crystals; hardness 5-6; specific gravity 4.7; color black; streak
brownish-red to black; luster metallic; opaque on thin edges.
When ilmenite occurs in crystals they are tabular and resemble hematite
in its darker varieties, but the streak readily distinguishes the two.
In masses it looks like magnetite, but the lack of magnetism serves to
distinguish these two minerals. It is very likely to be associated with
cassiterite, rutile, or magnetite in grains which have weathered out of
the original rock, and have resisted solution and wear. Sands with a
large amount of the above mentioned minerals are termed “black sands,”
some of which are important for one or another of these minerals.
Ilmenite is a mineral formed at high temperatures, and probably often
deposited from hot vapors. It is found in granites, syenites, and
gneisses. Among the better known localities are Orange, N. Y.,
Litchfield, Conn., Florida, California, etc.
Platinum
This metal is steel-gray in color, very malleable and ductile, almost
infusible and resists the action of acids. It is one of the “noble”
metals, much rarer than gold, and so has become popular for jewelry. It
is also used in the manufacture of sulphuric-acid, in nitrogen-fixation
plants, for chemical utensils, in the electrical industries, and in
dentistry. Platinum in its occurrence is associated with the certain
other equally rare elements, like iridium, palladium and osmium. Its use
has increased rapidly of late, but the supply has not kept up with the
demand, so that, whereas in 1906 platinum and gold were about equally
valuable, now the platinum brings about five times as much as the gold.
Platinum
Pt
Occurs in grains or nuggets; hardness 4.5; specific gravity 19 (21 if
pure); color steel-gray; luster metallic; opaque.
This rare metal is mostly found in placer-deposits, often with gold. It
comes originally from dark igneous rocks, like peridotite, pyroxenite,
etc., and platinum is found to be associated with the nickel ores of
Sudbury, Canada. While formerly 90% of the world’s supply of platinum
came from placer mines in the Ural Mountains, today more than half is
produced in Canada and about a fifth in Russia. In the United States it
is found in California, Oregon, Nevada, and Alaska.
The Magnesium Group
Magnesium is a silvery-white metal, easily tarnished by exposure to
moist air. Because of its light weight, less than twice the weight of
water, and strength, it is being substituted for aluminum, especially in
airplanes, where the question of weight is crucial. It is also used in
automobile and ship production and other machine industries, and in the
manufacture of flares and incendiary bombs. Magnesium is obtained
chiefly from magnesite, dolomite, and in the United States as a result
of a recently developed process, from sea water. Magnesium has a
considerable number of minerals, of which three are taken up here and
several more under the head of silicates, where both magnesium and
silicon are combined in a mineral.
Spinel
MgAlO₄
Pl. 29
Occurs mostly as isometric octahedrons; hardness 8; specific gravity
3.5; color, red, yellow, green, or black; streak white; luster vitreous;
transparent on thin edges.
This is a rather rare mineral, but, when in clear crystals is considered
one of the gems. It was early confused with corundum, and the red
variety called ruby, as it was found in the same gem-bearing sands in
Ceylon, Burma, and Siam. However the form of the isometric octahedron as
compared with the hexagonal prism of the corundum, together with the
lesser hardness are sufficient to distinguish the two easily. The
crystals are usually octahedrons, but may have the corners cut or the
edges beveled. Twins are not uncommon.
The standard color is a clear deep-red, and such a spinel is known in
the gem trade as a _spinel-ruby_. If the color is rose-red, it is a
_Balas ruby_; if orange, it is _rubicelle_, if of a violet tinge,
_almandine_. When small quantities of other elements replace the
magnesium, the color is greatly changed. For example a little iron
present gives the crystals a dark-green to black color, and the spinel
is known as _ceylonite_. If there is both iron and chromium present, the
color becomes yellowish or greenish-brown, and this variety is
_picotite_. When the impurities are iron and copper, the color becomes
grass-green, and it is called _chlorospinel_. A form, in which the
magnesium is completely replaced by iron, is black in color and termed
_hercynite_, and occurs fairly abundantly in Westchester Co., N. Y. From
Amity, N. Y., to Andover, N. J., there is a belt of granular limestone
in which spinel of all colors is found. St. Lawrence Co., N. Y., is also
a rich locality. Bolton, Mass., Newton, Sterling, and Sparta, N. J.,
North Carolina, Alabama, and California all yield spinel.
Magnesite
MgCO₃
Occurs in cleavable or compact porcelain-like masses; hardness 4;
specific gravity 3.1; color white to gray; luster vitreous; translucent
on thin edges.
Magnesite is white and brittle, and cleaves perfectly parallel to the
faces of the rhombohedron, but it seldom occurs in crystals. It will
effervesce in warm hydrochloric acid and has some resemblance to
calcite, but can be distinguished by the greater hardness. It is still
more like dolomite, both having the same color and cleavage, both
effervescing in warm hydrochloric acid; but the magnesite has half a
point greater hardness and the porcelainous appearance. Magnesite is
used in toilet preparations, paper making, and mixed with asbestos, as a
covering for heating pipes.
Magnesite is found in Massachusetts, Pennsylvania, Texas, and in large
deposits in California and Washington.
Dolomite
(MgCa)CO₃
Pl. 19 & 29
Occurs in crystals, or in cleavable or granular masses; hardness 3.5;
specific gravity 2.8; color white to pink or gray; streak white; luster
vitreous; transparent on thin edges.
Dolomite crystallizes in the hexagonal system, in rhombohedrons
(hemihedral form), which are more or less modified by faces on the
corners or edges. The cleavage is parallel to the rhombohedron, and it
will effervesce in warm hydrochloric acid. Sometimes the crystal faces
are curved, and when this is the case, dolomite is easily determined.
Usually however dolomite resembles both calcite and magnesite. From the
calcite it is distinguished by the greater hardness, and from magnesite
by lesser hardness and not being porcelainous in appearance. Some of the
commoner forms are shown on Plate 29, crystals like C being found
embedded in anhydrite and gypsum.
Magnesium is a common element and is likely to be present wherever lime
is being deposited, so dolomite crystals are common, and much of the
limestone is dolomitic.
It may be found in almost any limestone section of the country. Some of
the finest crystals of dolomite however come from Roxbury, Vt.,
Smithfield, R. I., Hoboken, N. J., Lockport, Rochester, and Niagara
Falls, N. Y., etc.
Silicon, Silica and the Silicates
Silicon is one of the non-metallic elements, and does not occur as such
in Nature. When isolated it is either a dark-brown powder, or steel-gray
crystals. However silicon is next to oxygen in its importance in making
the crust of the earth. Forty-seven per cent of the surface rocks are
composed of oxygen, and 28% of silicon, the latter appearing in a host
of minerals. The oxide of silicon is termed silica (SiO₂), its crystal
form being quartz, the commonest of all minerals. In non-crystalline
form silica is also widely distributed, as chalcedony and opal, even
appearing in the tissues of animals and plants, as in the feathers of
birds, the shells of certain Protozoa (Radiolaria), the spicules of
sponges; and in plants, as the shells of diatoms, and in the stalks of
grasses, especially cereals and bamboo. Silica in the form of sand is
widely used in making glass, porcelain, china, etc., and in the various
cements.
Then there are a considerable number of acids of silicon, which do not
occur in Nature, but their salts do, and make a host of minerals, which
are known as the silicates, such as mica, feldspar, hornblende, etc.
Either as quartz, or as silicates, silicon is represented in most all
the igneous and metamorphic rocks and in many of the sedimentary rocks.
Quartz
SiO₂
Pl. 30
Occurs as hexagonal crystals, or in grains or masses; hardness 7;
specific gravity 2.65; colorless when pure; luster vitreous; transparent
on thin edges.
Quartz is not hard to identify. Its hardness and the crystal-form
separate it from most all other minerals. It is the most common mineral,
making 12% of the earth’s crust. The usual crystal form is a hexagonal
prism with the sides horizontally striated, and a six-sided pyramid on
one or both ends. This six-sided pyramid is really two rhombohedrons, a
right-handed one and a left-handed one, so that the alternate faces of
the pyramid may show peculiarities, for instance three may be large and
three small, as in Fig. B, Plate 30, or the alternate ones may be duller
or etched in some manner. The crystals are clear and when pure
colorless, but there is a tendency for some slight impurity to color
them almost any hue.
The most perfect double-ended crystals form only where growth is
possible in all directions, as in clay. In cavities and caves there is
an opportunity for the crystals to grow in toward the open spaces, and
in such places, one finds fine large crystals; the Alps, Brazil, Japan,
and Madagascar being especially famous localities. The largest quartz
crystal on record is one 25 feet in circumference which came from
Madagascar. In this country the caves at Little Rock, Ark., have
furnished some very fine large crystals. Smaller, but very clear
crystals, come from about Herkimer, N. Y. Some of these have been used
as “Rhine-stones” and as cheap imitations of diamonds. Clear quartz is
beautiful enough to be a gem, but it is too common to interest people as
jewelry, however many objects of art have been carved from it. One of
these took the form of crystal balls, which, through the Middle Ages
particularly, developed into a form of mysticism. The gazing into the
crystal ball was supposed to give some people supernatural vision. It
seems to be a form of hypnotism, gazing at the bright reflecting surface
tiring the eye, and making possible visions, which are subjective rather
than anything external.
Silica is slightly soluble in water, especially when it is alkaline; so
that most river-, lake-, and sea-waters have some silica in solution,
and are carrying it from one place to another. The waters, which
percolate through the rocks, carry even more, and when they come out
into open spaces, they give up some of the silica, making crystals
lining these openings, whether fissures or cavities. Not infrequently
these silica-bearing waters dissolve out some other crystal, and then
deposit in its place silica, thus making a crystal which has the form of
what was dissolved, rather than that of quartz. Such a form is known as
a pseudomorph.
When molten masses of igneous rock were cooling the quartz crystals had
their faces interfered with as they grew, and we have resulting
crystalline quartz, simply filling in the spaces between the other
crystals, such as feldspar and mica, in the granite. Quartz is a large
component in many igneous rocks, also in metamorphic rocks, and certain
sedimentary rocks like sandstone are almost wholly made up of quartz
grains. Quartz is also the gangue mineral in many veins. In this case it
seems to have been deposited from hot water or vapors, as they rose from
cooling magmas. With it are associated all sorts of metallic ores as has
been suggested.
Quartz has been largely used to make imitations of other much rarer
minerals, sometimes in its crystalline form to imitate the diamond, at
other times ground and made into a “paste,” which is colored to imitate
other gems. This paste is a mixture of about 4 parts of quartz, 5 parts
of red lead and 1 part of potassium carbonate, melted and cooled slowly.
It is clear and has a brilliant luster like the diamond. If some
coloring matter is put into it it can be used for rubies, sapphires,
etc. When there is any reason to think that this is being used, it is
easily detected by being so much softer than any of the true gems, and
even than true quartz. Quartz will scratch glass readily, but this
imitation has only the hardness of very soft glass, or about 5.
Varieties of Quartz
Rock crystal is the term applied to quartz when it is clear and
colorless.
Milky quartz is the milky variety, the whiteness being due to
imperfections in the crystallization, such as cracks, bubbles, etc.
Smoky quartz is the cloudy brown-colored variety, which results from the
presence of small quantities of organic matter (hydrocarbons) in the
quartz. If the color is so dark as to be almost black it is termed
morion. In the above cases the color will disappear if the stone is
heated. Pebbles of smoky quartz from Cairngorm, Scotland, have been so
widely used as semiprecious stones that they have come to be known as
cairngorms.
Citrine, or false topaz, is a clear yellow variety, the color again due
to the presence of organic matter. It is distinguished from true topaz
by the lesser hardness, this having the hardness of 7, while true topaz
has a hardness of 8.
Amethyst is quartz with a violet color, due to the presence of small
quantities of manganese. To be suitable for cutting into gems, the color
must be deep or the small pieces will appear almost colorless. It is
widely used today as a semiprecious stone in jewelry; and in the
fifteenth century it had the traditional virtue of making the wearer
sober-minded, whether he had taken too freely of wine, or was over
excited by love-passion.
Rose quartz gets its pale-red color from the presence of a small amount
of titanium. It is widely distributed, but is more abundant in the Black
Hills of South Dakota.
Aventurine is quartz which has inclosed tiny scales of mica or hematite
giving it a spangled appearance.
Prase is a green quartz, the color being due to the inclusion of fibrous
crystals of green actinolite.
Cat’s Eye is a quartz which has inclosed silky fibers of asbestos. When
this is cut parallel to the fibers, the effect is opalescent. The colors
are greenish, yellowish-gray, and brown. This form, however, is not to
be confused with the true or Oriental Cat’s Eye, which is chrysoberyl
and has the hardness of 8.
Chalcedony
SiO₂
Non-crystalline, occurring in botryoidal, stalactitic or concretionary
masses; hardness, 7; specific gravity, 2.65; color white when pure;
luster waxy; translucent to transparent on thin edges.
In addition to the crystalline form, silica is freely deposited in an
amorphous or cryptocrystalline form which has the same properties as
quartz, except the crystal faces. This is called chalcedony, and it
occurs in seams, cavities and free surfaces. When the surface of a
chalcedony deposit is free it has a waxy luster. It is generally very
brittle and breaks in a peculiar splintery manner. Like quartz it also
has a great many varieties, according to the impurities present. Its
wide distribution, hardness, and the manner in which it can be chipped
have made this a most important stone in the history of the development
of civilization. The early men first broke it into rough tools, such as
knives, axes, spear points, etc., and used these as cutting tools, of
one sort or another, because they held their edge better than most
stones. We apply, to the people who used only these chipped stones as
tools, the term “_Men of the Old Stone Age_,” or the period is termed
the _Palæolithic Age_. Later men learned how to grind the edge to a
smoother outline, and this much shorter period is termed the _Neolithic
Age_. The use of flints for the first tools is world-wide, and the
American Indian when discovered was still using chalcedony in its
rough-hewn state.
“There the ancient Arrow-maker
Made his arrow heads of sandstone,
Arrow heads of chalcedony,
Arrow heads of flint and jasper,
Smoothed and sharpened at the edges,
Hard and polished, keen and costly.”
Chalcedony is the proper term to use when the color is white to
translucent, in which case the surfaces are usually botryoidal and waxy.
Carnelian is chalcedony which is clear red in color and translucent.
This is one of the first stones used for ornamental purposes and for
engraving. Carnelians with figures engraved on them were used by the
Egyptians, Assyrians and The Children of Israel, at least 2000 B.C.; and
the Egyptian scarabs of the fifth or sixth century B.C., were often
carved from this variety of chalcedony, as well as from jasper and
agates.
The brownish varieties are termed _sard_.
Chrysoprase is an apple-green variety of chalcedony the color being due
to the presence of nickel oxide. This is by no means as common as most
of the varieties of chalcedony, and was long prized as a gem.
Plasma is chalcedony with a leek- to emerald-green color, and the same
stone when it has small red spots of jasper in it is termed
_blood-stone_, or _heliotrope_. These red spots are said by tradition to
be drops of the blood of Christ.
Jasper is a deep red chalcedony, the color being due to hematite, which
is so abundant as to make it opaque. A brown variety colored by limonite
is also called jasper, and even green jaspers are found. In all cases
the opaque character is common.
Flint is an impure brown chalcedony, usually forming concretions. The
color is due to organic matter. Flint is mostly found in limestone or
chalk, and the concretions are the result of the small particles of
silica scattered through the rock being dissolved, and then
reprecipitated about some organic center. Generally the silica was
obtained by the dissolution of small fossils, like the shells of diatoms
or sponge spicules.
Hornstone and Chert are simply impure varieties of flint, brown in
color, and with a splintery fracture.
Agate, Plate 32, is a banded or cloudy chalcedony which has formed in a
cavity, the layers of different color representing deposition from
water, carrying first silica with one impurity, then later, silica with
another impurity. Gradually the cavity has been thus filled with silica;
and when the mass is freed by the weathering away of the surrounding
rock, these banded masses are found. Sometimes the manner of deposition
has changed, and while the outer part of the cavity was filled with
chalcedony, the central part will contain quartz crystals. On account of
the beauty of the colors, and the unusual way in which they may be
developed, agates are widely used for semiprecious jewelry and objects
of art, and this has been true since ancient times, the name itself
coming from the River Achates in Sicily. The center for cutting and
polishing agates is at Oberstein, Germany, where this work has been
carried on since the middle of the fifteenth century. In spite of the
many fine natural colors in agates, they are sometimes artificially
colored, in many cases by methods which are kept as “trade secrets.” The
color seldom penetrates far; so that even slight chipping reveals
whether an inferior agate has been taken and colored up, or whether the
stone is natural. Moss agates are chalcedony which has inclosed
dendritic masses of some one of the manganese compounds as shown under
manganite, p. 73.
Onyx is a variety of agate where the bands are alternately black and
white; while sardonyx is agate with red or brown bands alternating with
the white. Such agates as these are especially desirable for cameo work,
where the figure is carved in the chalcedony of one color, and the other
color makes the background.
Silicified or _agatized wood_ is a form of chalcedony, where silica has
replaced wood, molecule by molecule; so that in good specimens, all the
structure of the wood is still retained, and when thin sections are made
it can be studied under the microscope almost as well as modern wood.
This takes place under water, usually, if not always, in fresh water.
Such fossilized wood is widely distributed in the western United States,
the most famous cases being the Fossil Forest of Arizona, now a National
Reservation, and the fossil trees in the Yellowstone National Park.
Opal
SiO₂·H₂O
Pl. 33
Non-crystalline, massive, stalactitic or nodular; hardness, 6; specific
gravity 2; all colors; luster vitreous, resinous, or pearly; transparent
on thin edges.
Opal differs from chalcedony in having water, usually about 10%,
incorporated in its structure. This is water of crystallization, and not
firmly held; so that, if opal is heated in a closed tube to above 100
C., it is given off as a vapor. Opal is distinguished from chalcedony by
its lesser hardness, and the resinous to pearly luster. It forms in
cavities, in layers often of extreme thinness.
Opal is originally the product of the dissolution of silicate minerals
in hot acid waters, the resulting gelatinous silica, when it is
deposited and hardened, becoming the opal. There are many varieties,
some of them highly prized as gems in spite of the moderate hardness and
opacity of the mineral. Gem-quality opal gets its opalescent character
from the successive deposition of thin films of opal, the light
penetrating and being reflected from different films. This breaks up the
white light and causes the play of colors which is the charm of this
gem.
Precious opal, in which the play of colors is finest, comes mostly from
Hungary, Mexico, and Queensland. The opal was a favorite stone from
before Roman times, and in its early history was a charm against the
“evil eye.” During the nineteenth century for some reason it came to be
considered an unlucky stone.
Fire opal is a hyacinth-red to honey-yellow variety, which has a
fire-like play of color, and is found in Mexico and Honduras.
Common opal does not have the play of color, but comes in a variety of
colors; is waxy or greasy in luster; and occurs mostly as fillings of
seams or cavities, especially those in igneous rocks, like the steam
holes in lavas, etc. It is found in Cornwall, Penn., in Colorado,
California, etc.
Opal-agate is a variety in which there are color bands, and it is widely
distributed.
Opalized wood is formed in exactly the same manner as agatized wood,
much of the fossil wood called silicified being really opalized.
Siliceous sinter is the porous mass of opal which is so frequently
deposited about hot springs and geysers. It is readily recognized by its
porous character.
The shells of the diatoms, which are microscopic plants, are made of
opal; and while they are so small, there is certainly no other plant so
abundant or omnipresent, living as it does in every pool, lake, or sea
by the millions. These shells are very indestructible so that they
accumulate at the bottom of ponds, bogs, and sea-bottoms, making at
times extensive deposits. This material in quantities is termed
diatomaceous earth, or tripolite (from Tripoli where it was first used
commercially). It is used as a polishing powder for metals, marble,
glasses, etc.
The Feldspars
The term feldspar is a family name for a large variety of very common
minerals, which altogether make up nearly 60% of the crust of the earth,
being the predominant part of granites, gneisses, and lavas. In
composition they are silicates of aluminum, together with potassium,
sodium and calcium, and their mixtures. They may be tabulated as
follows:
1. KAlSi₃O₈, _orthoclase_, the silicate of aluminum and potassium.
2. NaAlSi₃O₈, _albite_, the silicate of aluminum and sodium.
3. CaAlSi₂O₈, _anorthite_, the silicate of aluminum and calcium.
4. Mixtures of 1 and 2 are _alkalic feldspar_.
5. Mixtures of 2 and 3 are _plagioclase feldspar_.
Orthoclase is monoclinic, but the rest of the feldspars are triclinic.
If crystals are available they may be short and stout, or tabular and
thin, but as the feldspars are mostly components of the igneous rocks,
where perfect crystals have not had a chance to grow, they are mostly
determined by their hardness and cleavage. The hardness of all the
feldspars is 6 or very close to it.
They all have three planes of cleavage, two of which are good and
intersect either at 90° as in orthoclase, or at about 86° as in the
plagioclase series; while the third cleavage plane is imperfect. In
figure 1, Plate 34, a and b are the two perfect cleavages, while c is
the imperfect one. Breaking into such cleavage masses as the one
illustrated is characteristic of feldspar. The specific gravity ranges
from 2.55 to 2.75. The luster is vitreous, and the color white, ranging
to various shades of gray and pink, and, sometimes in recent lavas,
colorless.
Twinning is very common and helps to distinguish orthoclase from the
plagioclase feldspars. In orthoclase the twins are simple, that is, only
two crystals growing together, and are united on one of the faces, as if
one of them had been revolved 180° with the other; or, while related to
each other as in the preceding case, they may seem to grow through each
other. On plate 34 are three orthoclase crystals showing this simple
type of twinning. The first (A) is a simple crystal; the second (B)
shows the simplest type of twinning where the left-hand crystal has
revolved 180° on the p face, and the end is composed, half of the upper
end of one crystal, and half of the lower end of the adjacent crystal.
The presence of reëntrant angles calls attention to the twinning. The
third figure (C) is a case of intergrowing crystals.
In the plagioclase feldspars twinning is multiple, a large number of
crystals, each thin, sometimes as thin as paper, growing side by side,
the first one in normal position, the next at 180° with it, the third
revolved 180° to the second and thus parallel to the first, and so on.
The result is first of all a striated appearance, and second that, as
plagioclase crystals have their prism faces intersecting at 86°, there
is a series of low roofs and valleys, which are best seen by holding the
piece of feldspar so the light reflects from a cleavage face, when it
will appear striated; then by tilting it about 8 degrees a second set of
reflections, also appearing striated, will appear. The light was first
reflected from one side of the roofs, and in the second case from the
other side. Figure D, Pl. 34, is a diagram showing the relation of the
individual crystals in a multiple twinned piece of plagioclase, in which
the crystals are represented as rather large. Plate 35, under
labradorite, shows a photograph of a cleavage piece, on which is readily
seen the striation which is characteristic of the plagioclase feldspars.
Mixtures of albite and anorthite occur in bewildering numbers, one or
the other predominating, and each mixture being uniform throughout the
crystal and in the whole mass; so each combination is a mineral, each
with its special properties; but the different plagioclase feldspars are
so similar in appearance, that by the naked eye it is impossible to
separate the closely related ones. This can be done under the microscope
by studying the angles at which light is cut off, and also by chemical
analyses. For our purposes six types will suffice to illustrate the
group, and their composition may be indicated as follows.
Albite is albite with up to 15% of anorthite mixed with it.
Oligoclase is albite with from 15-25% of anorthite mixed with it.
Andesite is albite with from 25-50% of anorthite mixed with it.
Labradorite is anorthite with from 25-50% of albite mixed with it.
Bytownite is anorthite with from 15-25% of albite mixed with it.
Anorthite is anorthite with up to 15% of albite mixed with it.
The best method for distinguishing these feldspars of the plagioclase
group is to measure the angle between the two perfect cleavage faces,
and even this requires careful measurement. The angles between these
faces are as follows:
Orthoclase 90°
Microcline 89° 30′
Oligoclase 86° 32′
Andesite 86° 14′
Labradorite 86° 14′
Bytownite 86° 14′
Anorthite 86° 50′
Orthoclase
KAlSi₃O₈
Occurs in granites, syenites, gneisses and light-colored lavas;
hardness, 6; specific gravity, 2.57; color white to gray or pink;
cleavage in two directions perfect and at 90°, in the third direction
imperfect; luster vitreous; translucent on thin edges.
Orthoclase is monoclinic, and when formed in cavities develops as
crystals, but it is usually a constituent of igneous rocks, in which
case the crystals have not had the opportunity to develop the crystal
faces, and the orthoclase is in grains or irregular masses; and the best
way of determining the mineral is the cleavage, the two perfect cleavage
planes intersecting at right angles. Twinning is frequent but of the
simple type, only two crystals being united, similar to either B or C on
plate 34.
It is found in granites, gneisses or lavas, wherever they occur, being
especially characteristic of the granites of the Rocky Mountains.
Microcline
KAlSi₃O₈
Pl. 35
Occurs in granites and gneisses as crystals or irregular masses;
hardness, 6; specific gravity, 2.56; color white to gray, pink, or
greenish; luster vitreous; translucent on thin edges.
Microcline has the same composition as orthoclase, but is in the
triclinic system, the c axis being inclined a half degree away from a
right angle with the b axis. This is best seen in the cleavage pieces,
the two perfect cleavage planes meeting at 89° 30′, and this is the only
test for determining this mineral by the unaided eye. Pike’s Peak is the
best known locality for microcline, and there it occurs in fine large
crystals of greenish color, which are known as _Amazon stone_.
Albite
NaAlSi₃O₈
Occurs in small crystals, or more often in lamellar masses in granites
or in seams in metamorphic rocks; hardness, 6; specific gravity, 2.62;
color white to gray; luster vitreous.
Albite may occur in simple crystals, in which case the two perfect
cleavage planes meet at an angle of 86° 24′. However, it is much more
frequently found twinned in the multiple manner, the individual crystals
often being as thin as paper. This gives rise to a fine striation on the
end of a crystal, or on the surface made by the imperfect cleavage
plane. Where the crystals are extremely thin, the surface may have a
pearly luster. Albite types of granite often inclose secondary minerals,
that are prized as gems, such as topaz, tourmaline, and beryl.
It is found at Paris, Me., Chesterfield, Mass., Acworth, N. H., Essex
Co., N. Y., Unionville, Penn., and in Virginia, and throughout the Rocky
Mountains.
Oligoclase
(NaCa)AlSi₃O₈
Generally found in cleavable masses in granites and lavas, rarely in
crystals; hardness, 6; specific gravity, 2.65; color white, greenish or
pink; luster vitreous; translucent on thin edges.
Oligoclase is a plagioclase feldspar and is distinguished by its two
perfect cleavage planes meeting at an angle of 86° 32′, but otherwise it
is very like albite. Crystals are not common, and it occurs mostly in
masses, making one of the components of granite or lava.
It is found in St. Lawrence Co., N. Y., Danbury and Haddam, Conn.,
Chester, Mass., Unionville, Penn., Bakersville, N. C., etc.
Labradorite
(NaCa)AlSi₃O₈
Pl. 35
Usually found in cleavable masses in granites and lavas; hardness, 6;
specific gravity, 2.71; color gray or white, often with a play of
colors; luster vitreous; translucent on thin edges.
Labradorite is distinguished by having the two perfect cleavage planes
meet at 86° 14′. The iridescent play of color is also very
characteristic and is generally present. It is due to the inclusion of
minute impurities. This feldspar is usually associated with granites or
lavas in which the dark minerals predominate. It gets its name from
being the feldspar of the granites of Labrador, and is also found in the
granites of the central part of the Adirondack Mountains and the Wichita
Mountains of Arkansas.
The Pyroxene Group
The minerals of this group are generally associated with feldspars, and
make the dark-colored component of granites, gneisses and lavas. This is
especially true of those which have some iron in the crystal. Pyroxenes
are salts of metasilicic acid (H₂SiO₃), in which the hydrogen (H) has
been replaced by calcium, magnesium, iron, etc. The commoner minerals
are orthorhombic or monoclinic, and all agree in their crystal habit,
being short stout prisms, with the vertical edges so beveled that a
cross section is eight-sided. The cleavage is good in two directions,
parallel to the beveling faces (m in figure b, Plate 36), and they
intersect at an angle of 87°. This is very characteristic, and if one
has a crystal broken across, it is easy to see and measure this angle of
intersection. These pyroxenes have the same chemical composition as the
corresponding series of amphiboles, but the two are distinguished by
several features. Pyroxenes are short and stout crystals, while
amphiboles are long and either blade- or needle-like; pyroxenes are
eight-sided in cross section, while amphiboles are six-sided; in
pyroxenes the cleavage planes intersect at 87°, while in amphiboles they
intersect at 55°. The minerals of this group are most frequently one of
the components of a lava or granite, and are less frequently associated
with metamorphic rocks. Three are common; enstatite, hypersthene, and
augite.
Enstatite
MgSiO₃
Usually occurs in lamellar or fibrous-lamellar masses in dark lavas;
hardness, 5.5; specific gravity, 3.3; color gray, bronze or brown;
luster vitreous, translucent on thin edges.
Enstatite rarely occurs in crystals, but when it does they are
orthorhombic. Usually it is in irregular masses with the cleavage
angles, typical of pyroxene. The color is light, that is gray or
brownish, and the streak white or nearly so. In most respects it is
similar to hypersthene, which has the same composition, except that a
large part of the magnesium is replaced by iron, and there are all sorts
of gradations between the two minerals. When some iron takes the place
of magnesium, the color darkens to, or towards bronze, until when about
a third of the magnesium is so replaced, and the color is fully bronze,
this variety is called _bronzite_. Bronzite is present in some of the
dark lavas like gabbro and peridotite. Enstatite is found in the
Adirondack Mountains, at Brewster and Edwards, N. Y., etc.
Hypersthene
(MgFe)SiO₃
Occurs in cleavable masses in dark lavas; hardness, 5.5; specific
gravity, 3.4; color dark-brown or greenish-brown; luster vitreous;
translucent on thin edges.
Hypersthene is a pyroxene in which magnesium and iron are present in
about equal quantities. It is similar to enstatite, except that the
color is darker, and the streak gray or brownish-gray in color. These
two minerals grade into each other, so that there are cases where it is
simply a matter of preference as to which name should be given to the
mineral. This form is associated with dark lavas, of the gabbro or
peridotite type, in such places as the Adirondack Mountains, Mount
Shasta in California, Buffalo Peaks, Colo., etc.
Augite
CaMg(SiO₃)₂, MgAlSiO₆ + Fe₂O₃
Pl. 36
Usually occurs in short stout monoclinic crystals; hardness, 5.5;
specific gravity, 3.3; color dark-green to black; luster vitreous;
translucent on thin edges.
Augite is a complex pyroxene having some iron and aluminum always
present in it, but the amount not a fixed quantity. It is by far the
commonest of the pyroxenes and has a wide distribution, both in the
sorts of lavas in which it appears, and in the world. It is commonly the
dark component of such lavas, as gabbros and peridotites, and also is
common in metamorphic rocks, especially impure crystalline limestones.
It is found at Raymond and Mumford, Me., Thetford, Vt., Canaan, Conn.,
in Westchester, Orange, Lewis and St. Lawrence Counties of N. Y., in
Chester Co., Penn., at Ducktown, Tenn., Templeton, Canada, etc.
The Amphibole Group
The amphiboles are a group of minerals made up of the same chemical
elements as the pyroxenes, but with the molecular arrangement different,
which appears in the forms of the crystals. The commoner ones are all
monoclinic but contrast with the pyroxenes as follows. Amphiboles are
long and slender crystals, while pyroxenes are short and stout;
amphiboles are six-sided, while pyroxenes are eight-sided; amphiboles
have the two perfect cleavages intersecting at 55° and 125°, while those
of pyroxene intersect at 87° and 93°. With the above in mind it is easy
to place the minerals in their proper group, but inside the group it is
not always so easy to distinguish one from another. This group is
associated rather with metamorphic rocks than with igneous rocks, with
which the pyroxenes are mostly associated. The three commoner minerals
of the group are tremolite, actinolite, and hornblende.
Tremolite
(CaMg)₃(SiO₃)₄
Pl. 37
Occurs in long prismatic crystals or in columnar or fibrous masses;
hardness 5.5; specific gravity, 3; color white to gray; luster vitreous;
transparent on thin edges.
The long prismatic crystals of tremolite occur especially where
dolomitic limestones have been altered by metamorphism. Sometimes these
crystals grow side by side, making fibrous masses, where the long
slender crystals can be picked apart with the fingers, and yet are
flexible, and tough enough so that they can be felted together. This is
termed asbestos, which, because it is infusible and a poor conductor of
heat, is much used to make insulators, fire-proof shingles, and all
sorts of fireproof materials. The varieties in which the crystals are
finer and silky in appearance, like the one illustrated on Plate 38 are
termed _amianthus_. There are other minerals, such as actinolite and
serpentine, which occur in the same manner, and are also called
asbestos, the serpentine variety being just now the most important
commercially.
Tremolite is found at Lee, Mass., Canaan, Conn., Byram, N. J., in
Georgia, etc.
Actinolite
(CaMgFe)₃(SiO₃)₄
Occurs in radiating crystals, or in fibrous masses; hardness, 5.5;
specific gravity 3; color pale- to dark-green; luster vitreous;
translucent on thin edges.
Except for its green color, this mineral is very like tremolite. The
difference between the two is due to the small amount of iron in the
actinolite. It is usually found in schists, and the radiating character
of the crystal groups is enough to determine the mineral, if it is
already clear that it is one of the amphiboles. Occasionally it occurs
with the crystals parallel to each other, making one of the forms of
asbestos.
Actinolite is found at Warwick, Edenville, and Amity in Orange Co., N.
Y., at Franklin and Newton, N. J., Mineral Hill and Unionville, Penn.,
Bare Hills, Md., Willis Mt., Va., etc.
Hornblende
(CaMgFe)₃(SiO₃)₄CaMgAl₂(SiO₄)₃
Pl. 37
Occurs in well-defined crystals, in grains and in masses; hardness, 5.5;
specific gravity 3.2; color black, dark-green, or dark-brown; luster
vitreous; translucent on thin edges.
In composition hornblende corresponds to augite, but occurs in long
slender, six-sided crystals with cleavage planes intersecting at 55°, so
that it is a typical amphibole. It occurs in a very wide range of rocks,
such as granite, syenite, diabase, and gabbro; and in such metamorphic
rocks as schists and gneisses; and sometimes igneous rocks are made up
almost entirely of hornblende, when they are known as amphibolites or
hornblendite. It is found all through the New England States, down along
the Piedmont Plateau, through the Blue Ridge Mountains, and in many of
the western mountainous areas.
The Garnet Group
The garnets are a series of double silicates, which occur with
surprisingly uniform characters. They are all isometric, and occur
either as dodecahedrons, or as the 24-sided figure (the trapezohedron),
which is formed by the beveling of the edges of the dodecahedron, and
developing these new faces to the exclusion of the dodecahedron faces.
Combinations of the dodecahedron and trapezohedron (36 faces) may occur.
All the garnets have a hardness of 7 to 7.5, and the specific gravity
runs from 3.2 to 4.3, according to the composition. In size they run
from as small as a grain of sand up to as large as a boy’s marble, and
occasionally even to four inches in diameter. The color varies with the
composition, from colorless to yellow, red, violet, or green. There is
no cleavage, and the luster is always vitreous.
Garnets are usually accessory minerals, found in metamorphic rocks,
though they are sometimes also present in granites and lavas. They are
always segregations which have taken place in the presence of high
temperatures. When clear and perfect several of the garnets are used as
gems. On the other hand some of the common garnets occur in such
quantities that they are crushed and used as abrasives, for such work as
dental polishes, or for leather and wood polishing.
The following is the composition of some of the commoner garnets.
Ca₃Al₂(SiO₄)₃ = grossularite
Mg₃Al₂(SiO₄)₃ = pyrope
Fe₃Al₂(SiO₄)₃ = almandite
Mn₃Al₂(SiO₄)₃ = spessartite
Ca₃Fe₂(SiO₄)₃ = andradite
Ca₃Cr₂(SiO₄)₃ = uvarovite
Grossularite is chiefly found in crystalline limestones, which have
resulted either from contact with lavas, or from general metamorphism of
impure limestones. These garnets are colorless to white, or more often
shades of yellow, orange, pink, green or brown, according to traces of
impurity which they may contain. The cinnamon-colored variety from
Ceylon is termed _cinnamon stone_, and is a fairly popular gem.
Pyrope is a deep-red color and when perfect is highly prized as a gem.
It is found in dark-colored igneous rocks, like lavas, or serpentines.
Some of the finest come from South Africa, where they are found in
company with the diamond.
Almandite is dark-red to brown in color, the brownish-cast
distinguishing it from pyrope. It is one of the garnets known as “common
garnet.” In some cases it is clear and deep colored enough to be used as
a gem, but mostly it is muddy in appearance. The name almandite comes
from Alabanda, a city of the ancient district of Caria, Asia Minor,
whence garnets were traded to ancient Rome. The finest garnets “Sirian
garnets” came from the city of “Sirian” in Lower Burma, and were
supposed to have been found near there, but careful investigation shows
that no garnets occurred near there, and this town was therefore, even
at that early time, a distributing point for garnets, found probably
further to the east. The “Sirian” garnet had a violet cast and now the
term is used to indicate a type of garnet, rather than a locality.
Spessartite is dark-hyacinth-red, or red with a violet-tinge, and is one
of the less-common garnets. It is usually found in granites. The finest
garnets of the type come from Amelia Court House, Va., which has yielded
some ranging from one up to a hundred carats.
Andradite is another garnet which is termed “common garnet.” It is red
in color, but with a yellowish-cast which distinguishes it from
almandite, but these two are not easy to separate. It is found mostly in
metamorphosed limestones. One variety is black in color and called
_malanite_. It is found in lavas. The common yellowish-red garnets are
found through New England and the Piedmont Plateau.
Uvarovite is a rare garnet of emerald-green color, found in association
with chromium ores.
The number of localities for garnets is so great that a list would
suggest most of the regions where metamorphic rocks occur, as all over
New England, throughout the Piedmont Plateau, the Rocky Mountains, etc.
Certain fine clear garnets, found in Montana, northeastern Arizona, and
northwestern New Mexico are sold under the trade name of “Montana,
Arizona or New Mexico rubies.” These are of fine quality and are mostly
collected by the Indians from the ant hills and scorpion’s nests of
those regions.
Garnets are among the earliest stones mentioned in ancient languages, as
would be expected from the way these hard and beautiful crystals weather
out of the much softer metamorphic rocks, like schists. In the past
they, with most any other translucent red stone, were included under the
name _carbuncle_. This, however, is not the name of any mineral, but
refers rather to a mode of cutting, _en cabochon_ or with a convex
surface.
Glucinum
Glucinum is a rare metal, silvery-white in color, malleable, and melting
at a fairly low temperature. It is found in the mineral beryl, from
which has come the alternative name _beryllium_. The name comes from the
sweet taste of its salts. Except for beryl its minerals are rare, and
the metal has found but few uses for man.
Beryl
Gl₃Al₂(SiO₃)₆
Pl. 39
Occurs in hexagonal crystals in granites, gneisses and mica schists;
hardness, 7.5; specific gravity, 2.7; color usually some tint of green;
luster vitreous; transparent on thin edges.
When this mineral occurs in coarse hexagonal prisms, with or without
faces on the ends, it is known as beryl; when the crystals are clear and
perfect and of a dark-green color, they are of gem value and are termed
_emerald_; when of a light-green color, they are _aquamarine_; and when
bright-yellow in color, they are the _golden beryl_. There is little
difficulty in determining beryl, for only apatite occurs in such
crystals, and is green, and this latter mineral has a hardness of only
5. There is an imperfect basal cleavage.
Ordinary beryl is fairly common in granites of the pegmatite sort, and
less common in gneisses and mica-schists. This type often furnishes
crystals of large size, up to two and three feet in diameter.
Beryl which is free from cracks and inclosures, so it can be used as a
gem, is so rare, that the emerald has a value above that of the diamond,
and second only to the ruby. It is one of the gems with a long history,
having been quarried on the west coast of the Red Sea at least 1650 B.C.
by the Egyptians. To early people it had a power to quicken the prophet
instinct and made the wearer see more clearly. The Spanish
conquistadores found fine emeralds among the treasures of both Mexico
and Peru. In the United States, Stony Point, N. C., was a notable
locality for these gems, but now seems to have been exhausted. The name
emerald has been applied to many other green stones, usually with some
geographical modification, as “Oriental emerald” which is green
corundum, “Brazilian emerald” which is tourmaline, etc.
Giant beryls have been found at Acworth and Grafton, N. H., and at
Royalston, Mass. Localities for ordinary beryl are Albany, Norway,
Bethel, Hebron, Paris, and Topsham, Me., Barre, Goshen and Chesterfield,
Mass., New Milford and Branchville, Conn., Chester and Mineral Hill,
Penn., Stony Point, N. C., and many other localities in the
Appalachians; also Mount Antero, Colo., and in the Black Hills of South
Dakota.
Sodalite
Na₄Al₃Cl(SiO₄)₃
Occurs in irregular masses, sometimes in dodecahedrons; hardness, 5.5-6;
specific gravity, 2.3; color deep-blue to colorless; streak white;
luster vitreous; translucent on thin edges.
This striking mineral, with its deep-blue to azure color, is not easily
confused with any other. It is characteristic of soda-rich igneous rocks
such as syenite and some lavas. In this country it is found at
Litchfield, Me., and Salem, Mass.
Zircon
ZrSiO₄
Pl. 39
Usually occurs in tetrahedral crystals in igneous rocks; hardness, 7.5;
specific gravity, 4.7; color brown; luster vitreous; translucent on thin
edges.
Zircon, the mineral of the rare earth element zirconium, nearly always
occurs in light-colored igneous rocks, like syenite. It may occur in
schists or gneisses, but in these rocks the crystals are of microscopic
size. Because of their great hardness and insolubility, zircon crystals
resist weathering and are often found, along with gold, cassiterite, or
magnetite, in sands which have resulted from the disintegration of
syenite rocks.
Zircon refracts and disperses light to a degree second only to the
diamond, so that clear crystals are sought as gems. They are often
called “Matura diamonds” because of their abundance at Matura, Ceylon.
When the crystals are colorless or smoky they are termed _jargons_ or
_jargoons_; when of a red-orange hue, they are _hyacinth_ or _jacinth_.
Most of the zircon of gem-quality comes from Ceylon, where it is picked
up as rolled-pebbles from the beds of brooks.
The most remarkable American locality for zircon is near Green River, in
Henderson Co., N. C., where it is found abundantly in a decomposed
pegmatite dike, from which many tons have been obtained. It is also
found at Moriah, Warwick, Amity and Diana, N. Y., at Franklin Furnace,
and Trenton, N. J., in the gold-bearing sands of California, etc.
Cyanite
Al₂SiO₅
Pl. 40
Occurs in long blade-like crystals in gneisses and schists; hardness, 7
at right angles to the length, and 4.5 parallel to the length; specific
gravity, 3.6; color blue; luster vitreous; translucent on thin edges.
There are only a few blue minerals, and the way in which cyanite occurs
in long thin blade-like crystals is entirely characteristic. If more is
still wanted to determine this mineral, its unique character in having
the great hardness 7 when scratched parallel to the length, and only 4.5
when scratched crossways, will settle any doubts.
The mineral _sillimanite_ has the same composition as cyanite, but is
fibrous in habit and has the hardness 6.5. If cyanite is heated to 1350°
C. it changes its character and becomes sillimanite.
Cyanite is found as an accessory mineral in metamorphic rocks, such as
gneiss and schist, at Chesterfield, Mass., Litchfield and Oxford, Conn.,
in Chester Co., Penn., in North Carolina, etc.
The Mica Group
The micas are very common minerals, easily recognized by their very
perfect basal cleavage, as a result of which thin sheets, often less
than a thousandth of an inch in thickness, readily split off. These are
tough and elastic, which distinguishes mica from the chlorite group in
which there is similar basal cleavage, but the sheets are not elastic.
Micas are complex silicates of aluminum, with potassium, iron, lithium,
magnesium and hydrogen. They are one of the principle components of many
granites, gneisses, and schists. This mineral is always crystalline,
being in the monoclinic system, but occurring in six-sided prisms. The
cleavage is so dominant a character that the crystal form is usually
overlooked, as it is seldom requisite in determining this mineral. The
size of the sheets of mica depend on the size of the crystals, the
larger sheets expressing great slowness in cooling from the original
magmas. Sometimes the crystals may be two or even three feet in
diameter. The hardness is not great, ranging between 2 and 3. The
specific gravity lies between 2.7 and 3.2. The color varies according to
the composition, from silvery-white, through gray, pink, and green to
black. The luster is vitreous to pearly, sometimes gleaming in the
darker-colored varieties. The commoner types of mica are as follows:
Muscovite, H₂KAl₃(SiO₄)₃ or potash mica.
Lepidolite, LiK(Al₂OH·F)Al(SiO₃)₃ or lithia mica.
Biotite, (HK)₂(MgFe)₂Al₂(SiO₄)₃ or iron mica.
Phlogopite, H₂KMg₃Al(SiO₈)₃ or magnesia mica.
Muscovite is colorless, silvery-white, gray or sometimes pale-green or
brown. It gets its name from Moscow where it was early used for window
panes, and it is still used for stove and furnace doors, as well as in
electric work, for a lubricant, etc.
The best crystals occur in granites, in the coarse varieties of which
large crystals may be obtained. It is found also as small scales in
gneisses and schists, and when weathered from its original rocks it may
be present in sandstones and shales. Muscovite is always in its origin
an elementary component of deep-seated igneous rocks, like granite; but
is never a component of extruded lavas. _Sericite_ is muscovite which
has been secondarily produced by the alteration of other minerals into
muscovite, as when feldspar, cyanite, topaz, etc., have been modified by
the presence of heat and hot vapors, when near lavas that have come in
contact with other rocks. Muscovite is very resistant to alteration by
weathering, but when it does change, the greater part of it becomes
kaolin. It is found at Acworth and Grafton, N. H., in plates, sometimes
a yard across at Paris, Me., Chesterfield and Goshen, Mass., Portland
and Middletown, Conn., at Warwick, Edenville, etc., N. Y., and all down
the Appalachian Mts., also in the Rocky Mts., the Cascade Range, etc.
Lepidolite is pink or lilac in color and occurs in scaly masses, mostly
in granites. It does not come in large crystals. Lepidolite is found at
Paris and Hebron, Me., Middletown, Conn., Pala, Calif., etc.
Biotite is dark-brown or black mica. Like muscovite it is very common,
making one of the chief components of granites, gneisses and schists;
and, unlike muscovite, it may occur in extrusive lavas, like trachyte,
andesite, and basalt. It resists weathering much less than muscovite, so
that, when the rocks of which it is a component disintegrate, biotite is
usually altered to kaolin and other compounds. It is likely to occur in
good-sized crystals, especially at Topsam, Me., Moriah, N. Y., Easton,
Penn., etc.
Phlogopite is pale-brown, often coppery in color, and is most likely to
occur in serpentines, or crystalline limestones or dolomites, often in
fine crystals, of good size. While one of the less abundant micas, this
is found at Gouverneur, Edwards, and Warwick, N. Y., Newton, N. J., and
Burgess, Canada.
Topaz
Al₂F₂SiO₄
Pl. 41
Occurs in crystals mostly; hardness, 8; specific gravity, 3.5; colorless
to pale-yellow; luster vitreous; transparent on thin edges.
Topaz may be colorless, but is more often some shade of yellow, and at
times brown or even blue. Its hardness is characteristic, there being
but few minerals as hard, and it is used to represent the hardness 8 in
the Moh’s scale. The crystals are orthorhombic prisms, with the edges of
the prism beveled and often striated. The ends of crystals usually
terminate with a basal plane, parallel to which there is good cleavage.
Between this basal plane and the prism faces there are usually several
sets of small faces as indicated on Plate 41.
This mineral, as is also true of most minerals containing fluorine, is
one of those which have crystallized out from hot vapors, escaping from
igneous magmas. It is associated with such minerals, as tourmaline,
beryl, fluorite, and cassiterite, and occurs mostly in cavities or
seams, in or near granites.
Ordinary topaz, which means crystals that are imperfect by reason of
tiny cracks and impurities is not very rare, but crystals which are
perfect and clear in color are considered gems. Most of the gem-topaz is
some shade of yellow, but may be brown or blue, never, however, pink, as
is often seen in jewelry. The “pinking” is artificial, and done by
packing yellow or brown topaz in magnesia, asbestos, or lime, and then
heating it slowly to red heat, after which it is cooled slowly. If
underheated the color is salmon, if overheated all color disappears.
Topaz has been a gem for centuries, the earliest records coming from
Egypt. The name comes from _topazios_, meaning to seek, because the
earliest known locality, from which it was gathered, was a little island
of that name in the Red Sea, and this island was often surrounded by fog
and hard for those early mariners to find. Here by mandate of the
Egyptian kings the inhabitants had to collect topazes, and deliver them
to the gem-cutters of Egypt for polishing.
Several yellow stones are called topaz, as the “Oriental topaz” which is
corundum and more valuable than topaz itself; and several varieties of
yellow quartz, which go under such names as “Saxon,” “Scotch,”
“Spanish,” and “smoky” topaz. When topaz occurs colorless as in Siberia,
the Ural Mountains, and in the state of Minas Geraes, Brazil, in all of
which places it is found as pebbles in brooks, it goes under the name of
“slave’s diamonds.” Brazil is today the chief source of gem-quality
topaz.
Ordinary topaz is found in this country at Trumbull, Conn., Crowder’s
Mt., N. C., Thomas Mts., Utah, in Colorado, Missouri, and California,
etc.
Staurolite
FeAl₅OH(SiO₆)₂
Pl. 41
Occurs in orthorhombic crystals; hardness, 7.5; specific gravity, 3.7;
color brown; luster resinous; translucent on thin edges.
This mineral occurs about equally abundantly in simple crystals similar
to the outline on Plate 41, and in twins which have grown through each
other either at 90° or at 60°. The color is either brown or
reddish-brown. In all cases it is an accessory mineral, occurring in
metamorphic rocks, usually schists, though less frequently in slates and
gneisses.
From the seventeenth century on, it has been used as a baptismal stone,
and worn as a charm, legends stating that it fell from the heavens. Fine
crystals have been found in Patrick County, Va., and there is in this
region the legend, that when the fairies heard of the crucifixion of
Christ, they wept and their tears falling crystallized in the form of
crosses, such as the one shown on Plate 41.
Staurolite is found in the schists of New England as at Windham, Me., or
Chesterfield, Mass., and all down the east side of the Appalachian
Mountains to Georgia.
Olivine
(MgFe)₂SiO₄
_Peridot_ or _Chrysolite_
Occurs in grains and irregular masses in dark lavas; hardness 6.5 to 7;
specific gravity 3.3; color bottle- to olive-green; luster vitreous;
translucent on thin edges.
Olivine rarely occurs in crystals, but when it does they belong to the
orthorhombic system. The dark-green grains or masses are recognized by
the color, considerable hardness and indistinct cleavage. Serpentine may
have a similar color, but its hardness is only 4. In hydrochloric acid
olivine decomposes to a gelatinous mass.
Olivine is typically one of the constituents of the dark lavas, like
basalt, gabbro, or peridotite. It is also a common mineral in
meteorites. Olivine, in the presence of water, alters to other minerals,
especially serpentine, with great facility.
It occurs fairly widely wherever the dark lavas are present, as in the
White Mountains of N. H., in Loudoun Co., Va., in Lancaster Co., Penn.,
and in many localities in the Rocky Mountains and Cascade Range.
Epidote
Ca₂(AlOH)(AlFe₂)(SiO₄)₃
Pl. 42
Occurs in grains or columnar masses; hardness, 6.5; specific gravity
3.4; color green, usually a pistachio or yellow-green; luster vitreous;
translucent on thin edges.
Rarely epidote occurs in crystals, which belong to the monoclinic
system, and may be either short like the diagrams on plate 42 or long
and needle-like. The color and hardness will suffice to determine this
mineral, as almost no other has the peculiar yellowish-green color which
is characteristic of this form.
Epidote occurs primarily in metamorphic rocks at or near the contact
with igneous rocks; or it may be a secondary mineral resulting from the
weathering of granites, especially along seams. It sometimes occurs with
hornblende in highly folded schists, as in New York City. It is often a
mineral which has resulted from the alteration of other minerals, as
pyroxene, amphibole, biotite, or even feldspars.
It is found at Chester and Athol, Mass., Haddam, Conn., Amity, Munroe
and Warwick, N.Y., East Branch, Penn., in the Lake Superior region, in
the Rocky Mountains, etc.
Tourmaline
(FeCrNaKLi)₄Mg₁₂B₆Al₁₆H₈Si₁₂O₆₃
Pl. 42 & frontispiece
Occurs in three-sided prismatic crystals; hardness, 7; specific gravity,
3.1; colorless, red, green, brown, or black; luster vitreous;
transparent on thin edges.
Tourmaline is readily distinguished from other minerals, as it always
occurs in long to short prisms, which are three-sided in cross section.
There is also a tendency for the sides to be curved as seen on the end
view of D, Pl. 42. Frequently the vertical edges of the prism are
beveled with one, two or three faces, grouped about each of the three
original edges, and there are often striations on the prism faces. The
ends are terminated by a low rhombohedron and again there may be a host
of modifying faces on the edges and corners of the end. The common
varieties are brown or black in color, but occasionally there may occur
green, red, yellow or almost any color. When the crystals are perfect,
that is free from impurities and without tiny cracks, tourmaline becomes
a gem of popularity and value.
Tourmaline is very complex in composition and may vary considerably, the
sodium, potassium, lithium, magnesium, and iron being either more or
less abundant or even lacking. The color is to some extent dependent on
the proportions of these elements present, the dark varieties having
more iron, and the light colored tourmalines lacking it. This mineral is
one of those which form from superheated vapors, escaping from molten
magmas. It will therefore occur in veins, often associated with copper
minerals, in crystalline limestones, or in cavities in granites, where
it is associated with such minerals, as beryl, apatite, fluorite, topaz,
etc.
If heated tourmaline crystals develop electricity, with the effect of
making one end a positive and the other a negative pole, and then will
attract bits of straw, ashes, etc. It was first introduced into Europe
about 1703 from India, and its vogue as a gem has greatly increased
since it was found on Mount Mica near Paris, Me. This Paris, Me.,
locality was discovered by two boys, amateur mineralogists, Elijah L.
Hamlin and Ezekiel Holmes, who in 1820 were returning home from a trip
hunting for minerals, when, at the root of a tree, they discovered some
gleaming green substance. It proved to be gem-quality tourmaline. A snow
storm that night buried their “claim,” but next spring it was visited
and several fine crystals found. Later this locality was systematically
worked, and over $50,000 worth of tourmaline taken from the pegmatite
seam in the granite, which lay under the crystals found on the surface.
The figure in the frontispiece is one of the crystals from there.
Well known localities are Paris and Hebron, Me., Goshen and
Chesterfield, Mass., Acworth and Grafton, N. H., Haddam and Munroe,
Conn., Edenville and Port Henry, N. Y., Jefferson Co., Colo., San Diego
Co., Calif., etc.
Kaolinite
H₄Al₂Si₂O₉
_Kaolin_
Usually found in whitish clay-like masses; hardness, 2; specific
gravity, 2.6; color white to grayish or yellowish; luster dull.
Kaolinite does not generally occur in crystals, though crystals of
microscopic size and monoclinic forms have been found. It is a secondary
mineral resulting from the decomposition by weathering of feldspars, the
calcium, potassium or sodium having been replaced by water. When found
in place it is generally white or nearly white, and is characterized by
its greasy feel.
As granites or other feldspar-bearing rocks are weathered away, the
kaolin is washed out by water, and with other fine material is carried
down into lakes or the sea, where it settles to the bottom and is known
as clay. Clay is kaolin with more or less impurities.
Pure kaolin is used for the manufacture of china and white porcelain
ware; but when it is impure, especially when it has iron in it, baking
causes the product to turn red or brown, so that it is only suitable for
making tile, bricks, etc.
It is found almost anywhere that feldspar rocks are, or have been,
exposed to weathering.
Talc
H₂Mg₃(SiO₃)₄
Occurs in scales, or in fibrous, scaly or compact masses; hardness, 1;
specific gravity, 2.7; color white, gray or pale-green; luster pearly;
translucent on thin edges.
This mineral is as soft as any, only graphite and molybdenite being of
the same hardness, but both these latter two have a black streak, while
the streak of talc is white. The greasy feel is also characteristic.
Talc is very seldom found in crystals, but if they are found, they will
appear like flakes and have a hexagonal cross section, though in reality
they belong to the monoclinic system.
Talc is a secondary mineral which usually results from the exposure of
magnesium silicates, such as pyroxenes or amphiboles, to moisture. In
this case, in-as-much as the original rocks were metamorphic in origin,
the talc therefrom will occur in old metamorphic regions. Some talc is
also formed by the action of silica-bearing waters on dolomite. This is
likely to be the case near the contact between dolomite and igneous
rocks. Talc is closely related to serpentine and likely to be found in
the same regions.
Talc has come to have a considerable use. Some of it is compact and then
called soapstone, and this was used by the ancient Chinese to make
images and ornaments; and our North American Indians used it to make
large pots, to serve as containers for liquids. Some of these pots have
been carved out with great skill, so as to be fairly light in proportion
to what they would hold. Pipes and images were also carved from
soapstone. Today we still cut soapstone into slabs to make mantels,
laundry tubs and sinks. The scaly and fibrous varieties are ground, and
used in making paper, paint, roofing, rubber, soap, crayons, toilet
powders, etc. The United States produce and use over half the world’s
production, our industries requiring over 100,000 tons of talc a year.
Of this 38% goes into paper, 23% into paint, 18% into roofing, and so on
down to toilet powder which uses 2½%, or 2,500 tons a year.
Talc is found in metamorphosed regions, that is in New England, all down
the east side of the Appalachian Mts., in the Rocky Mts., and the
Cascade Ranges, with a large number of local occurrences. New York State
is the leading producer.
Serpentine
H₄Mg₃Si₂O₉
Pl. 43
Occurs in compact, granular or fibrous masses; hardness, 3; specific
gravity, 2.6; color green; luster greasy; translucent on thin edges.
Serpentine is never in crystals. Its color and hardness serve to
distinguish it. Like talc it is a secondary mineral resulting from the
alteration, in the presence of moisture, of pyroxenes, amphiboles, and
especially, olivine. As these are often in metamorphic rocks, the
serpentine is likely to be associated with metamorphic rocks. Some
serpentine is also the result of the action of silica-bearing water on
dolomite, and this is likely to occur in areas of sedimentary rocks. The
fibrous variety of serpentine, _chrysolite_, usually occurs in seams or
veins, and when the fibers are long, it is used as asbestos. This form
of asbestos is the one most used commercially today, as there are
remarkably large deposits of it in the Province of Quebec, which provide
the major part of the world supply. In the United States it is also
found in California and Arizona but only in moderate quantities.
Massive serpentine is used in considerable quantities as an ornamental
stone, the green color varied with streaks and blotches of white, yellow
and red, due to various impurities, making it very effective. It is,
however, only suitable for interior work as the weather quickly spoils
the polished surface. This is further discussed under serpentine rock,
page 245.
Serpentine is found at Newfane, Vt., Newburyport, Mass., Brewster,
Antwerp, etc., N. Y., Hoboken, N. J., in Pennsylvania, Maryland, etc.
Chlorite
H₈(MgFe)₅Al₂(SiO₆)₃
Pl. 43
Occurs in monoclinic crystals of six-sided outline, or in scaly flakes
or masses; hardness, 2; specific gravity 2.8; color green; luster pearly
on cleavage faces; translucent on thin edges.
Chlorite is a family name, covering a series of closely related
minerals, so similar in appearance that they are best considered under
this common name. In many respects they resemble mica, in the shape of
the crystals and the remarkable basal cleavage. At first glance it is
easy to confuse the two, but chlorite scales are not elastic, and when
bent, stay bent, instead of snapping back like mica. In fact they look
like more or less rotted micas. This is more than appearance, for
chlorites form as a result of the alteration of micas in the presence of
moisture. They are then secondary, and will be found where mica-rocks
have been weathered, as in granites and schists.
They may be expected anywhere that micas have been long exposed, as in
New England, the Rocky Mountains, or the Sierra Nevada or Cascade
Ranges. Special localities are Brewster, N. Y., Unionville and Texas,
Penn., etc.
The Zeolites
The zeolites are a group of white minerals, with a pearly luster, light
weight, and easy solubility in acids; which, because their contained
water is lightly held, readily boil before the blowpipe. They are all
secondary minerals, which result from the decomposition of feldspars,
when exposed to weathering. They are almost universally found in seams
and cavities of disintegrating lavas. From a group of a dozen or so,
three are common enough to be considered here. They may be found by
watching such places, as where trap rock is being quarried for road
material, or being blasted for any reason.
Analcite
Na₃Al₂Si₄O₁₃ + 2H₂O
Pl. 44
Occurs as trapezohedrons in seams and cavities in lavas; hardness, 5.5;
specific gravity, 2.2; colorless, white or pink; luster vitreous;
transparent on thin edges.
Analcite usually occurs in the 24-sided form, known as a trapezohedron,
as illustrated in figure A, Pl. 44; but it may also occur in cubes with
the three faces of the trapezohedron on each corner. Small crystals are
often colorless, but the larger ones are either white or pink, and are
opaque. While the form is the same as that of garnets, the color, lesser
hardness, and the occurrence in lavas will serve to distinguish this
mineral. If placed in hydrochloric acid analcite dissolves to a
gelatinous mass.
It is always found in seams and cavities in lavas, as at Bergen Hill and
Weehawken, N. J., Westfield, Mass., in the Lake Superior region, etc.
Natrolite
Na₂Al₂Si₃O₁₀ + 2H₂O
Plate 44
Occurs as bristling crystals in seams and cavities in lavas; hardness,
5.5; specific gravity, 2.2; colorless; luster vitreous; transparent on
thin edges.
Natrolite occurs as beautiful bristling tufts of needle-like crystals,
each crystal an orthorhombic prism with a very low pyramid on the end.
This mineral is so easily fusible that it can be melted in a candle
flame, giving to the flame the characteristic yellow color due to
sodium. In hydrochloric acid it dissolves to a gelatinous mass. It is
always a secondary mineral in cavities and seams in disintegrating
lavas, and the tuft-like manner of growth is so characteristic, that
once seen, it will always be recognized.
Natrolite is found at Weehawken and Bergen Hill, N. J., at Westfield,
Mass., in the Lake Superior region, etc.
Stilbite
H₄(CaNa₂)Al₂(SiO₃)₆ + 4H₂O
Pl. 44
Usually occurs in sheaf-like bundles of fibrous crystals; hardness, 5.5;
specific gravity 2.2; colorless to white, yellow or brown; luster
vitreous; transparent on thin edges.
Stilbite crystals are really monoclinic, but on account of almost
universal twinning, appear as if orthorhombic. Like the two foregoing
minerals, stilbite is found in the seams and cavities of disintegrating
lavas. It is readily recognized by its habit of forming in sheaf-like
bundles of fibrous crystals. It may also, but more rarely, occur in
radiating masses. In hydrochloric acid it is completely dissolved. It is
found in lavas, at Weehawken and Bergen Hill, N. J., in the Lake
Superior region, etc.
Calcium
Calcium is one of the most abundant of metals, but never occurs as such
in nature, nor is it used as a metal by man. In its metallic form it is
yellowish-white, and intermediate between lead and gold in hardness.
Exposed to air it soon tarnishes by oxidation, and in water rapidly
decomposes the water, forming the oxide. However, it has a great
affinity for other elements, and makes a large number of minerals, the
most important of which are calcite, aragonite, gypsum and fluorite,
while it is an essential component of some garnets, anorthite, epidote,
amphibole and pyroxene. It is very widely distributed as limestone, and
is found in solution in most all natural waters, and in the shells and
bones of many animals and some plants.
Calcite
CaCO₃
Pl. 45
Occurs in well defined crystals in incrustations, and in stalactitic,
oolitic, and granular masses; hardness, 3; specific gravity 2.7;
colorless to white, or when impure, yellow, brown, green, red or blue;
luster vitreous to dull; transparent on thin edges.
Next to quartz, calcite is the most abundant of all minerals, and occurs
in an almost endless variety of forms, over 300 having been described.
It belongs to the hemihedral section of the hexagonal system, the form
of the crystals being all sorts of variations of the rhombohedron, and
combinations of left and right handed rhombohedrons. The cleavage is
entirely uniform, in three directions, parallel to the faces of the
rhombohedron, and at an angle of 74° 55′ with each other. Crystals may
occur in the form characteristic of the cleavage, but not often. The
commonest forms are a more or less elongated scalenohedron, made by
combining right and left handed rhombohedrons, so that the resulting
pyramid is six-sided, as in figure C, Plate 45. Such a scalenohedron may
be combined with other forms in a great variety of ways. The six-sided
prism with the ends terminated by one or more sets of rhombohedral faces
is also fairly common. Twinning occurs occasionally.
The quickest way to determine calcite is by the hardness (3), combined
with the fact that it effervesces, when hydrochloric acid is dropped
upon it.
An interesting feature of this mineral is its marked property of
deflecting light rays, so that a line or object placed behind a piece of
clear calcite appears double. It was with pieces of calcite from Iceland
that this was first seen; so that large transparent crystals of calcite
are still called _Iceland spar_; and such calcite is used to make the
Nichol’s prisms for microscopes, which are so useful in the study of
minerals. This power of refracting light is present in all minerals, but
not to such a marked degree as in calcite. The elongated scalenohedrons
of calcite are often called “dog-toothed spar” from a fancied
resemblance between them and the dog’s tooth.
Calcite is present in solution in the water of the sea and most streams,
from which it is withdrawn by many animals and some plants, to make
their shells, and bones. The foraminifera, some sponges, the
echinoderms, corals and molluscs all draw large quantities from the
water in which they live, and build more or less permanent structures
from it. These shells when they fall to the bottom, or after being
broken to bits, accumulate on the bottom and make limestone, which is
widely distributed over the country. This same limestone, when
metamorphosed and crystalline, is marble.
Calcite then is readily soluble in water, and streams flowing along
crevices and fissures in limestone dissolve out great cavities or caves,
like the Mammoth Cave of Kentucky. Other water, percolating through the
limestone, comes to these cavities saturated with lime in solution and
drips from the roofs and walls; then as part of the water evaporates, it
deposits part of its lime in icicle-like masses, hanging from the roof.
Such masses of non-crystalline calcite are called _stalactites_. Below
on the floor of the cave, conical masses are built up in the same manner
where the dripping water falls on the floor. These are _stalagmites_. In
these limestone caves and in smaller cavities many of the most beautiful
crystals grow. Somewhat similarly, when hot water from deep springs
comes to the surface, it cools and can not carry as much lime, and so
around the spring is laid down layer after layer of non-crystalline
calcite making a mass known as _travertine_. Sometimes this is colored
by iron or other impurities and a banded effect results. Such travertine
as the “Suisun marble” from California, “California onyx,” “Mexican
onyx,” and “satin spar” all belong to this class.
The coral animals, especially in tropical waters precipitate an enormous
amount of lime, until whole reefs are built of lime in this
non-crystalline form. In places it is hundreds of feet thick and
hundreds of miles in extent. Some of this coral has become popular for
personal adornment. This is particularly a small, fine-grained variety,
_Corallum rubrum_, which lives almost exclusively in the Mediterranean
Sea. This coral is red in color, varying all the way from a deep red to
white. It grows in small masses, three pounds being a good sized mass,
in water 60 to 100 feet deep, requires some ten years to develop a
full-sized mass. The making of this into beads and ornaments is an
Italian industry. The demand is growing, while at the same time the
supply is diminishing, and search is being widely made for more such
coral, but up to the present time with little success. This precious
coral is much worn as a protection against the “evil eye” and is widely
imitated, apparently with as much protection to the wearer. When coral
beads are offered cheap, they are probably something else, red gypsum
being much used. This and all imitations can be readily detected by
trying a drop of acid in the bead. Coral will effervesce, but gypsum and
other substitutes will not.
The bulk of the shells of most molluscs is made of lime, but the
mother-of-pearl layer inside is usually aragonite. The chalk of the
cliffs on either side of the English channel is lime, and composed of
the shells of single celled animals. See p. 213. When lime is deposited
in loose porous masses, as around grass, etc., and below hot springs,
this mass is termed _calcareous tufa_.
Calcite will be found almost everywhere, some of the localities for the
finest crystals being Antwerp and Lockport, N. Y., Middletown, Conn.,
the caves of Kentucky, Warsaw, Ill., Joplin, Mo., Hazel Green, Wis.,
etc.
Aragonite
CaCO₃
Pl. 46
Occurs in crystals, in columnar or fibrous masses, or incrustations;
hardness, 3.5; specific gravity, 2.9; colorless, white or amber; luster
vitreous; transparent on thin edges.
Aragonite has the same chemical composition as calcite, but it
crystallizes in the orthorhombic system, either in simple forms like A
on Plate 46, or twinned, so as to make forms which seem hexagonal. When
in simple crystals its form easily distinguishes it from calcite and
dolomite, but when twinned it appears much like either of these two
minerals. From calcite it can then be distinguished by its greater
hardness and the fact that it has cleavage in one direction only, and
that imperfect. The cleavage is the only easy method of distinguishing
it from dolomite. However, aragonite is most always easily distinguished
by its habits, for it generally forms long slender crystals, which
appear more like fibers than crystals. Neither calcite nor dolomite is
at all fibrous.
Aragonite is much less abundant than calcite, and has resulted, either
from deposition from hot waters, or from waters having sulphates in
solution as well as lime. Much of the travertine, and many stalagmites
and stalactites are composed of aragonites, forming as outlined under
calcite. The mother-of-pearl layer in the shells of bivalves is
generally aragonite. The pearly luster of this layer is due to its being
formed by the successive deposition of one thin layer upon another; so
that light falling on the mother-of-pearl, penetrates, part of it to one
layer and part to another, and is then reflected. Certain molluscs have
this layer composed of especially thin layers, one, the _Unios_ or
freshwater clams, the other, the “pearl oysters” or _Aviculidæ_, these
latter, however, being only distantly related to the edible oysters. In
the cases, where molluscs of either of these two families are of large
size, large pieces of mother-of-pearl can be recovered, and are used for
buttons, handles, and various ornamental objects. A further peculiarity
of these same molluscs is the formation of pearls in the sheet of flesh,
lining the shells. The pearls are round or rounded concretions of
aragonite. At the center there is a grain of sand, or more often a tiny
dead parasite. Either was an irritant to the mollusc, and to be rid of
it, a layer of aragonite was secreted around it. Then as the mollusc
continued to grow and secrete layers for its shell, it also added each
time another layer around the sand-grain or parasite, until in time a
pearl of noticeable, and then of considerable size resulted. These have
all the pearly luster of the mother-of-pearl in a sphere which tends to
make the luster even more marked.
Pearls were in use as ornaments in China some twenty-three centuries
before Christ, and in India over 500 B.C. They were very highly prized
by the Romans and since their times the rulers of India have shown a
remarkable fondness for them. Today the finest come from the Gulf of
Persia and the Red Sea, while still others are found about Australia and
in the Caribbean Sea. In the United States not a few are collected every
year from the fresh water clams, some of them beautifully tinted with
pink or yellow.
Aragonite is found widely, as at Haddam, Conn., Edenville, N. Y.,
Hoboken, N. J., New Garden, Penn., Warsaw, Ill., etc.
Anhydrite
CaSO₄
Pl. 46
Occurs in cleavable or granular masses, rarely in crystals; hardness,
3-3.5; specific gravity, 2.9; color white, gray, bluish or reddish;
luster pearly on cleavage faces; transparent on thin edges.
When anhydrite occurs in crystals, they are orthorhombic, like the
diagram on Plate 46. Usually, however, it is found in beds or layers,
which were deposited by the evaporation of sea water, and so it is
associated with salt. Anhydrite has three cleavage planes which are at
right angles to one another, which produce rectangular or cube-like
forms. Mostly anhydrite is associated with gypsum, from which it differs
by its greater hardness, pseudo-cubic cleavage, and the fact that
anhydrite is not readily soluble in acid, while gypsum is. Chemically it
differs from gypsum in not having water of crystallization, which gypsum
does have. The anhydrite is likely to occur as veins and irregular
masses in beds of gypsum. Calcium sulphate is precipitated from sea
water when 37% of the water has been evaporated, and it may be deposited
either as anhydrite or as gypsum, the factors, which decide as to which
of these two minerals it will be, being as yet unknown. After
deposition, if exposed to moisture, the anhydrite may change to gypsum,
irregular masses often remaining unchanged.
It is found in salt mines in Elsworth Co., Kan., in limestone cavities
at Lockport, N. Y., in veins in Shasta Co., Calif., etc.
Gypsum
CaSO₄ + 2H₂O
Pl. 47
Occurs in crystals, in cleavable masses, or in fibrous masses; hardness,
2; specific gravity, 2.3; colorless, white, amber, gray, or pink; luster
vitreous, silky or pearly; transparent on thin edges.
Gypsum crystals are monoclinic as seen on Plate 47, the perfect ones
usually occurring in clay, as at Oxford, O., or in cavities; while
crystals of less perfect outline, but with fine cleavages, are found in
Utah, Kansas, and Colorado. The cleavage is very perfect in one
direction, making it possible to strip off thin sheets almost like mica,
and less perfect in two other directions, which appear on the smooth
surface of the first cleavage as lines intersecting at 66° 14′. Twinning
is also common in such a way, that the two united crystals make forms
similar to arrowheads. These cleavages and the twinning show nicely in
the photograph of gypsum on Plate 47.
Gypsum is distinguished from anhydrite by its lesser hardness, its
cleavage and by being soluble in acids.
Most gypsum occurs in beds or granular masses which were deposited from
evaporating sea-water, coming down when 37% of the water was lost. Such
beds are often very extensive and are quarried as a source of gypsum to
make plaster of Paris, stucco, neat plaster, Keene’s cement, plaster and
wall board, partition tiles, etc. The use of the gypsum for plaster of
Paris and all these other uses is based on its affinity for water of
crystallization. The gypsum is first heated to about 400° C., which
drives off the water of crystallization, and causes it to crumble to a
powder, which is the plaster of Paris. When water is added, it is taken
up and the powder “sets,” or recrystallizes back to gypsum. This simple
reaction has made it very useful, for making moulds, casts, hard finish
on walls, as stucco, etc.
When the granular type of gypsum is fine grained, it is known as
_alabaster_, which is used for carving vases, statuettes, ornaments,
etc. The fibrous variety is called _satin spar_, and is sometimes used
for cheap jewelry and ornaments, but it is very soft and quickly wears
out. At Niagara Falls there is a considerable trade in objects carved
from this satin spar, tourists buying them on the assumption that the
mineral is native and comes from under the falls. Most of it, however,
comes from Wales, the small amount of gypsum of that region being mostly
granular.
Gypsum is found all across the United States, as in New York, Michigan,
Virginia, Ohio, Alabama, South Dakota, Wyoming, Colorado, Utah,
California, etc.
The Strontium Group
Strontium is a pale-yellow metal, ductile and malleable, but oxidizing
quickly when exposed to the air. It does not occur in its native state
in Nature, but always as some compound, usually either the carbonate or
sulphate. It resembles barium, but differs in giving to the flame a
brilliant red color, on which account the compounds of strontium are
used mostly in making red fire in fireworks.
Strontianite
SrCO₃
Occurs in needle-like crystals, or in columnar or fibrous masses;
hardness, 3.5-4; specific gravity, 3.6; color white, pale-green or pale
shades of yellow; luster vitreous; transparent on thin edges.
Strontianite is orthorhombic, but appears as if hexagonal, since its
general habit is to have three twin crystals grow together in such a way
as to make a six-sided double pyramid. In this it is very like
witherite, both these minerals appearing externally much alike. They can
be readily distinguished, however, by holding a piece in the flame. If
it gives a red color to the flame it is strontianite, if green, it is
witherite. It effervesces readily in hydrochloric acid.
Strontianite is found in veins and cavities in limestone, where it has
been deposited after being leached from the limestone by percolating
waters. Though known at several localities it is not now being mined in
this country, what we use being imported mostly from Germany.
It is found at Schoharie, Chaumont Bay and Theresa, N. Y., in Mifflin
Co., Penn., etc.
Celestite
SrSO₄
Occurs in crystals, cleavable masses and fibrous; hardness, 3; specific
gravity, 3.9; colorless, white, pale-blue, or reddish; luster vitreous;
transparent on thin edges.
Celestite, the sulphate of strontium, is very like barite in external
appearance and habit. It is orthorhombic and occurs in tabular crystals.
Its cleavage is perfect on the basal plane, and imperfect in one other
direction. The ready way of distinguishing celestite from barite is to
hold a piece in the flame. If it is celestite it will color the flame
red, if barite, green.
Celestite is mostly found in veins or cavities in limestone, where it
has been deposited by percolating waters, after having been leached from
the limestone. Some years ago an important deposit of celestite was
found on Strontian Island in Lake Erie, but that was soon worked out and
now no veins are being worked in this country. It is also found at
Chaumont Bay, Schoharie and Lockport, N. Y., in Kansas, Texas, West
Virginia, Tennessee, etc.
The Barium Group
Barium is another metal which does not occur in its native state in
Nature. It has only been isolated as a yellow powder, which, exposed to
air or water, soon changes to one of the oxides. Both barium and its
compounds are peculiar in causing a green color, whenever exposed to the
flame. Two of its compounds are fairly abundant, the sulphate, barite,
and the carbonate, witherite. The former is the more abundant and has
come to be fairly widely used, something over 100,000 tons being
annually consumed in the United States, to make the body in flat finish
paints for interior work and light colors, for a filler in rubber goods,
linoleum, oil cloth, glazed paper, and for a wide range of chemical
compounds.
Barite
BaSO₄
Pl. 48
_heavy spar_
Occurs in crystals or in lamellar, nodular or granular masses; hardness
3; specific gravity, 4.5; colorless, white or almost any color; luster
vitreous; transparent on thin edges.
Barite occurs in orthorhombic crystals, which are tabular in form, and
usually have the edges beveled, as in figure A, Plate 48. There is
cleavage in three directions, a rather perfect basal cleavage, and two
less perfect cleavages, which are at right angles to the basal cleavage
plane, and intersect each other at 78°.
The tabular form, the cleavage, the heavy weight, and the fact that a
piece of barite put into the flame colors it green, all serve to
distinguish this mineral.
Barite is a secondary mineral of aqueous origin, which has been
deposited in veins and cavities in igneous, metamorphic, or sometimes
sedimentary rocks. It is most likely to occur in veins in igneous or
metamorphic rocks, the barium having been dissolved from certain
feldspars and micas by percolating water, and then redeposited in the
fissures, as the water came into them. If in sedimentary rocks, the
barite veins are usually in limestones. Barite is quite likely to be a
gangue mineral for the ores of lead.
It is found at Hatfield and Leverett, Mass., Cheshire, Conn., Pillar
Point, N. Y., Cartersville, Ga., in Virginia, North Carolina, South
Carolina, Missouri, Kentucky, Tennessee, Alabama, Illinois, Wisconsin,
Nevada, California, Alaska, etc.
Witherite
BaCO₃
Pl. 48
Occurs in crystals, or in granular or columnar masses; hardness, 3.5;
specific gravity, 4.3; color white to gray; luster vitreous; translucent
on thin edges.
Witherite is not an abundant mineral. Its crystals are really
orthorhombic, but they are usually twinned, three crystals growing
through each other in such a manner that the resulting crystal appears
like a six-sided double pyramid, similar to the one figured on Plate 48.
The commonest mode of occurrence is in compact masses. Witherite
effervesces when cold acid is dropped upon it, which, with its heavy
weight, and the green color it gives to the flame, serves to distinguish
the mineral. It is used for medicines, in chemical industries, and a
considerable amount is made into rat poisons. The chief locality for
witherite is in northern England, but in this country it is found along
with barite, especially at Lexington, Ky., and in Michigan.
Carbon
Carbon is an element widely distributed in nature, occasionally
appearing in its elementary form, as graphite or the diamond, but much
more important in its compounds. Small quantities are present in the air
as carbon dioxide, CO₂, immense quantities occurring in the carbonate
minerals, which have been considered under their respective metallic
salts, as calcite, malachite, siderite, cerrusite, smithsonite,
witherite, etc., and still other large quantities being represented in
organic compounds, which occur as rocks under the heads of petroleum,
coal, etc. The occurrence of limestones, graphite, coal or petroleum is
always indicative of the activity of living organisms, and in some cases
is the only indication of life in the earlier rocks.
Graphite
C
_Plumbago_
Occurs in hexagonal scales or flakes, in layered masses, or earthy
lumps; hardness, 1; specific gravity, 2.1; color black or steel-gray;
streak gray; luster metallic; opaque on thin edges.
Like the diamond graphite is pure carbon, but in this case it is in
non-crystalline form. It occurs in both igneous and metamorphic rocks.
In the former case it is either in flakes in the rock, or in veins, and
has been derived directly from the molten magmas, having either
precipitated in the hardening granite or lava, or having been carried
into the fissures and there precipitated to make the veins of graphite.
In either case the graphite probably represents organic deposits which
have been melted into the igneous magma at the time of its formation.
Graphite may also occur in metamorphic rocks, beds of coal or other
organic deposits being altered by the heat. Such beds are often of
considerable extent and economic importance.
The extreme softness, greasy feel, and the dark-gray streak readily
distinguish graphite.
It is widely used in making crucibles and furnace linings for foundries,
lead pencils, paint, lubricating powders, etc.
Graphite is found at Brandon, Vt., Sturbridge, Mass., Ashford, Conn., in
Essex, Warren and Washington Cos., N. Y., Clay, Chilton and Coosa Cos.,
Ala., Raton, N. M., Dillon, Mont., etc.
Diamond
C
Occurs in octahedral crystals; hardness, 10; specific gravity, 3.5;
colorless to yellow, brown, blue, etc., luster adamantine; transparent
on thin edges.
Like graphite the diamond is pure carbon, but in this case in crystal
form. It is the hardest of all minerals, and as brilliant as any; so
that in spite of being by no means the rarest, it may easily be
considered the most popular of all gems. Tiny diamonds have been made
artificially under great heat and pressure; so that this mineral is
thought of as forming in Nature in dark igneous lavas at great depths.
The diamond has good cleavage parallel to the octahedron faces, and, in
spite of some traditions to the contrary, is brittle.
There are not many diamond localities, the most famous being the
Kimberley district of South Africa, which produces many times as many
diamonds as all the others put together, though all the time some are
being found in Borneo and Brazil. A very few have been found in the
United States, only one locality however yielding them in the original
matrix. That is at Murfreesboro, Ark., where they are mined in a
disintegrating peridotite (a dark lava, mostly peridot), which has been
extruded through the sedimentary rocks of that region. This matrix is
similar to the “blue earth,” the matrix of the diamonds of South Africa,
which occurs in “pipes,” representing the necks of ancient volcanoes.
The American diamonds are of small size, averaging considerably less
than a third of a carat in weight, which does not allow great value to
the individual diamonds.
From time to time, especially large diamonds have been found in
different parts of the world, the largest being the Cullinan diamond,
found at the Premier Diamond Mine of South Africa. It weighed 3025
carats or about a pound and a quarter, and was valued at over
$3,000,000. It was presented to King Edward VII, who had it cut into 11
brilliants, four of which are larger than any other diamond yet found.
Other famous diamonds, like the Kohinoor, 106 carats, found in India in
1304; the Regent, 136 carats, also found in India; the Orloff, 193
carats, set in the eye of an Indian idol; the South Star, 125 carats,
the largest ever found in Brazil; the blue Hope, etc., have in many
cases romantic and interesting stories woven about them.
Though for ages diamonds have been highly prized gems, it is only in
comparatively recent times that cutting and polishing have been resorted
to, for the purpose of enhancing their brilliancy. This is done by
grinding reflecting faces on the original stone, by the aid of discs of
iron or tin in which diamond dust has been embedded. Diamond chips and
cloudy or imperfect diamonds are used for making tools for cutting
glass, rock drills, etc.
Phosphorus
The element phosphorus at ordinary temperatures is an almost colorless,
faintly yellow, solid substance of glistening appearance and waxy
consistency. In Nature it does not occur pure, but always as one of its
compounds. It is of great importance to man for it is one of the
essentials for plant growth and also for the higher animals, being
required for the bones and to some extent for nervous tissue. Originally
it is found in all the igneous rocks. Some of the phosphorus is removed
by solution and carried to other regions and to the sea. From this
distribution it comes into the sedimentary rocks, and, when they are
altered by heat, into the metamorphic rocks. Thus it has a wide, though
by no means even, distribution. The soils formed by disintegration of
these rocks probably all have some phosphorus in them; but where there
is vigorous plant growth, it soon tends to become exhausted, and must be
renewed. For this reason the use of phosphates has become of prime
importance in Agriculture. The possession of beds of rock carrying
phosphorus has come to be of international importance. The United States
is particularly fortunate in this respect, and produces over 25% of the
world’s supply of phosphates. Most all the phosphorus is recovered
either from phosphate minerals, the most important of which is apatite,
or from the non-crystalline and impure mixtures of phosphate minerals
and other substances, discussed under phosphate rock.
Apatite
Ca₅F(PO₄)₃
Pl. 49
Occurs in crystals, concretionary nodules, or in bedded masses;
hardness, 5; specific gravity, 3.2; color reddish-brown or green, rarely
white or colorless; luster vitreous; translucent on thin edges.
Apatite occurs in hexagonal prisms, usually with the ends truncated by a
basal plane, and with one or more sets of pyramidal faces between the
prism and the basal plane. Crystals range in size from tiny to over a
foot in diameter. There is but one cleavage and that is basal. The
crystal form, cleavage, and hardness will easily determine this mineral.
Apatite is usually associated with igneous or highly metamorphic rocks,
such as granites, gneisses, and crystalline limestones. While the
phosphoric acid of apatite is highly desirable for use in fertilizers,
the crystals do not occur in sufficient abundance to make them
commercially available, and non-crystalline phosphate rocks are resorted
to for this purpose.
Crystals of apatite are found at Norwich and Bolton, Mass., Rossie and
Edenville, N. Y., Suckasunny and Hurdstown, N. J., Leiperville, Penn.,
Wilmington, Del., etc. Templeton, Canada, is perhaps the best known
locality for fine apatite.
Turquois
H₅[Al(OH)₂]Cu(OH)(PO₄)₄
Occurs in seams and incrustations; hardness, 6; specific gravity, 2.7;
color bluish-green; streak blue; luster waxy; translucent to opaque on
thin edges.
In this country this complex phosphate of aluminum and copper is found
in streaks and patches in volcanic rocks, but in Persia comes from
metamorphic rocks. To the Persians it was a magical stone, protecting
the wearer from injuries, and among the Pueblo Indians it was regarded
as of religious value in warding off evil. The best turquois comes from
Persia, but it has been found at several points in the United States, as
in Los Cerrillos and Burro Mts., N. M., in Mohave Co., Ariz., San
Bernardino Co., Cal., in Nevada and Colorado.
Fluorine
At ordinary temperatures the element fluorine is a colorless gas, which
was not obtained pure until 1888, because it could not be contained in
vessels of glass, gold, platinum, etc. At that time it was made and kept
in a vessel composed of an alloy of platinum and iridium. Its most
important compound is hydrofluoric acid, a fuming liquid, which is
mostly used to etch or dissolve glass. It occurs in several minerals,
like tourmaline, turquois, etc., but the only one used to obtain the
hydrofluoric acid is fluorite.
Fluorite
CaF₂
Pl. 50
_Fluor spar_
Occurs in crystals and cleavable masses; hardness, 4; specific gravity,
3.2; colorless or some shade of violet, green, yellow, or rose; luster
vitreous; transparent on thin edges.
Fluorite usually occurs in beautiful cubic crystals, often with the
edges and corners beveled by smaller faces, and occasionally in twins,
which seem to have grown through each other. There is perfect cleavage
parallel to each of the octahedral faces, which often, as in the
illustration on Plate 50, show as cracks cutting off the corners.
Since fluorite loses weight and color on heating, it is concluded that
the colors are due to the presence of hydrocarbon compounds. The red and
the green fluorite when heated to above 212° F. become phosphorescent,
as may be seen if they are thus heated and exposed to the light, then
taken into the dark.
Fluorite is quite commonly the gangue mineral associated with metallic
ores, and is also likely to occur with topaz, apatite, etc. It is
generally in such places that it seems to have been deposited from hot
vapors, rising from igneous magmas.
It is the only mineral at all common from which fluorine can be
obtained, and is used for making hydrofluoric acid, and other chemical
compounds of this element. It is, however, of much greater importance as
a flux in reducing iron, silver, lead and copper ores. In the industries
it finds a place, being used to make apochromatic lenses, cheap jewelry,
and for the electrodes in flaming arc lamps.
Fluorite is widely distributed, some of the better known localities
being Trumbull and Plymouth, Conn., Rossie and Muscalonge Lake, N. Y.,
Gallatin Co., Ill., Thunder Bay, Lake Superior, Missouri, etc.
Halite
NaCl
Pl. 50
_Salt_
Occurs in crystals, and in cleavable and granular masses; hardness, 2.5;
specific gravity, 2.1; colorless to white; luster vitreous; transparent
on thin edges.
Halite is common salt, occurring in cubic crystals, with perfect cubic
cleavage. Its form, hardness, taste, and solubility in water make it
easy to determine.
Halite is the most abundant salt in sea water, making about 2.5% out of
the total of 3.5% of solids in solution. It is also a prominent, when
not the leading, salt in solution in the waters of inland lakes, like
Great Salt Lake, or the Dead Sea, there being 20% of halite in the
former and 8% in the latter, though the total of solid in solution in
the water of the Dead Sea is greater than that in Great Salt Lake.
The great salt deposits are mostly the result of the evaporation of the
water of arms or isolated portions of former oceans; the salt, gypsum,
etc., left by the drying sea, having been buried beneath later
sediments. Other bodies of salt represent the disappearance of ancient
lakes. There are also the curious “salt domes” of Louisiana and Texas,
which are immense, roughly circular, subterranean masses of salt
extending to as yet unknown depths which are thought to have been formed
by masses of salt from some deep source bed pushing their way upward
through the overlying formations by plastic flowage. As the upthrust
took place the sediments were arched into domes. Some of these domes are
today important sources of rock salt.
There are extensive beds of salt under parts of New York, Michigan,
Ohio, Oklahoma, Kansas, etc., which are mostly worked by drilling wells
into the salt layer, then introducing hot water to dissolve the salt.
The brine thus formed is pumped to the surface, and the salt recovered
by evaporation in pans. During the process, skeleton crystals of salt
with concave faces may form, but in Nature the crystals are uniformly
solid cubes.
Boracite
Mg₇Cl₂B₁₆O₃₀
Occurs in small crystals or granular masses; hardness of crystals, 7; of
the masses, 4.5; specific gravity 3; colorless to white; luster
vitreous; transparent to translucent on thin edges.
Small crystals, associated with salt and gypsum, occur in the beds and
incrustations, which result from the drying up of alkaline lakes,
especially in Nevada and southern California. The crystals are
orthorhombic, but appear like perfect cubes, with the edges beveled and
part of the corners cut. They are not easily dissolved in water, but
quickly go into solution in hydrochloric acid.
Colemanite
Ca₂B₆O₁₁ + 5H₂O
Occurs in crystals or compact masses; hardness, 4.5; specific gravity,
2.4; colorless to white; luster vitreous; translucent on thin edges.
The crystals when they occur, are monoclinic; but usually colemanite is
a bedded deposit, which has resulted from the drying up of a saline
lake. It was first found in Death Valley, Cal., in 1882, then near
Daggett, Cal., and since then in several similar locations in Nevada and
Oregon. The deposits are of all grades of purity, the colemanite being
mixed with varying quantities of mud. Today this mineral is the chief
source of borax, which is used in medicines, cosmetics, colored glazes,
enamel, and as a preservative.
Borax
NaB₄O₇ + 10H₂O
Occurs in crystals or in powdery incrustations; hardness, 2; specific
gravity, 1.7; colorless to white; luster vitreous; translucent on thin
edges.
The crystals are tiny and monoclinic, this mineral being usually
obtained by the evaporation of the saline waters of such lakes as Clear
and Borax Lakes in southern California, or from the muds of salt
marshes, like Searles Borax Marsh in California. Originally most of our
borax came from a large saline lake in Tibet, but now most of it is
obtained from colemanite. Borax is soluble in water, giving it a
sweetish taste.
Sulphur
S
Pl. 51
Occurs in crystals, incrustations or compact masses; hardness, 2;
specific gravity, 2; color yellow; streak yellow; luster resinous;
translucent on thin edges.
Aside from the numerous compounds, such as the sulphides of the metals
like pyrite, galena, sphalerite, etc., and the sulphates, like gypsum,
barite, anglesite, etc., sulphur occurs in its elemental form in Nature.
In this case it may be in crystals, which are orthorhombic and usually
occur as octahedrons, with the upper and lower ends truncated, either by
a basal plane, or by a lower octahedron, or by both. Incrustations and
compact masses are, however, much the commoner mode of occurrence. The
incrustations are found mostly about volcanic regions, where the sulphur
has risen from the molten lavas as a sublimate, and on cooling has been
deposited in crevices or on the adjacent surfaces. Irregular masses of
sulphur are often found where sulphide minerals, like pyrite or galena
have been decomposed in such a way as to leave the sulphur behind. The
extensive beds of sulphur are usually associated with gypsum, and are
thought to be the result of water, containing bituminous matter, so
acting on gypsum as to remove the calcium and oxygen as lime, and leave
the sulphur. Finally many waters carry sulphates in solution, from which
the sulphur may be precipitated by certain sulphur bacteria, making thus
incrustations on the bottom of ponds or lakes.
Sulphur is used for making matches, gunpowder, fireworks, insecticides,
in medicine, vulcanizing rubber, etc. It is widely distributed, however,
most of the present world’s production is from deposits associated with
the “salt domes” of Texas and Louisiana. A “caprock” of gypsum and
anhydrite overlies many of these which often contains elemental sulphur.
Wells are drilled into this, and the sulphur is melted by the
introduction of hot steam. This melted sulphur is then pumped to the
surface and run into molds.
Some of the best known localities are Sulphurdale, Utah, Cody and
Thermopolis, Wyo., Santa Barbara Co., Cal., Humboldt Co., Nev., and
about the hot springs of the Yellowstone Park.
Ice
H₂O
Pl. 51
_water_
Occurs solid as ice, snow and frost, or liquid as water; hardness, 2;
specific gravity, .92; colorless to white; luster adamantine;
transparent on thin edges.
Though we seldom think of ice, and its liquid form, water, as a mineral,
still it is one, and perhaps the most important of all minerals, as well
as the most common. Ice melts at 32° F. and vaporizes at 212° F., being
then termed steam. Because it is so common and liquid at ordinary
temperatures it acts as a solvent for a host of other minerals, and is
therefore the agent by which they are transported from place to place
and redeposited in veins and beds.
Not only does water act as a transportation agent for minerals in
solution, but is also the agent of erosion and weathering. Water
vaporizes slowly when exposed to the air at all temperatures above
freezing, and so it is slowly rising from the surface of the sea or
lakes or moist ground into the air, where it would accumulate until the
air was saturated, if the air would only keep still and at a uniform
temperature. The air will hold a given amount of water vapor, which is,
for example, 17 grams per cubic meter when the temperature is 68° F.,
but at 59° F. it will hold only 12½ grams, or at 50° F. only 9 grams.
Thus the air is more or less completely saturated at higher
temperatures, and when the temperature is lowered the air can not hold
all it has taken up, and it is precipitated in dew, rain or snow, most
often as rain. When the rain falls it mechanically carries away, and
more or less slowly transports to other places particles of rock, being
thus the agent of erosion; and when it is slowed down, as on entering
the quiet water of a lake or the sea, it drops the mechanically carried
sediment and makes sedimentary deposits.
Another very important and unique feature of water is that on freezing
it expands about ¹/₁₁th of its former bulk, so that, as a result, ice
floats, and also wherever water in crevices is frozen, the crevices are
enlarged. In locations where this freezing and melting take place
repeatedly throughout a year, there the breaking up of rocks is rapid.
This is hardly the place to take up a complete discussion of water, but
its action as a solvent, mechanically, and in freezing, melting, and
vaporizing is the basis of a large part of the study of geology.
When water crystallizes, as in forming ice, it is in the hexagonal
system. It tends to twinning and a snow-flake is made up of a large
number of twinned crystals, each diverging from the other at 60°. When
ice is formed in the air or on the surface of water it forms these
complex and beautiful multiple twins, of which but a couple are
suggested here. Beneath the surface the hexagonal crystals grow downward
into the water, parallel to each other, making a fibrous structure,
which is very apparent when ice is “rotten,” which is the time at which
the surfaces of the prisms are separating, because the molecules leave
the crystal in the reverse order to which they united with it. Frost in
marshy or spongy ground will often show this fibrous growth beautifully.