance of the soil. True earths are
white. Strontia and baryta are also earths, and the latter is used
in firework manufactories. Our chief assistants are ALUMINA, which
furnishes us with bricks and slate; LIME, which gives us marble or
stones for building in a carbonate form. Quicklime, by which is meant
lime freed from the carbonic acid, is well known; and plaster of Paris
is only lime and sulphuric acid in combination. The SILICATES, such as
sand and flint, are in daily demand. Agate, cornelian, Scotch pebbles,
rock-crystal, etc., belong to the same family. Even our gems are
crystallized earths, and, as already stated, diamonds are merely carbon.
Stone, as we know, is quarried; that is, it is dug out of the earth.
But perhaps many readers do not know why a stone-mine is called a
“quarry.” Most kinds of stone (granite and marble are the exceptions)
are found in layers, or strata, rendering them easy of removal. The
blocks of stone are cut with reference to these layers in a more or
less square manner, and “squared up” before they are carried away. Thus
the term “quarry,” from an old French word, _quarré_, or _carré_, as
now written, signifying a square. In granite quarries the stone being
very hard is bored, and loosened by means of gunpowder or dynamite
blasting. Slate, on the contrary, is easily divided into slabs. We will
now resume the subject of Crystals.
[Illustration:
{1.—Emerald. 3.—Garnet. 5.—Diamond.
Fig. 434. {
{2.—Agate. 4.—Ruby. 6.—Rock crystal.]
We have said that crystals vary in size, and this variety may be traced,
in the cases of crystallization from fluids, to the slowness or the
rapidity of the cooling process. If the work be done slowly, then the
crystals obtain a size commensurate with the time of cooling, as they
are deposited one upon the other. The form of minerals is the first
important point, and to ascertain their forms and structure we must
study CRYSTALLOGRAPHY. We shall find faces, or _planes_,—the lines of
contact of any two planes,—called _edges_, and the _angles_ formed
where these planes meet. We may add that crystals have, at least, four
planes, making six edges and four angles. Nearly all crystals have
more than this, for the forms are, if not infinite, very numerous, and
are divided into six (by some writers into seven) different systems or
fundamental forms from which the varieties are derived. The axis of a
crystal is an imaginary line drawn from an angle to the opposite one.
The first form, the _monometric, or cubic system_, with three equal
axes at right angles, is represented by fig. 436. This crystal is
limited by eight equilateral triangles. It has twelve edges and six
angles. If we describe a line from any one angle to an opposite one,
that line is called an _axis_, and in the case before us there are
three such axes, which intersect each other at right angles.[23] Such
crystals are regular octohedra. There are irregular forms also, whose
axes do not come at right angles, or they may be of unequal length.
The substances which we find crystallized in this form or system are
the diamond, nearly all metals, chloride of sodium (salt), fluor-spar,
alum, etc.
[Illustration: Fig. 435.—Stone quarry.]
When we say in this form we do not mean that all the minerals are
shaped like the illustration (fig. 436). We shall at once see that the
system admits of other shapes. For instance, a regular crystal may have
been cut or rubbed (and the experiment can be made with a raw turnip).
Suppose we cut off the angles in fig. 436; we then shall have a totally
different appearance, and yet the crystal is the same, and by cutting
that down we can obtain a cube (fig. 437). Take off its angles again we
obtain a regular octohedron once more, as shown in the diagram opposite.
[Illustration: Fig. 436.—Regular octahedron—first system.]
We will exhibit the gradations. Suppose we cut fig. 437; we will obtain
(fig. 438) the cube. The next is merely the cube with angles and edges
cut off; and if we proceed regularly we shall arrive at fig. 442, the
rhombic dodecahedron, or twelve-sided figure, whose equal planes are
rhombs.
We can, by taking away alternate angles or edges situated opposite,
arrive at other secondary crystals. From the original octohedron we
can thus obtain figs. 443 and 444. These are known as _tetrahedron_.
The _pentagonal dodecahedron_ is another secondary form (fig. 445).
[Illustration: Fig. 437.—Octohedron angles removed.]
[Illustration: Fig. 438.—The cube.]
[Illustration: Fig. 439.—Cube with angles removed.]
The cube, or hexahedron, the octohedron, and the rhombohedron are
all simple forms, being each bounded by equal and similar faces, or
surfaces. We can thus understand how certain primary or original
natural forms of crystals can be changed in appearance by connection.
Of the various substances crystallizing in this system we find salt,
iron pyrites, gold, silver, copper, and platinum, and the sulphide
of lead called _galena_, in the cube or hexahedron form. The diamond
and fluor-spar, alum, etc., appear in the first form (I), fig. 436
(octohedron). The cube, we see, has six equal _faces_, eight equal
_angles_, and twelve equal _edges_. Galena, as will be observed from
the illustration herewith, shows this peculiarity in a very marked
manner (fig. 446).
[Illustration:
Fig. 440.—Another intermediate form of octohedron between figs. 436
and 438.]
[Illustration: Fig. 441.—Cube deprived of edges and angles.]
[Illustration: Fig. 442.—Rhombic dodecahedron (garnet crystal).]
[Illustration: Figs. 443 and 444.—Secondary forms of first system.]
[Illustration: Fig. 445.—Pentagonal dodecahedron.]
The _second_ crystalline form is the HEXAGONAL, and in this system
three of the four axes are equal and in the same plane, inclined at an
angle of 60°, with a principal axis at right angles to the others. In
crystals of this system are found quartz and calc-spar.
The _third_ system is termed the QUADRATIC or the _diametric_. This
form has three axes, all at right angles, two being equal and the other
longer or shorter than the former two. In this system crystallize
sulphate of nickel, zircon, oxide of tin, etc.
[Illustration: Fig. 446.—Galena, or sulphide of lead.]
[Illustration: Fig. 447.—Oxide of tin.]
The _fourth_, or RHOMBIC system, or the _trimetric_. Here we have three
rectangular axes, all unequal and intersecting at right angles. The
sulphate and nitrate of potassium crystallize in this system.
[Illustration: Fig. 448.—Rock crystal—second system.]
[Illustration: Figs. 449 and 450.—Quadratic, or third system.]
[Illustration: Fig. 451.—Prism of quadratic system.]
The _fifth_ is the _oblique_, or MONOCLINIC system, which displays
three unequal axes, two of which are at right angles; the third, or
principal axis, is at right angles to one and oblique to the other of
the preceding. Ferrous sulphate, tartaric acid, and gypsum crystallize
in this system.
[Illustration: Fig. 452.—Rhombic, or fifth system of crystals.]
[Illustration: Fig. 453.—Crystals of the fifth system.]
The _sixth_, or TRICLINIC system, or the _doubly oblique_. In this
system we have three axes differing in length, and all forms which can
be arranged about these unequal and oblique axes. Sulphate of copper
will be found in this group. The system has been called anorthic, or
triclinic, because the axes are unequal and inclined, as in the oblique
prism based upon an obliqued angled parallelogram. Axinite crystal, as
annexed, will show one form in this system.
[Illustration: Fig. 454.—Sixth system.]
As may be gathered from the foregoing, it is not easy to determine
a crystalline form with certainty,—a great part of the crystal may
be invisible. A crystalline mass is a mineral, which consists of an
arrangement of crystals heaped together. If it does not possess these
the mineral is _amorphous_, or shapeless. We will now endeavour to
describe some of the physical characteristics of minerals.
[Illustration: Fig. 455.—Wollaston’s Goniometer, an instrument for
measuring the angles of crystals.]
The GONIOMETER (_see_ fig. 455) is the instrument used for measuring
the angles of crystals. Wollaston’s reflecting instrument is most
generally used. It consists of a divided circle, graduated to degrees,
and subdivided with the vernier. The manner of working is easy, though
apparently complicated. The vernier is brought to zero, when an object
is reflected in one face of the crystal. The crystal is turned till the
same object is viewed from another face. The angle of reflection is
then measured, and can be read off on the circle.
We have already referred to the physical characteristics of the
minerals, and one of these attributes is _cohesion_. When we find a
substance is difficult to break, we say it is “hard.” This means that
the cohesion of the different particles is very great. Minerals vary in
hardness; some are extremely difficult to act upon by force, and a file
appears useless. At the other side we find some which can be pricked
or scratched with a pin; and these degrees of hardness being put as
extremes, we can in a manner relatively estimate the hardness of all
other minerals. We can test this by scratching one against another;
whichever scratches the other is the harder of the two, and thus by
taking up and discarding alternately, we can at length arrive at a
comparative estimate of the hardness of all. Such a scale was arrived
at by Mohs, and arranged in the following order. The softest mineral
comes first:—
1. Talc.
2. Gypsum (rock-salt).
3. Calcareous spar.
4. Fluor-spar.
5. Apatite-spar.
6. Felspar.
7. Quartz.
8. Topaz.
9. Corundum.
10. Diamond.
Talc, we see, is the softest, and diamond the hardest. Thus “diamond
cut diamond” has passed into a proverb expressive of the difficulties
one “sharp” person has to circumvent or “cut out” another. Diamond
is used by glass-cutters. When geologists wish to express the degree
of hardness of any substance, they mention it with reference to the
foregoing list; and if the substance be harder than fluor-spar, but
not so hard as felspar, they determine its hardness five, or perhaps
between five and six, or between four and five, according as it is
harder or less hard than apatite. Thus hardness, or power of cohesion,
resistance to exterior force and pressure, is a prime characteristic of
the mineral kingdom. The file is the best test.
We now come to another phase of the physical character of our
minerals—_cleavage_. This is the term employed to express the
facility of cutting in a certain direction which in the mineral is
its direction of cleavage. Take mica, for instance. There is no
difficulty in separating mica into thin layers; we can do so with our
fingers. The layers, or flakes, or laminæ are so arranged that they
exhibit less cohesion in one direction than when tried in other ways.
We cut with the grain, as it were in the direction of the fibre when
wood is concerned. Here we have another popular saying expressive of
this,—“against the grain,”—which signifies an act performed unwillingly
and unpleasantly. Cleavability, therefore, means cutting with the
grain, as it were, and various minerals are possessed of different
degrees of cleavage. It sometimes happens that electric excitement is
observed when cleavage takes place. One place will become positive, and
the other negative. Mica, arragonite, and calcareous spar will exhibit
this action after cleavage or pressure. When a crystal of tourmaline
is heated, it will develop positive electricity at one end of its
principal axis, and negative at the other. Even if it be broken, the
extremities of the fragments will exhibit similar phenomena, and so
far like a magnet, which, as we have seen, possesses this attribute
of “polarity.” But a curious fact in connection with this is that, if
the heating cease the polarity ceases for a second or two, and yet as
cooling goes on the polarity is restored, with the difference that the
positive end has become negative, and the end previously negative has
come over to the opposite pole. Electricity, therefore, must be hidden
away in every portion of our globe, and will some day be proved to be
the mainspring of all life.
_Fracture_ in minerals is also to be noticed. Those substances which we
cannot laminate we are obliged to break, and we may require to break a
mineral in a direction different from or opposed to its direction of
cleavage. Under such circumstances we must break it, disintegrate it,
and observe the fracture. Sometimes we shall find the surfaces very
even, or uneven, or what is termed _conchoidal_. This is observable in
the breaking of flint. There are various ways in which minerals display
fracture, and the particular manner and appearance denotes the class to
which the mineral belongs.
We may pass over the question of the specific gravity of minerals, as
we have in a former part explained this. It is important, however, to
ascertain the specific gravity. As a general rule, minerals containing
heavy metals are of high specific gravity.
But the relation of minerals (crystals) with regard to light is of
great interest and importance. When we were writing of polarization,
we mentioned the faculty a crystal has for double refraction, by which
it divides a ray of light into two prolonged rays taking different
directions, the plane of vibration of one being at right angles to
that of the other. This property is not possessed by all crystals.
Some act as ordinary transparent media. Some crystals transmit only
one polarized ray, and tourmaline is called a polarizer; and if light
be passed through it to another polarizer, it will be transmitted
if the latter be similarly held; but if the second be held at right
angles to it the ray will be stopped. We can easily understand this
if we suppose a grating through which a strip of tin is passed; but
the strip will be stopped by bars at right angles to it. The coloured
rings in crystals can be observed when a slice of a double refracting
crystal is examined. The rings are seen surrounding a black cross in
some instances, and a white cross in another. The effect when examined
in the polariscope is very beautiful. Selenite is probably the best
crystal for exhibiting colours.
Minerals sometimes reflect, sometimes refract light; they are said
to possess lustre and phosphorescence. All these properties may
be considered as belonging to the crystals which are transparent,
semi-transparent, translucent, or opaque, according to the degrees
in which they permit light to pass through them. All minerals are
electric or non-electric, and the variety can be ascertained by
rubbing and placing the mineral near the electrometer. But all do
not exhibit magnetic properties. Taste and smell are strongly marked
in some minerals—salts, for instance, and sulphur; some are soapy to
the touch, some appear cold to the fingers. Chemistry is very useful
to us in determining the nature of the mineral, and the amount of it
enclosed in the substance under examination. These delicate operations
are termed qualitative and quantitative analysis. The application of
heat is increased by means of the blowpipe, which is in effect a small
bellows. We can thus, and particularly by means of the oxy-hydrogen
blowpipe, obtain a very intense heat with little trouble. When the
fragments of a mineral are held in the flame by platinum “tweezers,” or
tongs, then the _fusibility_ of the substance, and the colour of the
blow-pipe flame will be of great assistance in determining the nature
of the mineral. It is also curious to observe the different forms into
which the various substances expand or contract under the influence
of the blowpipe. We may have a rugged slag, an enamel, or a glass,
or a bead, or “drop” of metal. The varied substances produce various
colours—yellow, green, orange, or red, according to circumstances.
Strontia is a vivid red, copper is green, lime orange, and so on.
[Illustration: Fig. 456.—The blowpipe.]
It is very little use to attempt a study of mineralogy without some
acquaintance with chemistry. In dealing with minerals, and in studying
geology, we must try to keep our knowledge of chemical science in our
minds, and thus fortified we can more easily understand the steps
leading to the classification of minerals. It is impossible to teach
mineralogy or geology from books. Nature must be studied, the specimens
must be seen, the earth must be examined. The advance in mineralogy may
be—probably will be—slow, but crystals will teach something; and when
we can pass a _viva voce_ examination in chemistry and crystallography,
expressing, by the symbols, the various substances under discussion, we
shall have made a considerable advance in the science. We shall have
an idea of the component parts of various substances, and be able to
class the various minerals according to their chemical constitution.
Beginning with the metalloids, we shall pass to the metals and various
compounds, salts, resinous substances, etc., such as amber.
It is impossible in the space at our command to describe all the
minerals, and yet it is necessary to enumerate the most important.
We may, therefore, take them in the following order. It should be
added that most of the simple minerals occur in comparatively small
quantities, but sometimes we find them in aggregate masses (rocks). We
append a table.
SYNOPTICAL TABLE OF THE MINERALS.
First Class.—Metalloids.
Sulphur.
Boron.
Carbon.
Silicium (Silicon).
Second Class.—Light Metals.
Potassium.
Sodium.
Ammonium.
Calcium.
Barium.
Strontium.
Magnesium.
Aluminum.
Heavy Metals.
Iron.
Manganese.
Cobalt.
Copper.
Bismuth.
Lead.
Tin.
Zinc.
Chromium.
Antimony.
Arsenic.
Mercury.
Silver.
Gold.
Platinum.
Third Class.
Salts.
Earthy resins.
SULPHUR is found in Sicily and Italy and other parts of Europe, in
a native state, but as such has to be purified. The crystals take
the form as shown in the margin. Cleavage imperfect; it is brittle.
Sulphuric acid is a very important combination, and a very dangerous
one in inexperienced hands. Sulphur combines with a number of elements,
which combinations are “Sulphides.” (_See_ Chemistry section.)
[Illustration: Fig. 457.—Crystals of sulphur.]
SELENIUM is a metalloid resembling sulphur, but less common. It is
inodorous.
BORON is usually found near volcanic springs, and in combination
with oxygen. It is soluble. Taste, acid bitter, and white in colour;
friable. It is known as SASSOLINE, or boracic acid. (_See_ Biborate of
Soda for one of the borates.)
CARBON is one of the most important of our minerals. In the form of
coal we have it in daily use, and in the form of diamond it is our most
valuable gem. In the latter form it is the hardest of all minerals, a
powerful refractor of light, lustrous, and transparent. It is found in
the East Indies and Brazil; more lately Cape diamonds have been brought
to Europe, but they do not equal the Eastern gem. Almost fabulous
prices have been given for diamonds, which, after all, are only carbon
in a pure state. Another form of carbon is _graphite_ (plumbago, or
blacklead). It is much used for pencils and in households. It is found
in Cumberland, and in many other localities in Europe and Canada.
Carbon appears in one or other of the above forms in regular
octahedrons or their allied shapes. _Anthracite_, another form of
carbon, is used as fuel for strong furnaces. It leaves little “ash,”
and is smokeless when burned. _Coal_, in all its forms, is evidently
derived from wood. Thousands of years ago vegetable matter must
have been embedded in the ground and subjected to carbonization.
There are different kinds of coal, all of which come under one or
other of the following heads: cubical coal, slate coal, cannel coal,
glance-lignite,—the last being, as its name implies, an imperfect
development of wood; it is a brown coal. We are not here concerned with
coal as a fuel. Charcoal is also a form of carbon prepared from wood
and finds a counterpart in coke, which is prepared from coal. Carbon,
as we have already seen, plays an important part in electric lighting
and in the Voltaic Battery. Peat, or as it is called in Ireland,
“turf,” is one of the most recent of the carboniferous formations.
It is much used as fuel. It is cut from moors (“bogs,” as they are
sometimes called), and the various deposits can be traced. Bog-oak
is no doubt the first step towards peat, as peat is the step towards
coal. The brown turf is newer than the black, and both kinds may be
seen stacked in small square “bricks” along the Irish canals and in the
yards of retailers of fuel.
[Illustration: Fig. 458.—Crystals of carbon.]
SILICON. Silica occurs generally in combination with alumina, and never
in a free state. In combination with oxygen it is called silicic acid.
Silica, when crystallized, is usually called _quartz_.
QUARTZ has several varieties. We need only enumerate them, they will
all be immediately recognized. We give illustrations of the crystals of
quartz (fig. 459):—
1. Rock crystal appears in beautiful six-sided prisms.
2. Amethyst is coloured by protoxide of manganese, supposed by the
ancients to be a charm against drunkenness.
3. Common quartz, or quartz rock, forms granite in combination, and is
also known as “cat’s-eye,” “rose” quartz, etc.
4. Chalcedony, sometimes termed _cornelian_: used for seals, etc.
5. Flint: much used in potteries. “Flint and steel” have been
superseded by phosphorus.
6. Hornstone: something like flint, resembling horn.
7. Jasper: of various colours; opaque and dull in appearance.
8. Silicious slate: a combination; used as a whetstone.
9. Agate: a mixture of quartz, amethyst, jasper, and cornelian; very
ornamental.
10. Opal: a peculiar variety, containing water. It is not found in the
form of crystal, but in vitreous masses. Its changeableness of hue is
proverbial. The “noble” opal is much prized.
11. Smoky quartz, or cairngorm.
12. Onyx and Sardonyx.
[Illustration: Fig. 459.—Quartz crystals in various forms.]
We now arrive at some minerals which contain metals.
POTASSIUM. This metal is so frequently combined in minerals with
alumina that we may refer to it with the latter in sequel. There are
two natural potassa salts—nitre, and sulphate of potassa. Nitre is
known as saltpetre, and is of great use in medicine. It is the chief
ingredient in the composition of gunpowder.
SODIUM. We have a number of minerals in this group—viz., _nitrate of
soda_ (nitratine), which occurs in large quantities in Peru; _rock
salt_, chloride of sodium, known as salt. It crystallizes in the cubic
system. Colour usually white, but it occurs in secondary rocks in
company with gypsum, etc. It is sometimes of a reddish colour, or even
green and yellow. Biborate of soda is _borax_, and is found in and on
the borders of a Thibetian lake. There are several other combinations
with soda: the sulphates of soda—viz., thenardite and glauberite,
anhydrous and hydrated respectively, carbonate of soda, and so on.
AMMONIA combinations occur in lava fissures, and are not often met with
in consequence of their volatile nature.
[Illustration: Fig. 460.—Spar crystal.]
CALCIUM. This forms an important group of the minerals, which are
very white in colour, and not very hard in substance. Calcium is the
metallic basis of lime. Fluoride of calcium, known as _fluor-spar_,
most frequently crystallizes in cubes in the first system. _Anhydrite_
is the anhydrous sulphate of calcium. The hydrated sulphate is called
gypsum. One variety of the hydrated sulphate is selenite, another is
known as alabaster. Apatite, or asparagus stone, and pharmacolite are
in this group.
[Illustration: Rhombohedron (_r_).
Primary rhombohedron (_r_).
Six-sided prism (_g_) regular.
Primitive rhombohedron (_r_), with acute form (_r½_).
Obtuse rhomtrahedron (_r½_), ending in prism (_g_).
Equal six-sided prism (_a_), ending in regular (_r_).
Obtuse rhombohedron.
Fig. 461.—CRYSTALS OF CARBONATE OF LIME.]
_Carbonate of lime_, not content with one system of crystals, makes its
appearance in two. It is therefore divided into two minerals—namely,
_calcareous spar_ and _arragonite_. In the former it possesses various
forms, as will be observed in the accompanying diagrams. It is a very
important mineral, as will be readily acknowledged; it enters largely
into the composition of all shells and bones. The minute shells,
deposited by millions at the bottom of the sea, have combined to raise
our chalk cliffs. Carbonate of lime is a constituent of water, as the
deposits at the bottoms of kettles, upon the sides and bottoms of
water-bottles, and the stalactites all testify. A little good vinegar
will quickly dissolve this deposit. Calc-spar is crystallized, and the
Iceland spar is celebrated. Marble, which is another form of carbonate
of lime, is white, hard, and granular. It is sometimes varied, but the
pure white is the most valuable. Chalk, we know well, is soft, and is
useful for writing. We have also _aphrite_, schiefer spar,—compact
limestone in various forms,—and finally, _arragonite_, called from the
place of its nativity, Arragon,—a colourless and somewhat transparent
vitreous crystal.
BARYTES. The sulphate of baryta is known as _heavy spar_; the crystals
are of tabular forms, but numerous modifications exist. One of the
forms is represented in the margin.
[Illustration: Fig. 462.—Tabular form of heavy spar.]
STRONTIUM is the metallic basis of strontia. Sulphate of strontium
is _celestine_, the mineral which colours the blow-pipe flame a fine
crimson. There are certain varieties. Strontia salts are chemical
preparations. A beautiful pyrotechnic “red fire” is produced by mixing
nitrate of strontia with sulphur, antimony, charcoal, and chlorate of
potassia.[24] There is a carbonate of strontia in the same crystalline
system.
MAGNESIUM. With this metal we have a large group of minerals.
_Magnesite_ is carbonate of magnesia, and occurs as talc-spar. The
magnesium limestone crystallizes as _bitter spar_. This dolomite
is like marble or common limestone, according to colour. Talc is a
combination of magnesia with silicic acid. The hydrated carbonate is
termed “white magnesia.” The sulphate of magnesia is found in Siberia,
and we have _boracite_, and native magnesia called _periclase_. The
sulphate is generally present in mineral waters, such as the Seidlitz
and Epsom Springs. Large masses have been found in the extensive
caverns of Kentucky and Tennessee, etc.
_Meerschaum_ is a hydrated silicate of magnesia. It is found in
Anatolia and Negropont, also in France and Australasia. Serpentine
is another similar composition. It is found in Cornwall, where it is
carved into various ornaments. It is sometimes called snakestone.
There are many other hydrated silicates of magnesia—viz., gymnite,
picrosmine, pycrophyll, etc.
[Illustration: Fig. 463.—Crystal of augite.]
There is another family allied to magnesia, called AUGITES. These
minerals are black or dark-green, and are contained in lava and basalt:
AUGITE and HORNBLENDE are the chief representatives of this family. The
former crystallizes in the fourth system (_see_ fig. 463), and there
are several varieties—diallage, bronzite, diopside, etc. HORNBLENDE
belongs to the same system, and is a large factor in the composition
of gneiss, syenite, and porphyry. _Tremolite_ is a hornblende, and
_asbestos_ (_amianthos_), and _mountain-cork_ are also varieties. The
attribute of asbestos for sustaining heat is well known, and may be
usefully employed for fire-proof purposes. The well-known _jade-stone_
of China and _calamite_ are other varieties.
[Illustration: Fig. 464.—Alum crystals.]
ALUMINUM, or ALUMINIUM, gives us a large class of minerals. It is the
metallic basis of alumina, which, combined with silica, is the chief
component of our clay. Silicic acid and this base combine to form many
minerals, and contains nearly all the precious stones. _Corundums_
consist of pure alumina, and crystallize in the hexagonal system. The
following stones are varieties of this mineral:—_Sapphire_, a beautiful
blue; _ruby_, a red oriental; _topaz_, yellow oriental; _amethyst_,
violet; all being sapphires more or less. The finest crystals are found
in the East Indies in the sands of rivers and diluvial soils. The
_common corundum_ is very hard, and is used for polishing. _Emery_ is
well known, and is found in mica-slate. It is of a bluish-grey colour,
and is also a polisher.
ALUM forms another family, of which we may first mention aluminite, a
“basic sulphate” of alumina and found in small quantities. _Alum-stone_
is found in Italy. _Alum_ occurs in large crystallized masses.
(_See_ illustration, fig. 464.) There are different minerals with a
composition very similar to alum, in which the potassa base of alum is
supplied by others. Thus we have the potassa alum, soda alum, manganese
alum, ammonia alum—all being very nearly of the same constituents, and
having similar crystals in the regular system, and are thus termed
isomorphous, or similarly-formed. The potassa, or potash alum, is the
commonest form, and is found abundantly in England, on the Continent of
Europe, and the United States. Soda alum is called _salfatarite_, and
magnesia alum _pickeringite_; manganese alum is _apjohnite_; phosphate
of alumina is _wavellite_.
There are compounds of alumina and magnesia called SPINELS. They are
hard minerals, and the same isomorphous changes take place with them as
are observable with the bases of alum. There are therefore varieties
such as the _spinel ruby_ found in the East Indies, very red in colour;
the _balas ruby_ not so red, and the orange-red, termed _rubicelle_.
Ceylon is remarkable for some fine specimens of spinels. _Chromite_ is
like the spinel, but is known as chrome iron.
ZEOLITES are principally compositions of silica and alumina. They
contain water, and are white, vitreous, and transparent. There are
several varieties of them—natrolite, stilbite, etc. We will now pass on
to the _Clays_, which are a very important family of the aluminum group.
There are a number of hard minerals which, when disintegrated, form
certain earthy masses. These we term clay, or clays, which possess
various colours and receive certain names, according to the proportion
of metallic oxides they contain. All clays have an affinity for water,
and retain it to a very great extent. The earth has also a peculiar
smell. Clay is used in various ways; pottery, for instance, we read
in the Bible as having been an employment from very ancient times.
One attribute of clays, the retention of water, is of the greatest
use to the world in providing moisture for plants and seeds. We may
mention other characteristics of clay. It absorbs oil very quickly, and
therefore is useful for removing grease-spots. It cannot be burned,
so we have fire-bricks and fire-clay in our stoves and furnaces.
There are various clays—pipe-clay, for instance, which is white;
potters’ clay is coarser. There is porcelain clay as well as porcelain
earth, of which more below. _Yellow ochre_ and _sienna_ are clays
used by artists. _Bole_ is a reddish clay; and _tripoli_ is employed
for polishing. There are, besides, _andalusite_, or chiastolite and
disthene, crystalline forms of clay.
* * * * *
Porcelain has been known to the Chinese for centuries. In 1701 it was
discovered in Germany by Böttcher, a chemist, who while endeavouring
to make gold by Royal command, found the porcelain, and was thereby
enriched. Porcelain earth is frequently found; is known in many places
as kaolin, and usually comes from the decomposition of felspar. But in
Cornwall we find it as decomposed granite, and the filtering process
can be viewed from the railway, while both gneiss and granulite have
been known to yield kaolin. It is also found in China, Saxony, and
France. It is free from iron, and when ground and mixed with silicic
acid, it is handed to the potter or moulder. After the vessels have
been dried in the air they are put into the kiln, and then become white
and hard. After that they are glazed in a mixture of porcelain earth
and gypsum, or ground flints and oxide of lead, made fluid with water
in the glazing of earthenware. The vessel is then put into the furnace
again, or “fired,” as the process is called, and then comes out white,
hard, and partly transparent.
[Illustration: Fig. 465.—Porcelain furnace.]
Earthenware utensils are made of a coarser material,—clay and powdered
flints,—from which all the gross matter has been eliminated. Flint
is not difficult to break, if made hot and thrown into cold water. A
stamper is then used to break the flints. They are first ground in a
mill and purified like the clay, then they are mixed and beaten, while
moist, into “putty,” and turned, or forced, into moulds. The handles
are fixed on afterwards. The ware is baked for two days and glazed. The
various colours are obtained by mixing different clays and oxides—iron
or manganese. Biscuit porcelain is made by pouring a creamy mixture of
porcelain earth into plaster-of-Paris moulds, and when a thin case has
formed within, the liquid is poured out again. It is then dried in the
mould and shrinks. The mould is taken to pieces, and the thin biscuit
porcelain is left.
[Illustration: Fig. 466.—Stampers.]
[Illustration: Fig. 467.—Flint mill.]
FELSPARS are very like the zeolites, except that the former contain no
water. Felspar crystallizes in a number of different forms. We annex
illustrations of specimens. This spar is found in rocks, granite,
gneiss, etc. One variety is the _moonstone_, of a peculiar lustre.
Felsite is amorphous felspar. _Albite_ contains soda instead of potash.
_Labradorite_ is nearly a pure lime felspar, and is remarkable for
its colours, like a pigeon’s breast. _Spodumene_ is like albite, and
leucite, soda-lite, etc., belong to this family.
[Illustration: Fig. 468.—Felspar.]
[Illustration: Fig. 469.—Felspar crystal.]
LAPIS-LAZULI is a felspar distinguished by its blue tint. It was used
for ultramarine colouring at one time, which colour can also be made
chemically. Lapis-lazuli is found in Siberia and China. It is a mixture
of mineral species. _Hauyne_ is something like it. _Obsidian_ is a sort
of black glass, and occurs in various colours in vitreous masses. It is
derived from the fusion of rocks, and is employed in the manufacture of
boxes, etc. _Pumice stone_ bears a resemblance in composition to the
foregoing, but is porous, and so called spongy. It contains both potash
and soda in some quantities. _Pearlstone_ and _pitchstone_ also attach
themselves to this family group.
The GARNETS possess many curious forms of crystals, which are coloured
and used as gems. _Tourmaline_ is a very particularly useful crystal,
and is used in the investigations concerning the polarization of
light. It is found of nearly all colours. The garnet and staurolite
crystals are shown (figs. 470, 471).
The former is silicate of alumina with the silicate of some other
oxide, which is not always the same. This change, of course, gives us a
series, as in the case of _alum_ above mentioned.
[Illustration: Fig. 470.—Garnet crystal.]
The red varieties, called _almandine_, or precious garnets, are
distinguished from the duller, “common” species by their clear colour.
Bohemia is the most productive soil for the garnets.
MICA includes, as we have already noticed, a group of minerals which
have a peculiarly _laminated_ structure. These layers are by no means
all alike, but they present a smoothness to the fingers which is
highly characteristic. The chief constituents are alumina and silica,
occasionally with magnesia. Mica slate is very common, and is often
used instead of glass in window-frames. _Muscovite_, _lepidolite_, and
_phlogopite_ are all micas of the “potash,” “lithia,” and “magnesia”
varieties.
In the list of minerals associated with the lighter metals, we need
only now mention the _Gems_, so well known. These stones are very hard
in many instances, infusible, and exhibiting beautiful colours. Amongst
them are diamonds, sapphires, and rubies, of which we have spoken; the
topaz, noticed under corundum. The chrysoberyl (of a pale green, or
occasionally reddish hue), of which the alexandrite of Siberia is a
variety, is a compound of glucina with alumina; the beryl, a silicate
of the same, and the emerald of beautiful green. Zircon is another gem,
and “hyacinth” is its most valued form. The latter is found in basaltic
rocks. The emerald crystallizes in the hexagonal system.
[Illustration: Fig. 471.—Staurolite crystal.]
We may now consider the minerals formed by the heavier metals, such as
Iron, Copper, Nickel, etc.
IRON. This well-known metal fills a very important place in our mineral
arrangements, for the substances formed with iron ores occur in great
variety of structure, and occasionally in very large masses. They are
highly magnetic, and very hard. Were we here treating of iron as a
metal, we could give some information respecting its extraction and
manifold uses. All we need mention here is the fact that iron occurs
in nature in various ores which are essentially composed of iron and
oxygen. The iron is extracted in the blast furnace, in which the
process is continued for years. The “slag,” or glassy scum, protects
the molten iron from the air; its presence is necessary in all blast
furnaces. The most important of the iron group of minerals are MAGNETIC
IRON (magnetite), or loadstone. This mineral occurs in Sweden and
North America, and is found in primary rocks, and in Scandinavia forms
mountains. It crystallizes in the regular (octahedron) system, and
often in the form in illustration in the margin. It is highly magnetic,
as its name implies.
[Illustration: Fig. 472.—Magnetic iron.]
Native iron very rarely occurs, and then only in thin layers. The most
extraordinary specimens are those termed meteoric iron, which fall from
the atmosphere in great masses; and the meteoric stones, which contain
ninety per cent of iron, together with other constituents in small
quantities—viz., nickel, cobalt, copper, manganese, carbon, sulphur,
arsenic, etc.
_Red hematite_ crystallizes in the hexagonal system. It possesses much
the same (chemical) constitution as corundum (_q. v._). It is brightly
metallic, and shows a red streak. It occurs in various forms, as
iron _glance_ or specular iron, which is found in Sweden and Russia;
micaceous iron, bloodstone, clay, ironstone, etc.
_Brown hematite_ has not been found in crystals, but brown ironstone
(fibrous) is crystalline. The earthy brown, containing clay, gives us
_yellow ochre_ and _umber_. _Pea-iron ore_ and “_morass_” or “bog” ore
also belong to this class. _Limonite_ is the name given to these more
recent formations, of which yellow ochre is a pure specimen.
[Illustration: Fig. 473.—Native oxide of iron.]
The combinations of iron with sulphur (pyrites) are also important.
_Iron pyrites_ and _magnetic iron pyrites_ are two which may be
mentioned. The latter first.
Magnetic iron pyrites (or _pyrrhotin_) crystallizes in six-sided
prisms, and is attracted by the magnet. The composition of this mineral
has not been exactly ascertained, and no chemical formula has been
found for it.
IRON PYRITES (bisulphide of iron) is known as _cubic pyrites_, _yellow
pyrites_, and _mundic_. It is generally found in the regular system
of crystals, either as a cube or as a pentagonal dodecahedron. (_See_
first system of crystals, _ante_.) Its colour is yellowish. It is
known also as _green vitriol_ when oxidised, and forms beautiful green
crystals (copperas). This salt is used in the preparation of Prussian
blue and violet dyes. With gallic acid it makes ink.
There are many other “ferruginous” minerals, such as _vivianite_,
_green ironstone_, white iron pyrites, arsenical pyrites, or mispickel,
etc.
A carbonate of iron, called _chalybite_, or spathic ironstone, is
very abundant in nature, and forms obtuse rhombohedrons. It is very
useful for the production of steel, as it forms the clay iron ore found
in coal districts in combination. In a fibrous form it is known as
_sphærosiderite_. It is a most useful mineral.
Chrome iron (chromite) is useful for the preparation of chromium
compounds. It crystallizes in the cubic system. It is magnetic,
especially when treated. Chromic acid forms scarlet “needle” crystals,
and by its assistance chromate of lead, or _chrome yellow_, is
prepared. (Chromate of lead is found in a native state as crocoisite).
_See_ Chromium.
MANGANESE is contained in several minerals. It usually occurs as an
oxide. It colours minerals variously. In a pure state manganese is
white and brittle. The chief minerals are—
_Pyrolusite_ (the binoxide of manganese of commerce) occurs in
crystals. It is black. It is used in the preparation of chlorine and
oxygen. The other minerals are known as _manganite_, which is also
found associated with pyrolusite, as are _hausmannite_ and _braunite_,
the other oxides.
NICKEL and COBALT are generally found together, both being similar,
and the minerals are compounds of arsenic or sulphur, and occur under
similar circumstances. The principal are of NICKEL and of COBALT—
Sulphide of nickel (ullmanite).
Arsenical nickel (nickeline).
Nickel glance (gersdorffite).
Nickel pyrites (siegenite).
Arsenical cobalt (smaltine).
Cobalt glance (cobaltine).
Cobalt bloom (erythrine).
Cobalt pyrites (linnæite).
Nickel ores are used for extraction of the metal, which is used
as a substitute for silver. The name is derived from the German,
_kupfernickel_, or false copper. It was discovered in 1751.
COPPER, again, forms a number of minerals, and the chief is the _red
oxide_ of the metal, called _cuprite_. It crystallizes in the cubic
system. Its colour is red, and tinges a flame green. _Cuprite_ yields
excellent copper, and is found in Cornwall, and in many places on the
continent. The _black_ oxide is rarely found. It is known as melaconite.
_Malachite_ (carbonate of copper) is remarkable for its beautiful green
colour. In Australia it is worked for copper. It is chiefly ornamental.
Siberia yields the finest specimens, but the mineral is found in
Cornwall and Cumberland, as well as on the continent. Chessylite (from
Chessy, in France) is frequently found with malachite. It has been
called blue malachite, or the azure copper ore. It is used as a paint.
Besides the above, copper unites with sulphur to form minerals, such
as the needle ore (bismuthic sulphide of copper), antimonial sulphide,
bournonite; purple copper, and copper pyrites, which is very abundant,
and furnishes us with most of our copper. There is also the “grey”
copper ore, which contains various metals; even silver is obtained from
it at times.
BISMUTH gives us only a few minerals, of secondary importance. Native
bismuth resembles antimony, but is reddish in hue. Bismuth ochre,
bismuth blende, and bismuthine are the chief combinations.
LEAD is more important, and is obtained from _galena_, the sulphide
of lead, which is very abundant, and the principal lead ore. It can
be at once distinguished by its high specific gravity and metallic
lustre; the “cubic cleavage” also is very easy. It frequently is
found containing silver, and even gold, antimony, iron, etc. There
are several suphantimonites of lead, such as zinkenite, geocronite,
etc., and the salts, such as sulphate of lead and white lead ore, or
carbonate of lead (cerasite). The chromate of lead is found in the Ural
Mountains.
TIN is not found in a native state, but as _tinstone_, or binoxide
of tin, named _cassiterite_. It is found largely in Cornwall, and
the mines there have yielded great quantities for generations. Tin
pyrites, a union of sulphides of tin, iron, and copper, is also found
in Cornwall.
ZINC is produced from the ore called (zinc) blende, or sulphide of zinc
(black Jack). Its colour is very variable, sometimes red, but when pure
is greenish-yellow. It is also found black and brown. The red oxide of
zinc (or spartalite) is also worked for zinc. The carbonate, or zinc
spar, is common, and used to make brass, as is _calamine_, which is
possessed of a remarkable lustre, and is even luminous when rubbed. It
is a silicious oxide of zinc, and is found in the sedimentary rocks.
When heated, it displays strong electric properties.
CHROMIUM occurs in very few mineral combinations; chromate of lead,
chrome iron, and chrome ochre, or sesqui-oxide of chromium are the only
important ones.
ANTIMONY minerals are very hard; the tersulphide is the most common,
and from this the metallic antimony is produced. Red antimony, the
oxide, is a rarer ore.
ARSENIC resembles antimony, and occurs in combination with many
metals. White arsenic, or arsenious acid, is found in Bohemia, Alsace,
Transylvania, etc. Orpiment and realgar are sulphides of arsenic, and
are employed as colouring matters in paint and fireworks. Arsenic is
very poisonous.
MERCURY is occasionally found native, but more generally as _cinnabar_.
Chloride of mercury (or calomel) is found associated with the cinnabar,
or hepatic ore. Cinnabar is easily volatilized, and possesses high
specific gravity. The Californian mines are very rich. Spain also
produces a large quantity. It is opaque, and carmine in colour.
SILVER occurs native, or in ores. The latter are as follows:—The
sulphide, or the vitreous silver (argentite); antimonial silver;
and the combined sulphides, of antimony and silver. There are many
silver minerals, such as the chloride (horn silver, or kerargyrite),
bromide, and carbonate of silver, bismuthic silver, etc. The bromide
and iodide are bromargyrite and iodargyrite. Silver occurs most
frequently associated with gold; natural alloys of these two metals are
found, containing from 0·16 to 38·7 per cent. of silver, which causes
considerable variations both of colour and density. In addition to this
alloy, we may mention _sylvanite_ (graphic tellurium), which contains,
besides gold and silver, one of the rarer metals—viz., tellurium.
[Illustration: Fig. 474.—Gold crystals.]
GOLD is our most precious mineral, and is generally found native. It
exists in sand and in certain rocks. It crystallizes in various forms,
and in Mexico it is found in companionship with silver and copper
sulphides. Australia and California render the most valuable supplies
of the metal.
PLATINUM is also found native, and rarely is crystals. It is often
alloyed with other metals, chiefly with iron or gold; also with
diamonds. We have already considered it as a metal. Little remains
to be said about _salts_ and _resins_, for with the exception of
those we have referred to under Chemistry, they are of little value.
The bitumens, rock oil, etc., which exude from the earth, are very
useful, and as asphalt and petroleum play an important part in the
civilized world, but scarcely come under the strict rule of minerals
as we consider them, and with this reference we close our sketch of
Mineralogy.
FOOTNOTES:
[23] A crystal should be held so that one of its axes is vertical to
the spectator. This axis is termed the principal axis, and when there
is inequality the longest axis is the principal.
[24] _See_ “Strontium” in Chemistry.