THE MINERALS
KEY TO THE MINERALS, BASED ON HARDNESS, COLOR, ETC.
OPAQUE COLORS
Color Hardness Streak Remarks Mineral
Red
scarlet 2.5 scarlet surface tarnishes prousite
black
2.5 vermilion surface scarlet to cinnabar
dark red
ochre 7 white non-crystalline jasper
6 ochre red color red to hematite
almost black
rose 4 white effervesces in rhodochrosite
warm acid
dark 4 orange zincite
2.5 purplish red surface tarnishes pyrargyrite
black
brownish 3.5 brownish red cuprite
Orange 3.5 white to pyromorphite
yellowish
1-1½ orange realgar
Blue 5.5-6 white in igneous rocks sodalite
azure 4 azure azurite
sky 7 & 4.5 white blade-like crystals cyanite
turquoise 6 blue non-crystalline turquois
2-4 white chrysocolla
Green
malachite 3.5 lighter green malachite
olive 6.5-7 white in igneous rocks olivine
3.5 white to yellow pyromorphite
2 white mica-like cleavage chlorite
1 white greasy feel, color talc
light to dark
olive green
yellowish 6.5 white epidote
2.5-4 white color yellow green serpentine
to olive
Yellow
golden 2.5 shining non-crystalline gold
brassy 6 greenish-black usually crystalline pyrite
6 greenish-gray color pale brassy marcasite
yellow, usually
non-crystalline
5.5 greenish-black colors nitric acid millerite
green
4 greenish-black color golden chalcopyrite
similar to gold
3.5 dark brown purplish tarnish tetrahedrite
on surface
bronze 5.5 pale color with coppery niccolite
brownish-black cast
4 dark gray-black with speedy black pyrrhotite
tarnish
3 gray-black brownish with bornite
bluish tarnish
2.5 shining coppery red color copper
sulphur 3.5 white to compact masses pyromorphite
yellowish
2 yellow sulphur
1-3 earthy masses carnotite
Brown
violet 1½ shining tarnishes black cerargyrite
yellowish 7.5 white 4-sided prisms zircon
6.5 gray cassiterite
5.5 ochre yellow compact to earthy limonite
masses
5 brownish-yellow goethite
4.5 black wolframite
3.5 yellowish-brown sphalerite
3.5 white siderite
grayish 7.5 white often twinned staurolite
6.5 pale brown rutile
3.5 white to earthy masses pyromorphite
yellowish
reddish 7 white dodecahedrons & garnet
trapezohedrons
Black 6.5 gray cassiterite
6 reddish-brown franklinite
6 black magnetic magnetite
5.5 dark brown chromite
5.5 black yellow precipitate wolframite
in sulphuric acid
5-6 black non-magnetic ilmenite
5-6 brownish-black compact masses psilomelane
5 brownish-yellow surface often goethite
brownish
3.5 dark brown tetrahedrons tetrahedrite
2.5 silvery fresh surfaces silver
silver color
2.5 scarlet fresh surfaces prousite
bright red
2.5 purplish red fresh surfaces red pyrargyrite
2 black earthy masses pyrolusite
1 steel gray greasy feel graphite
Metallic 2.5 black tarnishes black, chalcocite
Gray bluish, or green
2.5 lead gray sectile argentite
2.5 lead gray cubic cleavage galena
2 lead gray long prismatic stibnite
crystals
1.5 bluish gray in scales molybdenite
steel 5.5 gray black rose color in smaltite
nitric acid
4.5 steel gray very heavy platinum
4 reddish black often in striated manganite
prisms
1 gray with greasy feel graphite
silvery 5.5 black arsenopyrite
2.5 silvery tarnishes black on silver
exposure
reddish 5.5 gray black rose color in cobaltite
nitric acid
pearly 1-1½ shining exposed surfaces cerargyrite
violet brown
White, with 4 white porcelainous magnesite
impurities masses,
effervesces in acid
grayish 2 white earthy masses, kaolinite
or greasy feel
yellowish
1-3 white earthy masses bauxite
1 white greasy feel, talc
fibrous or scaly
TRANSPARENT OR TRANSLUCENT COLORS
Color Hardness Remarks Mineral
Colorless or with faint tinges of color due to impurities
10 in octahedrons diamond
9 in hexagonal prisms corundum
8 in hexagonal prisms topaz
7 in three-sided prisms tourmaline
7 in hexagonal prisms quartz
7 non-crystalline chalcedony
7 or 4.5 cubes with beveled edges boracite
6 non-crystalline, pearly luster opal
5.5 rhombohedrons willemite
5.5 trapezohedrons analcite
5.5 tufts of needle-like crystals natrolite
5.5 sheaf-like bundles of crystals stilbite
5 hexagonal prisms with basal cleavage apatite
5 effervesces in acid smithsonite
5 becomes jelly-like in acid calamine
4.5 monoclinic prisms colemanite
4 in cubes fluorite
3.5 effervesces in acid, but one cleavage aragonite
3.5 effervesces in acid, heavy cerrusite
3 effervesces in acid, rhomboidal calcite
cleavage
3 no effervescence, but soluble in anglesite
nitric acid
2.5 in cubes tastes of salt halite
2 soluble in water, sweetish taste borax
2 1 perfect cleavage, and two imperfect gypsum
cleaves at 66 with each other
White or with faint tinges of color due to impurities, such as pink,
bluish, etc.
7 hexagonal prisms quartz
7 non-crystalline chalcedony
7 or 4.5 cubes with beveled edges boracite
6 non-crystalline, pearly luster opal
6 cleavage in 3 directions, good in 2 feldspar
and imperfect in the other
5.5 short eight-sided prisms pyroxene
5.5 long six-sided prisms amphibole
5.5 trapezohedrons analcite
5.5 tufts of needle-like crystals natrolite
5.5 sheaf-like bundles of crystals stilbite
5.5 rhombohedrons willemite
5 effervesces in acid smithsonite
5 becomes jelly-like in acid calamine
4.5 & 7 cubes with beveled edges boracite
4.5 monoclinic prisms colemanite
4 effervesces in acid, porcelainous magnesite
3.5-4 effervesces in acid, heavy, red color strontianite
in flame
3.5 effervesces in acid, heavy, green witherite
color in flame
3.5 effervesces in warm acid, rhomboidal dolomite
cleavage
3.5 effervesces in acid, cleavage in one aragonite
direction only
3.5 effervesces in acid, heavy, does not cerrusite
color flame
3-3.5 no effervescence, cleavage in three anhydrite
directions at right angles
3 effervesces in acid, rhomboidal calcite
cleavage
3 tabular crystals, heavy, green color barite
in flame
2-3 cleaves in thin elastic sheets mica
2.5 cleaves in cubes cryolite
2.5 cubes, soluble in water, salty taste halite
2 1 perfect cleavage, and 2 less perfect gypsum
ones
2 cleaves in thin non-elastic sheets chlorite
2 soluble in water, tastes sweet borax
1 greasy feel talc
Green 9 hexagonal prisms oriental
emerald
8 octahedrons spinel
7.5 hexagonal prisms beryl
7 three-sided prisms tourmaline
7 dodecahedrons or trapezohedrons garnet
7 non-crystalline prase or
plasma
6.5-7 non-crystalline, olive color olivine
6.5 yellow green color, rather opaque epidote
6 non-crystalline, pearly luster opal
5.5 short eight-sided prisms pyroxene
5.5 long six-sided prisms amphibole
5 hexagonal prisms apatite
4 cubes fluorite
3.5 effervesces in acid cerrusite
2.5-4 somewhat greasy feel, massive or serpentine
fibrous
2 in mica-like scales, non-elastic chlorite
1 greasy feel, fibrous or scaly talc
Red 9 hexagonal prisms ruby
8 octahedrons spinel
7 three-sided prisms tourmaline
7 dodecahedrons or trapezohedrons garnet
7 hexagonal rose quartz
7 non-crystalline jasper or
carnelian
6 pearly luster fire opal
4 cubes, rose tints fluorite
2-3 pink mica-like scales lepidolite
Blue 9 hexagonal prisms sapphire
7 & 4.5 blade-like crystals cyanite
6 non-crystalline masses turquois
5.5-6 in igneous rocks sodalite
4 azure color azurite
3.5 effervesces in acid, heavy cerrusite
2-4 earthy masses, turquoise color chrysocolla
Violet 7 hexagonal prisms amethyst
4 cubes fluorite
Yellow 9 hexagonal prisms oriental
topaz
8 octahedrons spinel
8 hexagonal prisms topaz
4 cubes fluorite
Brown 9 hexagonal prisms corundum
8 octahedrons spinel
7.5 four-sided prisms zircon
7 hexagonal prisms smoky quartz
7 three-sided prisms tourmaline
7 non-crystalline flint
6 non-crystalline opal
5.5 short eight-sided prisms pyroxene
5.5 long six-sided prisms amphibole
2-3 cleaves into thin sheets mica
Black 9 hexagonal prisms corundum
8 octahedrons spinel
7 three-sided prisms tourmaline
5.5 short eight-sided prisms pyroxene
5.5 long six-sided prisms amphibole
2-3 cleaves in thin sheets mica
The Gold Group
Gold was undoubtedly the first metal to be used by primitive man; for,
occurring as it did in the stream beds, its bright color quickly
attracted the eye, and it was so soft, that it was easily worked into
various shapes, which, because they did not tarnish, became permanent
ornaments. The metal is associated with the very earliest civilizations,
being found in such ancient tombs as those at Kertsch in Crimea and in
northern Africa and Asia Minor. It was used in the cloisonné work of
Egypt 3000 years B.C. In America the Indians, especially to the south,
were using it long before the continent was discovered.
Of all the metals gold is the most malleable, and its ductility is
remarkable, for a piece of a grain’s weight (less than the size of a pin
head) can be drawn out into a wire 500 feet long; and it can be beaten
into a thin leaf as thin as ¹/₂₅₀₀₀₀ of an inch in thickness, and thus a
bit, weighing only a grain, can thus be spread over 56 square inches.
It forms very few compounds, but has a considerable tendency to make
alloys (_i.e._, mixtures with other metals without the resulting
compound losing its metallic character). In Nature gold is never
entirely pure, but is an alloy, usually with silver, there being from a
fraction of 1% up to 30% of the silver with the gold, the more silver in
the alloy, the paler the color of the gold. Australian gold is the
purest, having but about .3% of silver in it, while Californian gold has
around 10% and Hungarian gold runs as high as 30% of silver. Another
alloy fairly abundant in Nature is that with tellurium, such as
_calaverite_ (AuTe₂) which is a pale brassy yellow, similar to pyrite,
but with the hardness of but 2.5. Another combination includes gold,
silver and tellurium, _sylvanite_, (AuAgTe₄) a silvery white mineral
with a hardness of but 2. Such combinations are known as tellurides and
the calaverite is mined as a source of gold at Cripple Creek, Colo.,
while the sylvanite is one of the important ores of gold in South
Africa. Occasionally gold is also found alloyed with platinum, copper,
iron, etc. Jewelers make several alloys, “red gold” being 3 parts gold
and 1 of copper, “green gold” being the same proportions of gold and
silver, and “blue gold” being the combination of gold and iron. Our gold
coins are alloys, nine parts gold and one of copper, to give them
greater durability. Most of the gold recovered from nature is found
native, _i.e._, the pure metal, or with some alloy.
Gold
Au
Pl. 5
Usually non-crystalline, but occasionally showing cube or octahedral
faces of the isometric system; hardness 2.5; specific gravity 19.3;
color golden yellow; luster metallic; opaque.
Gold is mostly found as the metal and is readily recognized by its
color, considerable weight, hardness, malleability, and the fact that it
does not tarnish. It usually occurs in quartz veins in fine to thick
threads, scales or grains, and occasionally in larger masses termed
“nuggets.” It is insoluble in most liquids so that when weathered from
its original sites, it was often washed down into stream beds, to be
found later in the sands or gravels, or even in the sea beaches. When
thus found it is termed “placer gold,” and its recovery is placer
mining. Most of the original discoveries of gold have been in these
placer deposits; and from them it has been traced back to the ledges
from which it originally weathered. In the placer deposits the size of
the particles varies from fine “dust” up to large nuggets, the largest
found in California weighing 161 pounds; but the largest one found in
the world was the “Welcome Nugget,” found in Australia, and weighing 248
pounds. When gold was discovered in California in 1848, this became the
chief source for the world, but later this distinction went to
Australia, and now belongs to South Africa, which today yields over half
the annual supply.
The ultimate source of gold is from the lighter colored igneous rocks,
like granites, syenites, and diorites, throughout which it is diffused
in quantities too small to be either visible or worth while to extract.
It becomes available only when it has been dissolved out by percolating
waters and segregated in fissures or veins, either in or leading from
these igneous rocks. Generally this transfer of gold has taken place
when the rocks were at high temperatures, and by the aid of water (and
perhaps other solvents) which was also at high temperatures. The
presence of gold in sandstones, limestones, etc., is secondary, as is
also its presence in sea water, in which there is reported to be nearly
a grain (about five cents worth) in every ton of water. Beside the
direct recovery of gold from gold mining, a great deal is obtained from
its association with iron, copper, silver, lead and zinc sulphides, in
which it is included in particles too fine to be visible, but in
quantities large enough to be separated from the other metals after they
are smelted.
In the United States gold is found in the Cordilleran region from
California to Alaska, in Colorado, Nevada, Arizona, Utah, the Black
Hills of South Dakota, and in small quantities in the metamorphosed
slates of North and South Carolina, Georgia, and in Nova Scotia.
The Silver Group
Though much commoner than gold, silver did not attract the eye of man as
early, probably because it tarnishes when exposed to air or any other
agent having sulphur compounds in it, and a black film of silver
sulphide covers the surface. Its first use was for ornaments, and some
of these found in the ruins of ancient Troy indicate its use as early as
2500 B.C. A thousand years later it was being used to make basins, vases
and other vessels.
Silver is next to gold in malleability and ductility, so that a grain of
silver can be drawn out into a wire 400 feet long, or beaten into leaves
¹/₁₀₀₀₀₀ of an inch in thickness. As a conductor of electricity it is
unsurpassed, being rated at 100% while copper rates 93%. Silver is also
like gold in the freedom with which it alloys with other metals, such as
gold, copper, iron, platinum, etc. All our silver coins, tableware,
etc., have some copper alloyed with the silver to give it greater
hardness and durability.
Unlike gold, silver freely enters into compounds with the non-metals,
which is the reason that it is not found primarily in its native state,
but usually as a sulphide. Its ultimate source is in the igneous rocks,
few granites or lavas, on analysis, failing to show at least traces of
silver. Before it is available as an ore, or mineral, it has been
dissolved from the original magma, and segregated in fissures or veins,
along with such minerals, as quartz, fluorite, calcite, etc. This seems
to have taken place while the igneous rocks were still hot, and by the
agency of vapors and liquids which were also hot. The presence of silver
in sedimentary and metamorphic rocks, or even in sea water, is
secondary.
The primary deposition of silver is usually in the form of sulphides,
the commoner of which are, argentite or silver sulphide, pyrargyrite or
silver and antimony sulphide, and prousite, or silver and arsenic
sulphide. Its occurrence as native silver, or the chloride, cerargyrite,
is secondary and due to the reactions which have taken place when
sulphide deposits have been subjected to weathering agents.
The United States produces about 25% of the world’s supply, Mexico some
35%. It is especially found along the Cordilleran ranges of both North
and South America.
Silver
Ag
Pl. 6
Usually non-crystalline, but occasionally showing cube or octahedron
faces of the isometric system; hardness 2.5; specific gravity 10.5;
color silvery white; luster metallic; opaque.
When found in its native state silver is usually in wirey, flakey, or
mossy masses; but sometimes masses of considerable size occur, the most
famous being an 800 pound nugget found in Peru, and another of 500
pounds weight found at Konsberg, Norway, and now preserved in
Copenhagen. When exposed to the air the surface soon tarnishes and takes
on a black color which must be scraped off to see the real color.
Like gold, silver is usually found associated with other metals, like
iron, copper, lead and zinc; and much of the silver recovered is
obtained in connection with the mining, especially of copper and lead.
Some lead ores have so much silver in them that they are better worth
mining for the silver; galena, for instance, under such circumstances
being termed argentiferous galena. Native silver is a secondary mineral,
having been formed by the reduction of some one of its sulphides by
water, carrying various elements which had a greater affinity for the
sulphur.
Silver is found along with copper in the Lake Superior region, and in
Idaho, Nevada, and California.
Argentite
AgS
Pl. 6
_silver glance_
Usually in irregular masses, but sometimes in cubes; hardness 2.5;
specific gravity 7.3; color and streak lead gray; luster metallic;
opaque on thin edges.
Argentite, the simple sulphide of silver, is the chief source from which
silver is obtained. It looks like galena, and has the same hardness,
streak and specific gravity, but can be distinguished by the galena
having a very perfect cubic cleavage while the argentite has no
cleavage. Argentite is easily cut with a knife (sectile). It is usually
found in irregular masses, but sometimes in cubes which make very choice
cabinet specimens; and is associated with such other minerals as galena,
sphalerite, chalcopyrite, pyrite, fluorite, quartz, and calcite.
It occurs in fissures and veins all through the Cordilleran regions,
especially in California, Colorado, Nevada (Comstock Lode), Arizona
(Silver King Mine) and about the shores of Lake Superior.
Pyrargyrite
Ag₃SbS₃
Pl. 7
_ruby silver_ or _dark red silver_
Usually occurs in irregular masses; hardness 2.5; specific gravity 5.8;
color dark red to black; streak purplish red; luster metallic to
adamantine; translucent on thin edges.
Pyrargyrite, the sulphide of silver and antimony, is distinguished by
its dark red color and the purplish streak. It may look like prousite,
but is easily distinguished from the latter which has a scarlet streak.
It also at times looks like hematite and cinnabar, but the hematite has
a hardness of 6, and the latter has the bright red color throughout,
while pyrargyrite turns black when exposed to the light, so that the
characteristic red color will be seen only on fresh surfaces. The
characteristic red color can only be kept on the mineral if it is
constantly protected from the light.
Sometimes pyrargyrite occurs in crystals and these belong to the
hexagonal system, and are prisms with low faces on the ends, as on plate
7, and the mineral is peculiar in that the faces on the opposite ends
are unlike.
Pyrargyrite is found mostly in fissures and veins of quartz, fluorite,
calcite, etc., and associated with pyrite, chalcopyrite, galena, etc. It
is fairly common in Colorado in Gunnison and Ouray counties, in Nevada,
New Mexico, Arizona, etc.
Prousite
Ag₃ AsS₃
Pl. 7
_light red_
_silver_
Usually occurs in irregular masses; hardness 2.5; specific gravity 5.6;
color scarlet to vermilion; streak the same; luster adamantine;
transparent on thin edges.
In general this mineral is very like pyrargyrite, but has the scarlet
color and streak which are entirely characteristic. It is likely to have
the surface tarnished black, which happens on exposure to light, so that
it is essential to be sure that fresh surfaces are being examined.
Occasionally it is found in crystals, of the same type as the preceding
mineral. It is generally found associated with pyrargyrite.
Cerargyrite
AgCl
_horn silver_
Usually found in irregular masses or incrustations; hardness 1 to 1½;
specific gravity 5.5; color pearly gray, grayish green to colorless, but
turning violet brown on exposure to light; luster resinous; transparent
on thin edges.
This mineral is usually found in thin seams or waxy incrustations, but
it may occur in crystals in which case they are cubes. It is very soft
and easily cut with a knife, which with its tendency to turn
violet-brown on exposure to light, makes it easy to identify.
Cerargyrite is a secondary mineral, resulting from the action of
chlorine-bearing water on some one of the sulphides of silver. It is
found in the upper portions of mines, especially those in arid regions.
The Copper Group
After gold the next metal to be utilized was copper. About 4000 B.C. our
early forefathers found that by heating certain rocks, they obtained a
metal which could be pounded, ground and carved into useful shapes.
Curiously enough the rocks which had the copper also had some tin in
them, so that this first-found copper was not pure, but had from five to
ten per cent of tin in it, making the resulting metal harder, and what
we call bronze. It was some thousands of years later before they
distinguished the copper as a pure metal, but it worked and made good
tools. The newly found metal was not as ornamental as gold; but, because
it could be made into tools, it had a tremendous influence on man’s
development. As the bronze tools began to take the place of the stone
implements, the “Age of Bronze” was ushered in. In America the Indians
in the Lake Superior region found native copper weathered out of the
rocks and later mined it, and they too pounded it into knives, axes,
needles, and ornaments, but probably never learned to melt it and mold
their tools. At any rate they were not as far advanced in using this
metal when Columbus landed as were the southern Europeans 6500 years
earlier. Since the use of iron became general, copper has not held such
a dominant place, but it still is “the red metal” which holds the second
most important place.
It is malleable and ductile, though not equal to gold or silver in these
respects. It is a good conductor of electricity and a very large amount
of copper is used in electrical manufacture, roofing, wire, etc. It
alloys with other metals; ten parts copper and one of tin being bronze,
ten of copper and one of zinc is brass, and copper with aluminum is
aluminum bronze.
Like silver and gold, copper is widely diffused through the igneous
rocks, but before it is available, it must be leached out by solvents
and concentrated in veins, fissures, or definite parts of the lavas or
granites. The primary ores are those which, while the igneous rock was
still hot, were carried by hot vapors and liquids into the fissures and
there deposited, mostly as sulphides. There is a long list of these, but
in this country, the following are the commoner ones; chalcocite the
sulphide of copper, chalcopyrite the sulphide of copper and iron,
bornite another combination of copper, iron and sulphur, and
tetrahedrite copper and antimony sulphide. When these primary ores are
near enough to the surface to come in contact with waters carrying
oxygen, carbon dioxide or silica in solution, they may give up their
sulphur and take some one of these new elements and we have such forms
as cuprite, the oxide of copper, malachite and azurite, carbonates of
copper, or chrysocolla, the silicate of copper. Native copper is also a
secondary deposit laid down in its present state by a combination of
circumstances which deprived it of its original sulphur. In general
copper mining can not be profitably carried on for ores with anything
less than a half of one percent in them; and the use of such low grade
ores has only been possible for a few years, as the result of inventing
most delicate processes in the smelting.
The United States produces about a quarter of the world’s supply of
copper, with Chile ranking second with about 17%.
Copper
Cu
Pl. 8
Usually in irregular masses; hardness 2.5; specific gravity 8.9; color
copper red; luster metallic; opaque. Native copper, easily determined by
its color and hardness, is generally found in irregular grains, sheets,
or masses, on which may sometimes be detected traces of a cube or an
octahedral face, showing that it belongs to the isometric system. The
most famous locality is the Upper Peninsula of Michigan which may be
taken as typical. Here, long before it was known historically, the
Indians found and dug out copper to make knives, awls, and ornaments.
In this region, beds of lava alternate with sandstones and
conglomerates. The copper was originally in the lavas, but has been
dissolved out, and now fills cracks and gas cavities in the lavas, and
also the spaces between the pebbles of the conglomerate. This locality
has been very famous both because of the quantity mined, and also
because of the strikingly large masses sometimes found. Today but little
of the ore runs above 2 percent copper, and it is mined if it has as
little as ½ of one percent.
While nowhere near as abundant, native copper occurs in the same way in
cavities and cracks in the trap rocks of New Jersey, and along the south
shore of the Bay of Fundy. It is also known from Oregon, the White River
region of Alaska, and in Arctic Canada.
Chalcopyrite
CuFeS₂
Pl. 8
_copper pyrites_ or _yellow copper ore_
Occurs in crystals of irregular masses; hardness 4; specific gravity
4.2; color bronze yellow; streak greenish black; luster metallic; opaque
on thin edges.
Chalcopyrite resembles pyrite, but its color is a more golden yellow,
and its surface tarnishes with iridescent colors. Then too the hardness
of chalcopyrite is but 4 as compared with 6 for pyrite. When in crystals
this mineral belongs to the tetrahedral system as the c axis is but .985
in length as compared with I for the two other axes. This difference is
so little that, to the eye, the octahedron appears to belong to the
isometric system. Chalcopyrite occurs in octahedrons and tetrahedrons
(as on plate 8), the latter being the form where but half of the
octahedral faces are developed. However by far the most frequent mode of
occurrence is in irregular masses.
This is the most important primary ore of copper, and is widely
distributed, being found either in lavas, or in veins, or in fissures
connected with igneous rocks. Apparently the deposits were made, either
at the time of eruptive disturbances or shortly afterward, from vapors
or hot solutions carrying the copper sulphides (and other sulphides)
from the molten igneous rocks. Chalcopyrite is usually associated with
pyrite, galena, sphalerite and chalcocite, as well as quartz, fluorite
and calcite. It is found in all the New England States, in New York, New
Jersey, Pennsylvania, Maryland, Virginia, North Carolina, Tennessee,
Missouri, and all the Rocky Mountain and Pacific Coast States.
Bornite
Cu₃FeS₃
_purple copper ore_
Occurs in granular or compact masses; hardness 3; specific gravity, 5;
color bronze-brown with a bluish tarnish; streak gray-black; luster
metallic; opaque on thin edges.
Bornite is also known as erubescite, blushing ore, variegated copper,
peacock copper, etc., all of which names refer to the highly iridescent
tarnish which fresh faces soon take on when exposed to the air. Though
usually in masses, it is sometimes found in rough cubes of the isometric
system. In this country it is not abundant enough to be used as an ore,
but is likely to be found with other ores like chalcopyrite or
chalcocite. In the east it has been found at Bristol, Conn., and near
Wilkesbarre, Penn., while in the west it may be expected to occur
wherever other sulphide minerals of copper are found.
Chalcocite
Cu₂S
Pl. 9
_copper glance_
Occurs in fine grained compact masses; hardness 2.5; specific gravity
5.7; color dark leaden gray; streak black; luster metallic; opaque on
thin edges.
Chalcocite is one of the important ores of copper, especially in Arizona
and the Butte District of Montana. It resembles argentite in color and
general appearance, but is readily distinguished by being brittle and
having a tendency to tarnish to bluish or greenish colors on fresh
surfaces. Occasionally it occurs in crystals which are in the
orthorhombic system; but the edges of the prism are so beveled that
there are six sides and the prism resembles a hexagonal prism (see page
16).
In the Butte, Mont., district, the most important copper region in the
United States, fully 50% of the ore is chalcocite, which is a derivative
of the originally deposited chalcopyrite, the latter having lost its
iron. In the veins of this district chalcopyrite, bournite,
tetrahedrite, and several other copper minerals not described in this
book, occur all together, and with them also gold, silver and arsenic
minerals. The gold amounts to about 2¼ cents per pound of copper, and
the silver is in somewhat less quantity. These veins were first opened
to get the silver ores, which were the more important ones down to a
depth of 200 to 400 feet. Below these depths the copper became much more
important. It was the weathering which had removed a large part of the
copper minerals in the upper levels of the veins, but had left a large
part of the silver. Chalcocite is also important in most of the Utah and
Arizona mines.
In the east it has been found at Bristol, Simsbury and Cheshire, Conn.,
and in the west it is found in all the Cordilleran States.
Tetrahedrite
Cu₃SbS₃
Pl. 9 & 10
_gray copper ore_
Occurs in irregular masses and in tetrahedrons of the isometric system;
hardness 3.5; specific gravity 4.7; streak dark brown; luster metallic;
opaque on thin edges.
In its crystalline form the tetrahedrite occurs in tetrahedrons, which
generally have faces formed by beveling the edges and by cutting the
corners, as in the two figures of plate 10. Chalcopyrite may also occur
in tetrahedrons, but its golden yellow color is entirely different from
the gray-black of the tetrahedrite. When in masses the hardness and the
streak which is dark brown, are very characteristic.
In England and Bolivia tetrahedrite is an important ore of copper, but
in this country it is simply a copper mineral which is widely
distributed, and associated with most of the mining enterprises, but is
in no case the important ore. It has been found sparingly through the
New England States, at the Kellogg Mines in Arkansas, and abundantly in
Colorado, Montana, Utah, Arizona, Nevada and New Mexico.
Cuprite
Cu₂O
Pl. 9 & 10
_red copper ore_
Occurs in isometric cubes, octahedrons, and dodecahedrons, or in masses;
hardness 3.5; specific gravity 6; color dark brownish-red; streak
brownish-red; luster metallic; translucent on thin edges.
When in crystals cuprite is easily determined, but when in masses its
fresh surfaces may suggest prousite, but the streak and hardness are
quite different in the two cases. Sometimes its color suggests hematite,
but the latter has the hardness of 6. When found it is often coated with
a thin film of green, which is malachite.
Except when found as native copper, the ore which contains the greatest
percentage of copper is cuprite with 88.8% of copper. It is likely to
occur in any of the deposits of copper ore, where they are in arid
climates and above the level of the underground water, and is very
frequently associated with malachite and azurite. In the Bisbee,
Arizona, district cuprite is one of the important ores.
Besides the normal occurrence described above, cuprite may be found in
two other varieties; one where the crystals have grown side by side and
so only the ends have been free for continuous additions of the mineral,
which has resulted in a fibrous mass known as “plush copper ore” or
chalcotrichite; the other an earthy mixture of limonite and cuprite,
which is brick red in color, and termed “tile ore.”
Cuprite is found sparingly in New England, more abundantly at such
places as Summerville and Flemington, N. J., Cornwall, Penn., in the
Lake Superior region, and fairly abundantly in the Cordilleran States.
Malachite
CuCO₃·Cu(OH)₂
Pl. 11
Usually occurs in nodular or incrusting masses; hardness 3.5; specific
gravity 4; color green; streak a lighter green; luster adamantine, silky
or dull; translucent on thin edges.
The vivid green of malachite is usually enough to determine it at once,
but one may be sure by trying a drop of acid on it, in which case it
effervesces as is characteristic of so many carbonates, but this is the
only carbonate which is vivid green. Generally the malachite is in
irregular masses, but crystals are occasionally found. These are
extremely small and needle-like, and belong to the monoclinic system. In
the Ural Mountains there is a locality where these crystals grow in
fibrous masses, usually radiating from the center. Malachite in such
nodules has a silky luster. These rare nodules have furnished the rulers
of Russia with a unique and much prized material for making royal gifts.
In European museums and palaces one finds many objects carved from this
form of malachite, and marked as gifts of the czars of Russia.
In the United States malachite is widely distributed, appearing as green
streaks and stains where copper minerals have been exposed to the air.
It is the green tarnish which appears on bronze and copper when exposed
to the weather. It is found in large quantities in New Jersey,
Pennsylvania, Wisconsin, Nevada, Arizona, Utah, New Mexico, etc. The
Bisbee mine in Arizona is the place that has furnished museums with so
many of the handsome specimens of malachite associated with azurite.
These are the most striking specimens for the vividness of their colors
that appear in any collections.
Malachite has been known since about 4000 B.C., the Egyptians having
mines where they obtained it between the Suez and Mt. Sinai. In those
early days it was particularly a child’s charm, protecting the wearer
from evil spirits. It is still used as a stone of lesser value in making
some sorts of jewelry.
Azurite
2CuCO₃·Cu(OH)₂
Pl. 11
Occurs as short prismatic or tabular crystals of the monoclinic system;
hardness 4; specific gravity 3.8; color azure blue; streak lighter blue;
luster vitreous; translucent on thin edges.
Azurite is another very striking mineral fully characterized by its
color and streak. Like malachite it effervesces in acid. It is very near
to malachite in composition, and by increasing its water content, can
and freely does change to the green mineral; so that few specimens of
azurite are without traces of malachite. It is found in the same places
as malachite, but is not as abundant in the east.
Azurite with the accompanying malachite is cut and polished to make
semi-precious stones for some forms of jewelry.
Chrysocolla
CuSiO₃·2H₂O
Never occurs in crystals, but in seams and incrustations; hardness 2-4;
specific gravity 2.1; color bluish-green; streak white; luster vitreous;
translucent on thin edges.
This rather rare mineral often appears in opal- or enamel-like
incrustations, and its color is variable ranging from the typical
bluish-green to sky-blue or even turquoise blue. This is a mineral
resulting from the action of silica bearing waters, coming in contact
with most any of the copper minerals, and is found accompanying cuprite,
malachite, azurite, etc. It is never in large enough quantities to be
used as an ore, but its striking color attracts attention and it can be
found fairly frequently, especially in the west.
The Iron Group
Pure iron is a chemical curiosity which looks very much like silver. As
obtained from its ores, or as it occurs in Nature, iron always has some
impurities with it, such as carbon, silicon, sulphur and phosphorus, and
these are highest in the crudest iron such as “pig-iron.” Its
malleability and ductility are only a little less than for gold and
silver, and so it has a wide range of qualities for use by man. It is
only rarely found native in minute grains in some of the dark lavas.
There is however one remarkable exception to this statement, in that on
Disco Island, Greenland, there is a basaltic rock, from which are
weathered great boulders of native iron up to 20 tons in weight. This
iron is very like that occurring in meteorites, and probably came from
great depths in the earth’s interior. The specific gravity of iron is
7.8. It makes up around 5% of the crust of the earth, and probably
occurs in much larger percentages in the interior of the earth.
Iron was discovered by man later than gold or silver or copper, about
1000 B.C.; but once found it was so much more abundant than any of these
that it soon dominated over copper, and from Roman times to the present
has been the basis of progress in civilization, and these times are well
called “the iron age.”
Iron unites freely with the non-metals, and occurs as sulphides, oxides,
carbonates, etc., and is also present as a secondary metal in that great
group of minerals known as the silicates (see page 97). It alloys with a
wide range of other metals, every combination altering the properties of
the iron, and thus making it useful in a still greater range of
manufacture. The introduction of ¼ to 2½% of carbon into iron makes
steel, which is harder (in proportion to the amount of carbon) and
stronger than the pure iron.
Iron compounds are among the most numerous and important of the colors
in Nature’s paint box, limonite furnishing the browns which color the
soil and so many of the rocks, hematite giving the red color to other
abundant rocks, and magnetite often coloring igneous rocks black, while
the chlorophyll which gives the green color to plants is an iron
compound, as is also the hemoglobin which gives the red to our blood.
Iron is present in all igneous rocks, and secondarily in the sedimentary
and metamorphic rocks. It is soluble in water, and so is being
constantly transferred from place to place, and changes from one
compound to another, according to the circumstances in which it is
placed.
The primary forms are pyrite, magnetite and the silicates. When in
weathered rocks the iron is changed to limonite, siderite or hydrated
silicates. Hematite is an intermediate oxide from which the water
contained in limonite has been driven off by moderate heat or bacterial
action.
Limonite
2Fe₂O₃·3H₂O
Pl. 12
Never crystalline, occurs in mammillary, botryoidal and stalactitic
forms, or in fibrous, compact, oolitic, nodular or earthly masses;
hardness 5.5; specific gravity 3.8; color yellow-brown to black; streak
yellow-brown; luster metallic to dull; opaque.
Limonite is a very common mineral, the color, streak and hardness
identifying it readily. Iron rust is its most familiar form. When
powdered it is the ochre yellow used in paints. Being so universally
distributed, it is to be expected it will occur in a variety of ways.
First, there is the fibrous type found lining cavities, in geodes, or
hanging in stalactites in caves. This has a silky luster, an opalescent,
glazed or black surface, and is in mammillated or botryoidal masses.
Second, it may occur in compact masses in veins, where it was deposited
by waters; which, circulating through the adjacent rocks, gathered it
from the rocks, and, on reaching the open seams, gave it up again.
Third, it may occur in beds on the bottom of ponds, where it was
deposited by waters which gathered it as they flowed over the surface of
the country rocks. Measurements in Sweden show that it may accumulate in
such places as much as six inches in the course of twenty years. In
ponds and swamps, the decaying vegetation forms organic compounds, which
cause the precipitation of the iron from the water, as it is brought in
by the streams. This sort of iron in the bottom of ponds or swamps is
also known as “bog iron.” Another form in which limonite may occur in
ponds, lakes, or even the sea, is in oolitic masses. In this case the
iron forms in tiny balls, with perhaps a grain of sand at the center,
and one coat of iron after another formed around it, like the layers of
an onion. If the resulting balls are tiny this is called oolitic (like
fish eggs), but if the balls are larger it is pisolitic (like peas).
Bacteria probably have a good deal to do with the precipitation of
limonite in this manner. Fourth, limonite occurs in earthy masses,
usually mixed with impurities like clay and sand, which are the residue
left behind, where limestones have been dissolved by weathering. The
fifth mode of occurrence is known as gossan, or “the iron hat,” which is
a mass of limonite capping a vein of some sulphide mineral, like pyrite,
chalcopyrite or pyrrhotite, which has been exposed to weathering; and in
these minerals the sulphur has been removed, leaving a mass of limonite
over the vein. This is particularly common in the west. Limonite is
quite easily fusible and so was probably the first ore from which early
man extracted iron.
Limonite is iron oxide, with 3 molecules of water of crystallization (or
constitution) associated with every 2 molecules of the oxide. If
limonite is moderately heated the water is driven out and the resulting
compound is hematite, the same oxide, but without the water. In this
case and many other similar cases, as gypsum, opal, etc., we have two or
more minerals resulting from the presence or absence of water in the
mineral. The water molecules have a definite place in the arrangement of
molecules which determines the structure of the mineral. Sometimes the
water is driven out at a temperature around 212 F., in which case it is
called, water of crystallization, but in other cases as gypsum, a
considerably higher temperature is required to drive out the water, and
then it is called, water of constitution. In all cases the removal of
the water changes the arrangement of molecules and a new mineral
results, with characteristics of its own.
In this case limonite is only one of a series of minerals which have the
Fe₂O₃ molecule as a basis, and that incorporate more or less water into
their molecular construction as follows:
Turgite 2Fe₂O₃·H₂O
Goethite Fe₂O₃·H₂O
Limonite 2Fe₂O₃·3H₂O
Xanthosiderite Fe₂O₃·2H₂O
Limonite Fe₂O₃·3H₂O
Of these goethite is crystalline, the others non-crystalline. They may
occur pure or in all sorts of mixtures, the mixtures usually being
lumped under limonite. The limonite is far the commonest of the series,
goethite is fairly common, but the others are rare as pure minerals.
Limonite is found in all parts of all states and in every country.
Though so common, it is by no means an important source of iron today,
only about one percent of the iron mined in this country coming from
this source, though in Germany, Sweden and Scotland it is relatively
much more important.
Goethite
Fe₂O₃·H₂O
Pl. 12
Occurs in lustrous brown to black orthorhombic prisms, usually
terminated by low pyramids; hardness 5; specific gravity 4; color brown
to black; streak brownish-yellow; luster imperfect adamantine; opaque.
Goethite, named for the poet Goethe, who was interested in mineralogy,
is much less abundant than limonite or hematite, but occurs with them,
when they are in veins. Its usual form is an orthorhombic prism with the
edges beveled, and a low pyramid on either end. The crystals usually
grow in clusters, making a fibrous mass, often radiated, in which case
it is known as “needle iron stone”; or the prisms may be so short as to
be almost scales; when, because of the yellowish-red color, it is called
“ruby mica”. It is found in many states, including Connecticut,
Michigan, Colorado, etc.
Hematite
Fe₂O₃
Pl. 13 & 14
_specular iron_
Occurs in compact, mammillary, botryoidal, or stalactitic masses of dark
red to black color, or in earthy masses of bright to dark red; hardness
6; specific gravity 5.2; color ochre red to black; streak cherry red to
dark red; luster metallic, vitreous, or dull; opaque on thin edges.
Hematite is readily distinguished from other red minerals by its
hardness and streak. It may occur in crystals, which belong to the
hexagonal system, and are usually hemihedral forms of the double
pyramid, or rhombohedrons. These rhombohedrons usually have the edges
beveled, as in Pl. 13, A; or are tabular in form as a result of the
beveling of two of the opposite edges to such an extent that a form like
Pl. 13 B results. However the usual occurrence is in non-crystalline
masses, which represent transformations from limonite by the loss of
water of crystallization on the part of the limonite. In such cases we
have fibrous, oolitic or compact masses, according to the form in which
the limonite occurred. The transformation from limonite into hematite
involves some heat to drive out the water of crystallization, but
nothing like what is involved in metamorphism.
Hematite is the source of 90% of the iron mined in this country. Part of
it comes from the famous Clinton iron ore, a layer a foot or more in
thickness; starting in New York State, and extending all down the
Appalachian Mountains to Alabama, where it is ten or more feet thick and
the basis of the Birmingham iron industries. Then there are tremendous
deposits of earthy to compact hematite, probably derived from limonite,
around the west end of Lake Superior. This latter region yields today
around 75% of the iron for this country.
Loose earthy masses of hematite are often known as “ochre red,” and were
used by the Indians for war paint. Today the same sort of material is
obtained by powdering hematite and using it for red paint. The red color
in great stretches of rock is due to the presence of small amounts of
hematite, acting as cementing material. The red of the ruby, garnet,
spinel, and the pink of feldspars and calcite are due to traces of
hematite.
This mineral is very common and found in every state.
Magnetite
Fe₃O₄
Pl. 14
_Magnetic iron ore_
Occurs in masses or in isometric octahedrons or dodecahedrons; hardness
6; specific gravity 5.8; color black; streak black; luster metallic;
opaque on thin edges.
Magnetite is another important ore of iron, and is peculiar in being
strongly magnetic; its name being derived, according to Pliny, from that
of the shepherd Magnes, who found his iron pointed staff attracted by
the mineral when he was wandering on Mount Ida. This magnetic property
has been repeatedly used to locate beds of magnetite, and is very
helpful in separating magnetite from the “black sands,” of which it so
often forms a part. These sands however generally have magnetite with so
much titanium in it that they are unfit for smelting.
Magnetite is found in association with igneous or metamorphic rocks, and
often represents limonite or hematite which has been altered as the
result of high temperatures. Some of it, in the igneous rocks
especially, was undoubtedly in the molten magma and has crystallized out
from the magma while it was still hot. It is the form of iron always
indicative of former high temperatures. It is an ore mineral for about
3% of the iron in this country, but in Scandinavia and some other
countries, it plays a leading role as the source of iron.
It is found in the Adirondack Mountains, in New Jersey, Pennsylvania,
Arkansas, North Carolina, New Mexico, and California.
Siderite
FeCO₃
Pl. 13 & 14
_Spathic iron_
Occurs in fibrous botryoidal masses or rhombohedral crystals, sometimes
with curved faces; hardness 3.5; specific gravity 3.8; color gray-brown;
streak white; luster vitreous; translucent on thin edges.
Like hematite this mineral belongs to the hexagonal system, and
crystallizes in hemihedral form, making the rhombohedron. Its faces are
often curved, which is rare in minerals, only a few forms like this and
dolomite having other than plane faces. When siderite crystals grow in
clusters, the crowding often results in growth on one face only, making
a mass of fibrous character, and in such cases the surface of the mass
is botryoidal in contour. The mineral is likely to oxidize, losing its
gray-brown color, and becoming limonite. In the United States it is
scarcely ever used as an ore for iron, but in Germany and England a
great deal of iron is smelted from this mineral.
It occurs in Massachusetts, Connecticut, New York, throughout the
Appalachian Mountains, and also in Ohio.
Pyrite
FeS₂
Pl. 15 & 16
_iron pyrites_
Occurs as cubes, octahedrons and pyritohedrons, or in compact masses,
scales or grains; hardness 6; specific gravity 5.1; color brassy yellow;
streak greenish-black; luster metallic; opaque on thin edges.
This is one of the commonest of all minerals. It is found in all kinds
of rocks, with all kinds of associations, in all parts of the world. Its
crystals are isometric, and cubes and octahedrons are abundant. The
pyritohedron is also a common form, and characteristic of this mineral.
It is a hemihedral form derived from a 24-sided form, _i.e._ the cube
with four faces on each side. On this 24-sided form each alternate face
has developed and the others have disappeared, resulting in a 12-sided
form, known as the pyritohedron, which differs from the dodecahedron in
that each of its faces is five-sided instead of rhomboidal. When in
crystals pyrite can not be easily confused with any other mineral; but
when in masses it is often mistaken for gold, chalcopyrite, pyrrhotite
or marcasite. From the first two, the color should be sufficient to
distinguish it, for they are golden yellow. Pyrrhotite is bronze yellow,
and marcasite is paler yellow. Then too in hardness pyrite is much
harder than any of these minerals except marcasite. This last is the one
which is most likely to cause real difficulty. Its lighter color, and
the fact that it usually comes in fibrous masses are the best
distinctions.
In spite of being so abundant pyrite is scarcely ever used as an ore for
iron, because the sulphur makes the metal “short,” or brittle, and the
sulphur is not easily gotten entirely out of the iron; but pyrite is
used largely in the manufacture of sulphuric acid, so important to many
of our industries.
Other sulphides are commonly mixed with pyrite, such as chalcopyrite,
arsenopyrite, argentite, etc.; but the most important impurity is gold,
which is often scattered through the pyrite in invisible particles, and
sometimes in quantities enough to make it worth while to smelt it for
the gold.
Pyrite is particularly the form in which the sulphur compounds of iron
appear in rocks which have been highly heated, and is to be expected in
metamorphic rocks and also igneous rocks, especially in fissures and
veins leading from the igneous rocks. It may occur in sedimentary rocks,
but in these last it is usually marcasite.
Marcasite
FeS₂
Pl. 15
_white pyrite_
Occurs in orthorhombic crystals, usually grouped to make fibrous or
radiating masses, or non-crystalline in masses; hardness 6; specific
gravity 4.8; color pale brassy-yellow; streak greenish-gray; luster
metallic; opaque on thin edges.
Marcasite has the same chemical composition, as pyrite, and looks like
it, but is lighter colored and usually occurs in fibrous masses. It is
the commoner form in limestones and shales, while pyrite is more likely
to occur in igneous and metamorphic rocks. It seems probable that
marcasite is due to a more hasty precipitation from cold solutions,
while pyrite is deposited more slowly from hot solutions.
Isolated crystals of marcasite are rare; but, if formed, they belong to
the orthorhombic system. Usually some form of twinning is present, and
because of the multiple character of the twinning, marcasite crystals
usually show a ragged outline, with reentrant angles. It is most
abundant in radiated masses, which appear fibrous on the broken
surfaces. It decomposes easily, taking oxygen from the air and forming,
even in museum cases, a white efflorescence or “flower,” which is iron
sulphate or melanterite. In moist air it takes water and decomposes to
sulphuric acid which may change the surrounding limestone to gypsum.
Marcasite is found wherever limestones and shales are the country rock.
Pyrrhotite
Fe₁₁S₁₂
_Magnetic pyrites_
Occurs in masses; hardness 4; specific gravity 4.6; color bronze; streak
grayish-black; luster metallic; opaque on thin edges.
Tabular crystals are known, but are very rare. They belong to the
hexagonal system. This form is easily distinguished from the other
yellow minerals by being magnetic. It is by no means as abundant as the
two preceding sulphides of iron, but does occur fairly frequently in
veins in igneous rocks, and less frequently in limestones, large
quantities of sulphuric acid being made from a deposit in limestone at
Ducktown, Tenn. It will be found in most states. When associated with
nickel it is an important source for the latter mineral, as at Sudbury,
Canada. Pyrrhotite is very like a substance found in meteorites, known
as troilite.
The Lead Group
After learning how to get iron from the rocks by rude smelting methods,
the early peoples tried heating various rocks, and some time around 500
B.C. stumbled upon lead, which is rather easily separated from its ores.
This metal was used through Roman times to make pipes, gutters, etc.
Lead is a soft metal, fairly malleable, but with little ductility, and
still less tensile strength. Though one of the commoner metals, it does
not occur as pure metal in Nature. It is diffused in minute quantities
through the igneous rocks, and also is found in the sedimentary rocks
and in the sea water. Its minerals are few, galena, the sulphide of
lead, being the commonest, and at the same time the form in which lead
is primarily deposited. Galena may also represent a secondary
deposition. The other minerals, cerrusite, anglesite, and pyromorphite
are results of modification of the galena when it lies near enough to
the surface to be acted on by weathering agents, like water and air.
Lead minerals are usually associated with zinc minerals, there being but
few places where the minerals of the one group occur without the other.
Most lead when first smelted from its ore, contains a greater or less
amount of silver in it, sometimes enough so that the lead ore is better
worth working for the silver than for the lead.
Lead is used in making pipes, gutters, bullets, etc., and in its oxide
forms in the manufacture of paints and glass. Eighty-three parts of lead
with 17 parts of antimony make type metal. Lead and tin alloy to make
solder. Lead and tin with small amounts of copper, zinc and antimony
make pewter. The United States produce about 20% of the world’s supply
of this metal.
Galena
PbS
Pl. 17
_lead glance_
Occurs in cubes or cleavable masses; hardness 2.5; specific gravity 7.5;
color lead-gray; streak lead-gray; luster metallic; opaque.
While there is quite a group of lead-gray minerals, galena is easily
identified by its cleavage, which is perfect in three directions
parallel to the cube faces. Even a moderate blow of the hammer will
shatter a mass of galena into small cubic pieces. The crystals often
have the corners cut by octahedral faces, and occasionally the edges are
beveled by dodecahedral faces. It is not uncommon to find crystals of
large size, several inches across. If galena has 1 to 2% of bismuth as
an impurity, curiously enough, the cleavage changes to octahedral, but
this is a rare occurrence.
Galena may occur as a primary mineral in veins associated with igneous
intrusions, or in irregular masses in metamorphic rocks; but it is more
often found in irregular masses in limestones, where the limestone has
been dissolved, and the cavities thus formed, filled with secondary
deposits of galena. It also occurs at the contact between igneous rocks
and the adjacent rock, whatever this may be. Sometimes it is found in
residual clays.
Among the most important lead deposits are the Cœur d’Alene district in
Idaho, where galena with a high percentage of silver is mined; the
Leadville, Colo., district where lead, silver and gold occur together in
veins; the Joplin, Mo., district, where lead and zinc ores occur
together in irregular masses in limestones; and the Wisconsin district
of similar character.
When found galena is usually associated with sphalerite, argentite
chalcopyrite, pyrite and calcite. It will be found in every state.
Cerrusite
PbCO₃
Pl. 18
_White lead ore_
Occurs in fibrous or compact masses, or in orthorhombic crystals,
usually on galena; hardness 3.5; specific gravity 6.5; colorless; streak
white; luster adamantine; transparent on thin edges.
While the crystals of this mineral simulate hexagonal, they are actually
orthorhombic, the simple form being an octahedron with two of its edges
beveled, making double six-sided pyramids (see Pl. 18 A.) Usually prism
faces are present. Twinning is common, both the simple contact sort, as
shown on Plate 18 B, and also the sort in which three crystals have
grown through each other, so as to make a six-rayed crystal. The
considerable weight, and the fact that it effervesces in acid serve to
identify cerrusite. When pure it is colorless, but impurities cause it
to appear white, gray or grayish-black, and sometimes it has a tinge of
blue or green.
It is likely to occur wherever galena is found, as a secondary mineral
derived from the galena. In this country it is not used as an ore, for,
as in the Leadville district, veins which have cerrusite near the
surface change at moderate depths, and galena takes the place of the
cerrusite. It is found all down the Appalachian Mountains, and in all
the Cordilleran States. Especially fine specimens have come from the
Cœur d’Alene district in Idaho.
Anglesite
PbSO₄
Pl. 18
Occurs in grains and masses, or in tabular and prismatic orthorhombic
crystals; hardness 3; specific gravity 6.3; colorless; luster
adamantine; transparent on thin edges.
Two modes of occurrence are characteristic, one in cavities in galena,
the other in concentric layers around a nucleus of galena. In the former
case fine crystals are developed, in the latter the mineral is in
masses. The crystals look like those of barite, but are soluble in
nitric acid while the barite is insoluble. Sometimes the crystals are
prismatic with pyramidal faces instead of the tabular form.
It is found in the lead mines associated with galena, and in this
country is not used as an ore for lead, but in Mexico and Australia it
is abundant enough to be mined as an ore. Exposed to water which has
carbon dioxide in it, and most surface waters have some, it readily
changes to cerrusite. It is found in Missouri, Wisconsin, Kansas,
Colorado, and Mexico.
Pyromorphite
Pb₅Cl(PO₄)₃
Pl. 17
_Green lead ore_
Occurs in small barrel-shaped hexagonal crystals, and in fibrous or
earthly masses; hardness 3.5; specific gravity 7; color green to brown;
luster resinous; translucent on thin edges.
Pyromorphite is found in the upper levels of lead mines, and is formed
by the decomposition of galena. Its green color (sometimes shading off
toward brown), considerable weight and resinous luster, serve to
distinguish this mineral. The crystal form is that of a simple hexagonal
prism, with the ends truncated. It is found in Phœnixville, Penn.,
Missouri, Wisconsin, Colorado, New Mexico, etc.
The Zinc Group
Zinc and copper made the brass of early Roman times; but even then, zinc
was not known as a separate metal, the brass being made by smelting
rocks in which both zinc and copper occurred, the zinc never being
isolated until much later. Some time in the later Roman times it seems
to have been obtained separately, but then and all down through the
Middle Ages zinc and bismuth were confused. Our earliest record of zinc
being smelted, as we know it today, was about 1730 in England. In those
earlier days, the product, zinc, or bismuth, or both together, were
known as “spelter,” and this name has clung to zinc in mining and
commercial circles; so that today, if one looks for quotations in the
newspaper, he often finds zinc under the head of spelter.
Zinc, like lead, is diffused in small quantities through all the igneous
rocks. In places it is segregated in fissures or veins leading from the
igneous rocks, along the contact between igneous rocks and either
sedimentary or metamorphic rocks, in limestones where solution cavities
have been formed and later filled with zinc minerals, and as a residue
where limestones have been weathered away. In all these places it is
closely associated with lead.
The sulphide, sphalerite, is the primary mineral, and the other
minerals, like zincite, smithsonite, calamine, willemite, franklinite,
etc., are secondary, resulting from modifications of the original
sphalerite. In connection with zinc minerals the region of Franklin
Furnace, N. J., is especially interesting, for at that place are found
two large metamorphosed deposits containing a wide range of zinc
minerals, several of which are not found anywhere else.
Zinc is soft and malleable, but is only slightly ductile, and has little
tensile strength. It alloys with several metals, and in this form is
most useful today; three parts of copper to one of zinc making brass;
four or more parts of copper and one of zinc, making “gold foil”; copper
and zinc (a little more zinc than copper) making “white metal”; three
parts of copper to one of zinc and one of nickel making German silver;
etc. Zinc is also used in large quantities in galvanizing iron, sheets
of iron being dipped into melted zinc and thus thinly coated. It is also
used in batteries and a wide range of chemical industries.
Sphalerite
ZnS
Pl. 19 & 20
_zinc blende, black jack_
Occurs in grains, in fibrous or layered masses, or in isometric
crystals; hardness 3.5; specific gravity 4; color yellow-brown to almost
black; streak light yellow to brownish; luster resinous to adamantine;
translucent on thin edges.
When in crystals sphalerite occurs most commonly either in dodecahedrons
or in tetrahedrons (hemihedral forms of the isometric octahedron). The
cleavage is fairly good and parallel to the faces of the dodecahedron.
The difficulty usually is to get large enough crystalline masses to see
this cleavage clearly, but by examining the angles between the faces of
cleavage pieces they will be found to be the same as those on a
dodecahedron. When the mineral is pure, it has the color of resin, but
sometimes it is reddish to red-brown, and then it is called “ruby zinc,”
more often it is dark brown due to the presence of iron as an impurity.
This is what the miners call “black-jack.” The presence of iron also
tends to make the streak darker. The hardness, streak and cleavage will
usually determine this mineral readily.
Sphalerite is the primary ore of zinc and is usually found in fissures
and veins leading from masses of igneous rocks, or along the surface of
contact where igneous rocks like granite or lavas come against such
metamorphic rocks as gneisses, schists, or crystalline limestones. In
the region of Joplin, Mo., however, the sphalerite is of secondary
character, having been gathered by waters circulating through the
limestones, and deposited in them in irregular pockets. This Joplin
district has produced more zinc than any other in the world. The United
States annually produces about 25% of the world’s supply of this metal.
Sphalerite is always associated with galena, and such other minerals as
argentite, pyrite, chalcopyrite, fluorite, quartz, calcite and barite,
are very apt to be present. It will be found in almost every state,
especially in fissures and veins, and less frequently in cavities in
limestones.
Zincite
ZnO
Pl. 19 & 20
_red zinc ore_
Usually occurs massive, but may be found in crystals; hardness 4;
specific gravity 5.6; color deep red; streak orange; luster
subadamantine; translucent on thin edges.
When in crystals zincite forms in hexagonal prisms with hexagonal
pyramids on the ends. This is rather rare, most of the zincite being
found in massive form. The cleavage is parallel to the prism faces and
perfect. The deep red color and orange streak are wholly characteristic.
This mineral is so common at Franklin Furnace, N. J., as to be an
important ore, but it is very seldom found elsewhere. This district, as
mentioned before, is a peculiar one for zinc minerals. The zinc beds are
in a metamorphosed limestone, and into this are intruded numerous dikes
of granite. Probably the zinc was originally present in the bed of
limestone as smithsonite, calamine and other secondary minerals of zinc.
When intruded by the hot granite the smithsonite (carbonate) may well
have been altered to the oxide, zincite; while the calamine (hydrous
silicate) became the simple silicate, willemite.
Willemite
ZnSiO₄
Pl. 20
Occurs in masses or in crystals; hardness 5.5; specific gravity 4.1;
color pale yellow when pure; luster resinous; translucent on thin edges.
Willemite is another of the minerals which are distinctively
characteristic of Franklin Furnace, and found elsewhere very rarely. It
is so common there as to be one of the principal ores, and mostly occurs
in irregular masses, but is also found in crystals. These are hexagonal
prisms, with a three-sided (rhombohedral) pyramid on the ends. The color
when pure is whitish or greenish-yellow, but with small amounts of
impurities it may be flesh-red, grayish-white or yellowish-brown. When
in crystals it is easily determined; but when massive it looks like
calamine, and can only be distinguished by placing a bit of the mineral
in a closed tube and heating it, in which case calamine will give off
water vapor, while willemite will not.
This mineral is one of those resulting from metamorphic alteration and
is derived from calamine, when the latter loses its water of
crystallization. It is common at Franklin Furnace, N. J., and also found
occasionally elsewhere, as at Salida, Colo., and in Socorro Co., New
Mexico.
Calamine
Zn₂(OH)₂·SiO₃
Occurs as crystalline linings in cavities, or as botryoidal or
stalactitic masses; hardness 5; specific gravity 3.4; colorless to
white; luster vitreous.
Calamine resembles both smithsonite and willemite when in
non-crystalline masses. From the smithsonite it is easily separated by
the fact that in nitric acid the smithsonite effervesces and the
calamine does not. From willemite it is harder to distinguish, but a
piece may be placed in a closed tube and heated. If it is calamine water
vapor will be given off, if willemite nothing happens. When calamine
occurs in crystals these are orthorhombic and mostly tabular, and the
crystals are peculiar in that the two ends are terminated differently.
Both this and smithsonite are secondary minerals and usually occur
together when zinc is found in limestones. It is abundant at Franklin
Furnace and Sterling Hill, N. J., and also found at Phœnixville, Penn.,
in Wythe Co., Va., and Granby, Mo.
Smithsonite
ZnCO₃
Pl. 21
_Dry bone_
Usually occurs as incrustations, grains, earthy or compact masses, and
as crystals; hardness 5; specific gravity 4.4; color white, yellow,
greenish or bluish; streak white; luster vitreous; transparent on thin
edges.
When pure this mineral is colorless, but, as it occurs, it is usually
white, or tinged with some shade of yellow, green, or blue, but in all
cases its streak is white. The crystals are rhombohedrons often with
edges beveled or corners cut by other faces. It resembles calamine and
willemite, but is readily separated from either of these by the acid
test, for smithsonite effervesces when acid is placed on it.
Next to sphalerite, smithsonite is the commonest of the zinc minerals.
It is a secondary mineral, resulting from the action of lime-charged
water acting on sphalerite, and so is likely to be found wherever zinc
minerals occur in a limestone region. In the Wisconsin-Illinois-Iowa
district it serves as a minor ore of zinc, and is termed here “dry
bone.” It is also found in the Missouri and Arkansas districts, and in
Europe is an important ore for zinc.
Franklinite
(ZnMn)Fe₂O₄
Pl. 21
Occurs in compact grains or masses, and in isometric octahedrons;
hardness 6; specific gravity 5; color black; streak reddish-brown;
luster metallic; opaque on thin edges.
This is a mineral peculiar to the Franklin Furnace region, from which it
gets its name. It looks like magnetite, but its reddish-brown streak and
lack of magnetism distinguish it. When it occurs in octahedrons, the
edges are rounded, while those of magnetite are sharp. It is a complex
and variable oxide of zinc, iron and manganese, which has resulted from
the metamorphism of the beds in which it occurred probably being
originally something quite different.
The Manganese Group
Though manganese was known in the mineral pyrolusite in early times, it
was then thought to be magnetite or magnetic iron ore. It was not until
1774 that it was isolated and recognized as a distinct element.
Manganese is one of the lesser elements in the crust of the earth,
making less than .07 of one percent, but as an alloy with other metals,
especially iron, it has attained a considerable importance to man. It is
used chiefly with iron, 20% of manganese making the alloy, spiegeleisen,
a combination which occurs in Nature in Germany, and from 20% to 80%
making ferromanganese. These alloys are in great demand because they
make an especially tough steel essential in the manufacture of
munitions. The sources for manganese are the oxide ores, manganite,
pyrolusite and psilomelane, which have been formed as secondary
minerals, as a result of the weathering of silicates which carry
manganese. They occur widely enough, but throughout the United States
the deposits are small, and this is one of the elements in which this
country is not self-sufficient. The largest producer of manganese is
Russia; however she consumes almost all of her output at home, and our
supply comes from the next largest producers, India, the Union of South
Africa, and the Gold Coast. A shift in trade may be expected when
Brazil’s recently discovered ore body in Matto Grosso is brought into
full production. Besides being used as an alloy, manganese is employed
in making paints and dyes, for clearing glass, and for some types of
electric batteries.
Pyrolusite
MnO₂
Occurs in earthy or fibrous masses; hardness 1-2; specific gravity 4.8;
color black; streak black; luster dull; opaque.
Pyrolusite occurs in soft masses and incrustations, usually leaving a
sooty mark on the fingers. Sometimes it seems to be in crystals, but
these are pseudomorphs which have the form of manganite, from which the
pyrolusite has formed as a result of the water having been driven from
the manganite. Frequently pyromorphite and manganite will be found
together, and in some cases the outer part of a mass or crystal will be
pyrolusite, while the center is still manganite. Psilomelane is another
oxide of manganese with water and may appear very like pyrolusite, but
both manganite and psilomelane have much greater hardness than does
pyrolusite. If there is difficulty in deciding about pyrolusite, it may
be placed in a closed tube and heated. It will not be affected by the
heat, while, under the same circumstances, both manganite and
psilomelane will give off water vapor.
Pyrolusite usually occurs in black streaks or pockets in residual clays
which have formed as a result of the decomposition of limestones. It may
also occur in dendritic forms in seams and crevices (see manganite). It
is found in Vermont, Massachusetts, Virginia, Arkansas, Colorado,
California, etc.
Psilomelane
MnO₂·H₂O
Occurs in compact botryoidal or stalactitic masses; hardness 5-6;
specific gravity 4.2; color black; streak brownish-black; luster
metallic; opaque on thin edges.
Psilomelane is very like pyrolusite, and often occurs with it. It is
distinguished by its greater hardness, and the fact, that when heated in
a closed tube, it gives off water vapor. From manganite it is more
easily distinguished, for it never occurs in crystals, while the
manganite is usually crystalline. This and pyrolusite are the principal
ores of manganese.
Wad is an impure form of psilomelane, having some iron oxide mixed with
the manganese oxide, usually limonite; or the impurity may take the form
of a copper, cobalt, lithium or barium oxide.
Psilomelane is found at Brandon, Vt., in Arkansas, Colorado, California,
etc.
Manganite
Mn₂O₃·H₂O
Pl. 22
Occurs in prismatic crystals, or in columnar or fibrous masses; hardness
4; specific gravity 4.4; color steel gray; streak reddish-black; luster
submetallic; opaque on thin edges.
This is the form taken by manganese oxide when it crystallizes in the
presence of moisture, and pyrolusite frequently changes to manganite
when exposed to moisture. The crystals are orthorhombic prisms, with
striated sides and the ends truncated. These prisms usually occur in
bundles and give the mineral a fibrous appearance. Manganite is not hard
to identify, the striations on the crystals and the streak being very
characteristic.
In seams and tiny crevices this mineral, and often pyrolusite, grows in
a branching manner, resembling tree-like or “mossy” masses. This is
termed dendritic, and the growths of manganese minerals are called
dendrites. One of the most curious of these is when the “mossy” growth
is inclosed in chalcedony, making the so-called _moss agate_. These moss
agates are abundant through the Rocky Mountains and are frequently cut
for semi-precious stones. The finest ones however come from India and
China.
Manganite is found in the Lake Superior region, Colorado, etc.
Rhodochrosite
MnCO₃
Occurs in compact cleavable masses; hardness 4; specific gravity 3.5;
color rose to dark red; streak white; luster vitreous; translucent on
thin edges.
This usually occurs in pink to red masses which cleave readily parallel
to the faces of the rhombohedron. When it is found in crystals, which
are rare, these too are rhombohedrons. It is usually found in veins as a
gangue mineral with copper, silver or zinc ores. Its beautiful color and
the fact that it effervesces in acid serve to distinguish this mineral.
It is found at Branchville, Conn., at Franklin Furnace, N. J., and in
veins with silver in Colorado, Nevada, and Montana.
The Aluminum Group
Though aluminum is one of the most abundant of all the metals, making
some 8% of the crust of the earth, its union with other elements is so
firm, that only recently have methods been found for getting the metal
free. It was first isolated in 1846, but up to 1890 the extraction of
aluminum was so expensive, that it could not be widely used. About that
time electrical processes were applied to its extraction, and since then
the price has steadily dropped, until now it is under $.20 per pound. It
is very malleable, and ductile, and has high tensile strength. Exposed
to the air, water or ordinary gases, it does not tarnish; and it is very
light, an equal bulk weighing about a third as much as iron. The
combination of lightness and strength, and the fact that it is a good
conductor of electricity, have made it available for a wide range of
uses, such as electrical apparatus, delicate instruments, boats,
aeroplanes, and domestic utensils.
It is an essential component of all the important rocks, except
sandstone and limestone, and combines to a greater or less degree in a
host of minerals. Though present in clays, shales, argillites,
feldspars, and micas, it is only from bauxite that it has been
successfully extracted. Aside from the small number of simple compounds
of aluminum grouped here, it also takes a part in the make-up of a large
series of minerals termed silicates, treated a little further on in this
book.
It alloys with other metals, especially copper. The union of copper and
a small amount of aluminum makes aluminum-bronze, which looks like gold
and is used for watch chains, pencil-cases, etc., and also for the
antifriction bearings of heavy machinery. A small amount added to steel
prevents air holes and cracks in casting.
Corundum
Al₂O₃
Pl. 23
Occurs in cleavable masses or in hexagonal crystals; hardness 9;
specific gravity 4; colorless, red, yellow, blue, or gray; luster
vitreous to adamantine; translucent to transparent on thin edges.
Corundum is readily recognized by its hardness, second only to that of
the diamond. The crystals may be simple six-sided prisms, hexagonal
pyramids or combinations of the two. The cleavage is usually described
as parting, for it is by no means perfect, but when it is recognizable
it is parallel to the faces of a rhombohedron, and cleavage pieces may
appear almost cubic.
When in clear and perfect crystals this mineral is one of the most
highly prized of all the gems. Clear and colorless it is known as the
“_Oriental white sapphire_”; when tinged with blue it is the _sapphire_;
when colored yellow, the “_Oriental topaz_”; when green, the “_Oriental
emerald_”; when purple, the “_Oriental amethyst_” and when red, the
_ruby_. Sapphires range from colorless to deep blue, the value depending
on the shade of the blue, and increasing as the color deepens. The
Oriental topaz can easily be confused with the true topaz, which is a
much commoner and less valuable gem, but can be distinguished by the
hardness, topaz having a hardness of but 8. The name emerald is applied
to several green gems, mostly to beryl, which is not so hard and is the
true emerald. The Oriental emeralds have a value about the same as
diamonds. Rubies of clear and deep color are the rarest of all gems,
ranging in value about three times as high as diamonds of equal size.
The most sought-for shade is the so-called “pigeon-blood red,” and the
value of a stone of this sort is almost dependent on the whim of the
buyer. The best of the rubies come from granites or metamorphosed
limestones in Burma; the best sapphires from Ceylon, though both of
these, and some of the other corundums of gem quality, have been found
in North Carolina and Montana.
Around these stones, which have been used so long among the Hindus,
Persians, Jews, Egyptians, and Christians, a wealth of lore has been
woven. The sapphire was Saturn’s stone, and a talisman to attract Divine
favor. Where tradition makes the stone on which the ten commandments
were written the sapphire, it is probable that, what was really meant,
is lapis lazuli, as is also the case when sapphires are mentioned as
building stones for the celestial gates. The ruby in ancient lore is
termed “lord of stones,” “gem of gems” etc., and so protected its wearer
that he was safe from injury in peace or war.
When corundum is colored brown by impurities of iron, it is termed
_corundum_, when black by greater quantities of iron, it is _emery_.
These varieties are far the commonest form in which corundum occurs, and
when ground to finer or coarser powder make the commercial emery. Emery
is likely to be found in sands, making so-called “black sands,” where it
has accumulated as a result of the weathering to bits corundum-bearing
rocks. In some one of its forms, corundum is found in Massachusetts,
Connecticut, New York, New Jersey, and all down the Appalachian
Mountains, also in Colorado, Montana, California, etc.
Bauxite
Al₂O₃·2H₂O
Occurs in grains, or oolitic or clay-like masses; hardness 1-3; specific
gravity 2.5; color white to yellowish-white or reddish-brown.
Bauxite never comes in crystals, but is usually in earthy masses, which
have resulted from the decomposition of granitic or volcanic rocks, in
circumstances where hot alkaline waters were present. This explanation
seems to apply especially to the deposits in France, which were first
the chief source of the bauxite, and may be applicable to those in
Georgia and Alabama. Some of the other deposits, however, do not seem to
have had any hot water available, and the deposit appears more like
simple decomposition of the underlying rocks by alkaline waters.
In many cases bauxite resembles limonite in being a mixture of two or
more aluminum oxides with water of crystallization, such as Al₂O₃·H₂O,
Al₂O₃·2H₂O and Al₂O₃·3H₂O. This is particularly true of the bauxite
which resulted from the decomposition of rocks by surface water.
Bauxite is the ore from which aluminum is obtained. The deposits are not
large, but the United States has its share of them. It is found in
Alabama, Arkansas, Georgia, Missouri, Tennessee, and California.
Cryolite
Na₃AlF₆
_Ice stone_
Occurs in pseudo-cubic crystals or massive; hardness 2.5; specific
gravity 3; color white; luster vitreous; transparent on thin edges.
Cryolite is a relatively soft mineral, colorless to white as snow; for
which reason, and partly also because it comes mostly from Greenland it
is called “ice stone.” It is really monoclinic but the inclination of
the c axis is so slight, that, unless examined carefully, the crystals
appear to be cubic. Until about 1900 great quantities of this mineral
were shipped from West Greenland, and from them the metal aluminum was
extracted. When bauxite was discovered, it was found to be considerably
cheaper to make the aluminum from that mineral, and now cryolite is no
longer sought. Aside from its occurrence in Greenland some cryolite is
found in Colorado, near Pike’s Peak.
The Arsenic Group
The metal, arsenic, is a dark steel gray in color, when the surface is
fresh, but it soon tarnishes. It is very brittle and easily powdered
under the hammer, and its only use as a metal, is for an alloy with lead
in making shot. Its compounds find a wider use. The white powder called
“arsenic” is arsenous acid, and is used mostly in making poisons, which
fortunately are easily detected in animal tissues. Copper arsenate,
(_Scheele’s green_) is a pigment used in making green paint, and
formerly in the green colors of wall paper. A combination of arsenous
acid, copper oxide and acetic acid is the well known _Paris Green_, so
much used for an insecticide. Beside these uses, arsenic serves a large
number of other purposes, as in making glass and enamel, embalming
fluids, and various medicines.
Curiously arsenic plays a double part, acting part of the time as a
metal, as in the two following minerals, and part of the time as a
non-metal, as in cobaltite, niccolite, etc.
Arsenopyrite
FeAsS
Pl. 24
Occurs in well formed crystals, grains, or masses; hardness 5.5;
specific gravity 6; color silver-white; streak black; luster metallic;
opaque on thin edges.
When in crystals, they are usually short prisms of the orthorhombic
system, either end being terminated with a low roof. Though usually
described as silver-white in color, there is always a brassy cast to the
color. Its appearance is much like cobaltite and smaltite, but it can be
easily distinguished from both these by putting a piece in nitric acid.
The arsenopyrite will not materially change the color of the fluid, but
the other two turn it rose-red, and all give off the smell of sulphur.
It looks sometimes like marcasite, but that is yellower, and has the
fibrous structure, not found in arsenopyrite.
It is found in veins or in metamorphic rocks, associated with argentite,
galena, sphalerite, chalcopyrite and pyrite. It is distinctly a mineral
formed by deposition from hot vapors or hot water rising from either
lavas, or in the course of metamorphism.
It is found in New Hampshire, Vermont, Massachusetts, Connecticut, New
York, New Jersey, California, etc.
Realgar
AsS
Pl. 24
Occurs in incrustations or scattered grains; hardness 1.5 to 2; specific
gravity 3.5; color orange; streak orange; luster resinous; opaque on
thin edges.
Crystals are very rare, but when found are short monoclinic prisms. The
color is aurora-red, changing to orange as soon as it is exposed to the
air. This and the streak are entirely characteristic. It is a mineral
associated with hot vapors or hot waters, and is found about volcanoes,
as deposits from the hot water of the geysers in Norris Basin,
Yellowstone Park, and in veins, associated with barite, stibnite,
quartz, etc., as in Massachusetts, Utah, California, etc.
Orpiment
As₂S₃
Occurs as incrustations or powdery masses; hardness 1 to 2; specific
gravity 3.5; color lemon yellow; streak yellow; luster resinous.
This mineral is very like realgar in its physical properties, and likely
to occur with it. It gives the lemon yellow color to the basins about
hot springs, as in the Yellowstone Park, and about volcanoes. It also
comes in veins with realgar.
Molybdenum
Molybdenum is a rare metal, silvery-white in color, brittle and very
difficult to fuse. It is used mostly as an alloy of steel, to make
certain grades of tool steel. The world’s greatest supply is obtained
from Climax, Colorado, where the principal ore mineral is molybdenite.
Molybdenite
MoS₂
Occurs in scales or scaly masses, occasionally in tabular hexagonal
crystals; hardness 1.5; specific gravity 4.7; color lead-gray; streak
bluish-gray; luster metallic; opaque.
This mineral is the chief source for the metal molybdenum. Its extreme
softness and greasy feel will distinguish it at once from any other
mineral except graphite, which has much the same qualities, but its
scaly character and the more bluish tinge in streak and color will
distinguish these two.
It occurs in granites, gneisses, and metamorphic rocks in Colorado, New
Mexico, Maine, Connecticut, New Hampshire, New York, Pennsylvania, etc.
Antimony
Antimony is another hard, brittle metal, of bluish-white color. Exposed
to the air at ordinary temperatures it does not tarnish; and this
combined with its hardness make it useful for such alloys as Britannia
metal, type metal, and pewter. Only one of its minerals, stibnite, is
common enough for mention.
Stibnite
Sb₂S₃
Pl. 25
_gray antimony_
Occurs in prismatic or needle-like crystals; hardness 2; specific
gravity 4.5; color lead-gray; streak lead-gray; luster metallic; opaque.
The crystals of stibnite are orthorhombic and usually elongated, the
sides striated and the ends with low pyramids on them. Sometimes the
long crystals are curved or even twisted. There is a well-developed
cleavage parallel to face b in the figure. While the color is similar to
that of galena, the form and cleavage are so different that stibnite is
easily determined.
The ancients used stibnite to color their eyebrows, now it is the source
for the metal antimony. Hungary and Japan are famous for the fine large
crystals they produce; but moderate sized crystals may be found in this
country. It occurs in veins along with pyrite, galena, cinnabar, and
realgar, with quartz, calcite or barite as gangue minerals.
Stibnite has been found in Arkansas, California, Nevada, and Utah.
The Nickel Group
Nickel as a metal is silvery-white in color, rather hard, and does not
tarnish when exposed to the air. When pure it is malleable and fairly
ductile. It is highly useful for plating other metals to protect their
surfaces. Alloyed with steel, it makes a product of extreme hardness.
Copper, zinc, and nickel make the well known German silver.
Nickel has a fairly large range of minerals, but they do not occur with
any abundance in the United States, so that we have to import most all
of our nickel. In the earlier days New Caledonia produced most of the
world’s supply, but recently since the finding of large nickel deposits
near Sudbury, Canada, this locality has not only outstripped New
Caledonia, but now produces four-fifths of the world’s supply. In this
country but two nickel minerals will be found at all common.
Niccolite
NiAs
Pl. 25
_copper nickel_
Occurs in masses; hardness 5.5; specific gravity 7.4; color pale
coppery-yellow; streak pale brownish-black; luster metallic; opaque on
thin edges.
Niccolite is very seldom in crystals, but if they do occur they are
hexagonal. The mineral looks a little like smaltite, but in case there
is any question of the determination, dissolve a piece in nitric acid,
and if niccolite, it will color the solution green.
Niccolite is usually associated with copper and silver ores, and in this
country has been found at Chatham, Conn., and Silver Cliff, Colo. It may
be associated with pentlandite, a sulphide of iron and nickel, which is
similar in appearance, but not so hard, and occurs in small grains
throughout dark lavas. The particles of pentlandite are however so
small, that they are seldom noticeable, but at Sudbury, Canada, this is
the chief ore of nickel.
Millerite
NiS
_capillary pyrites_
Occurs in needle-like or fibrous crystals; hardness 3.5; specific
gravity 5.5; color brass-yellow; streak greenish black; luster metallic;
opaque on thin edges.
The fibrous crystals of millerite belong to the orthorhombic system. The
color and streak suggest pyrite, but the crystals are long and slender,
while pyrite is in cubes, octahedrons, etc. If there is any doubt of the
identity of this form, place a piece in nitric acid, and if it is
millerite, it will color the acid green.
It may occur in veins associated with cobalt and silver minerals, or as
a secondary mineral as at Gap Mine, Penn., or in cavities in sedimentary
rocks. In the last case it usually is in needle-like crystals growing
through calcite crystals, as at St. Louis, Mo., Keokuk, Iowa, and
Antwerp, N. Y.
The Cobalt Group
As a metal, cobalt is hard, brittle, and of a grayish color, tinged with
red. It was not recognized as a separate element until 1735, and even
today is one of the minor metals. Cobalt, chromium and a little tungsten
make the alloy stellite, which has come into large use in making
high-speed tools. The oxide of cobalt (CoO) is “smalt,” used to give the
blue color to porcelain, pottery, glass, tiles, etc. Invisible ink is
made by diluting cobalt chloride in a large quantity of water. This
solution is a faint pink color and practically invisible on paper, but
if heated it loses water and turns blue in color, and is perfectly
visible.
Cobalt is another of the metals, of which the United States does not
have an adequate supply. Sweden, Norway and India were the chief sources
of supply until cobalt was found near the town of Cobalt in Ontario,
Canada, and now this district furnishes 90% of the world’s supply.
Cobaltite
CoAsS
Pl. 26
_cobalt glance_
Usually crystalline in cubes, pyritohedrons or octahedrons; hardness
5.5; specific gravity 6.1; color reddish silver-white; streak
grayish-black; luster metallic; opaque on thin edges.
In color cobaltite may appear very like arsenopyrite, especially if the
reddish tinge is not strong, in which case the mineral can be definitely
determined by putting a piece in nitric acid. If it is cobaltite the
solution will be colored rose-red, if arsenopyrite there will be no
change of color. The forms of the crystals are the same as those of
pyrite, but the color will easily distinguish cobaltite from pyrite.
This pink color is characteristically present either in or about cobalt
minerals, being sometimes called “cobalt bloom.” It is a
cobalt-arsenic-oxide with water of crystallization (Co₃As₂O₈·8H₂O),
which results from the exposure of cobalt and arsenic minerals to air
and moisture. It is the pink color on the figures of both cobaltite and
smaltite. In Sweden, Norway and India, this is the chief ore for cobalt,
but in the United States it is rather rare, but is found in Oregon, and
at Cobalt, Canada.
Smaltite
(CoNi)As₂
Pl. 26
_gray cobalt ore_
Usually occurs in masses; hardness 5.5; specific gravity 6.2; color
tin-white to steel-gray; streak grayish-black; luster metallic; opaque
on thin edges.
While very like cobaltite, smaltite is almost never found in crystals,
but when crystals are found, they are cubes. The color is tin-white but
there is usually a pink tinge visible due to the presence of small
amounts of “cobalt bloom.” If in any doubt about the determination of
this mineral, put a piece in nitric acid. If it colors the acid
rose-pink, and is non-crystalline it is pretty surely smaltite; if the
acid is not affected it is arsenopyrite.
Smaltite is found in Kentucky, Missouri, Colorado, Idaho, California,
and at Cobalt in Canada.
Chromium
This metal gets its name in recognition of the many colors (_chroma_
“color”), in which its compounds appear. Chromic oxide is a vivid green,
used to color porcelains, pottery, tiles, etc., and also as a substitute
for the arsenical greens formerly used in wall-paper. The chromate of
lead is the pigment, well known to artists as “chrome yellow,” and the
bichromate of potassium is bright red. The metal is obtained in at least
two different forms; one hard, brittle and so resistant to heat as to be
infusible at temperatures which would volatilize platinum; the other as
a powder which burns brightly if heated in air. While used in paints,
dyes, etc., its greatest importance is for the making of ferro-chrome
steel, which is used where resistance to sudden shock is required, as in
armor plate, automobile springs, ball bearings, etc. With tungsten and
cobalt it makes the alloy, stellite, as noted above.
Chromium was used in relatively small quantities before the first world
war, and we imported our supplies from Turkey, India, New Caledonia, and
Rhodesia. During the last war we started a large-scale development of
low-grade ores in Montana, and can now supply all of our needs from this
source.
Chromite
FeCr₂O₄
_chromic iron_
Occurs in grains, masses, or isometric octahedrons; hardness 5.5;
specific gravity 4.4; color black; streak dark-brown; luster
submetallic; opaque on thin edges.
In form, color and streak chromite resembles magnetite and franklinite.
From the magnetite it is distinguished by being non-magnetic; from the
franklinite, by being insoluble in hydrochloric acid, while the
franklinite is soluble. Chromite furnishes practically all the chromium
used in the arts and manufactures. It is a mineral associated with high
temperatures, and therefore found in dark lavas, serpentine, and
olivine. It occurs in Pennsylvania, Maryland, New Jersey, Montana,
Oregon, Wyoming, and California.
Tungsten
This element is obtained either as a heavy dark-gray metal, which is
very hard and difficult to fuse, or as a dark-gray powder. It is used as
an alloy with iron, one part of tungsten to nine of steel, to make the
ferrotungsten, which has extraordinary hardness, and is used mostly for
high-speed tools. Tungsten is also one of the three metals (cobalt,
chromium and tungsten) which are alloyed together to make stellite. Some
of the tungsten supply is also used to make the films in incandescent
lamps, and in some of the chemical industries. It has but one important
ore, wolframite, and this is found in the United States in but small
quantities; so that we ordinarily have to import the greater part of
what we use. During the last war, under the stimulus of high prices and
the urge of necessity, we did find and produce substantial quantities of
tungsten. China is the world’s largest producer of tungsten ore with
Burma second, and the United States a poor third.
Wolframite
(FeMn)WO₄
Occurs in monoclinic crystals or in crystalline masses; hardness 5.5;
specific gravity 7.4; color dark-brown to black; streak nearly black;
luster submetallic; opaque on thin edges.
If in crystals the form will serve to distinguish this mineral from
cassiterite and ilmenite, the two which it most resembles; but if it is
massive the only sure way to decide is to put a piece in strong
sulphuric acid; if it dissolves and throws down a yellow precipitate
(tungstic acid) it is wolframite.
Like the two other minerals mentioned above it occurs in veins in
igneous rocks, being associated with high temperatures. As it is almost
insoluble in water, like cassiterite and ilmenite, it is likely to occur
with them in the sands which are the result of the disintegration of the
rocks which carried the minerals; and so a large part of the supply
today comes from placer deposits.
It is found in Connecticut, North Carolina, Missouri, Colorado, and
California.
Radium, Uranium and Vanadium
These three metals are all rare and occur together. Radium, discovered
in 1898, is a heavy metal which has proved very useful because of its
radio-activity, that is, its power of giving off or radiating tiny
particles of matter known as _X-rays_, part of which are charged with
positive electricity, and part of them with negative electricity. The
ability of these rays to pass through other substances has made possible
photographing the denser substances within those less dense, as the
bones within the flesh, or metal within leather or wood, etc. The rays
have proved of great value medicinally, and are also used to make
objects luminous in the dark. These X-rays are also used in the study of
the ultimate structure of matter, as it can be thus obtained in such
small units.
Uranium is another element which is radio-active and can be used for
many of the same purposes as radium.
Vanadium, the third of these associated metals, and the commonest of the
group, is not radio-active. It is a silvery-white metal, mostly used as
an alloy with steel to give it great hardness.
Carnotite
K₂O·2U₂O₃·V₂O₅·3H₂O
Pl. 27
Occurs in earthy masses; color yellow.
This mineral is included here, not because it is common, but because it
is of such great interest. It is the chief source of supply in the
United States of radium, uranium and vanadium. It is a lemon-yellow
earth or powder, which looks a little like orpiment. It is however found
in a sandstone, instead of where hot waters have deposed minerals. From
a ton of this ore about 10 pounds of uranium oxide, 55 pounds of
vanadium and ¹/₁₀₀₀th of a gram of radium are obtained. Carnotite is
found in south-west Colorado and south-east Utah, and on Carrizo
Mountain on the line between Arizona and New Mexico.
Mercury
Mercury, or quicksilver, is the only metal which is liquid at ordinary
temperatures. It is silvery-white in color, with a striking metallic
luster, and at the low temperature of 662° F., boils and changes to a
colorless vapor. Mercury alloys with certain metals, these alloys being
known as amalgams. In this way it is especially useful for the recovery
of gold and silver, the mercury being added to crushed ore, the gold or
silver uniting with the mercury in a liquid amalgam, which is then drawn
off and heated to a temperature above 662° F., at which temperature the
mercury volatilizes and is recovered, while the gold or silver remains
behind. Mercury also forms a solid amalgam with tin which is used to
coat glass, the high metallic luster making the most effective looking
glass. It is also used in medicines (calomel, corrosive sublimate,
etc.), for scientific instruments (thermometers, barometers, etc.), in
cosmetics, in paints for ship bottoms, etc.
Though there are some 25 minerals of mercury, only one is common or
important as a source of the metal, cinnabar. The United States is
self-sufficient as far as mercury is concerned, producing just about as
much as it uses. The leading producers are Spain, Austria, Italy, and
the United States. Commercially mercury is quoted as quicksilver, and in
flasks of 75 pounds each.
Cinnabar
HgS
Pl. 27
Occurs in massive or earthy form, or in minute crystals in cavities;
hardness 2.5; specific gravity 8; color scarlet to dark red; streak
vermilion; luster adamantine; translucent on thin edges.
The bright-red color and the streak are usually enough to identify this
mineral at once, but some of the darker varieties resemble hematite or
zincite in appearance, but both these have much greater hardness. When
in crystals they are tiny hexagonal prisms with pyramids on the end.
Cinnabar is usually found in or near metamorphic or igneous rocks,
either in veins leading from the igneous rocks, or in metamorphic rocks,
or it may occur disseminated through metamorphic rocks. It is associated
with quartz or calcite, and may occur with other sulphides like pyrite,
galena, argentite, etc. It is most abundant in California, but is also
found in Oregon, Washington, Idaho, Arizona, Nevada, Utah, Texas, and
Montana.
Tin
Tin has been known since early Roman times, and the mines at Cornwall,
England, were worked from that time all through down to the present, but
now they are becoming of minor importance as they approach exhaustion.
The metal is silvery-white, does not easily tarnish, is malleable, but
has little ductility and little tensile strength. Tin is mostly used in
making tin plate, a thin sheet of steel covered with tin, the tin being
only 1 to 2% of the total weight. This tin plate is mostly made into tin
cans, and used as containers for food. Some tin is used in making
solder, tin-foil, tubes for paste, vaseline, etc., and around 1000 tons
per year for weighting silk. This “weighting” makes the silk heavier by
about 25% and gives it a “rustle,” which, while much in evidence, is
really indicative that the silk is not pure. The United States produces
very little tin, most of the world’s supply coming from the Malay
Peninsula, Dutch East Indies, China, and Bolivia, with small amounts
from several other countries.
Cassiterite
SnO₂
Pl. 28
_tin stone_
Occurs in tetragonal crystals, massive, or in grains and pebbles;
hardness 6.5; specific gravity 7; color black or dark-brown; streak
gray; luster adamantine; translucent on thin edges.
The crystals are short prisms with pyramidal ends. Twinning is common.
Cassiterite also occurs in fibrous masses, and when it is weathered from
its original location, is so insoluble and hard, that it remains as
grains and pebbles, making placer-deposits, from which today three
quarters of the supply is obtained. If pure, the crystals would be
colorless, but impurities of iron and titanium give it the dark-brown to
black color. Cassiterite may appear very like rutile, the crystalline
forms being identical, but the reddish tinge of color in the rutile will
separate the two.
Cassiterite is one of those minerals which result from deposition at
very high temperatures, probably from vapors, and is found in the veins
in igneous rocks, such as light-colored granites, gneisses, syenites,
etc. While not mined in this country it is found in small quantities in
Maine, Massachusetts, New Hampshire, Virginia, Alabama, Wyoming,
Montana, and California.
Titanium
Titanium, as a metal, is a heavy, gray, iron-like powder, which is
chiefly useful as an alloy with iron, giving it toughness, and
preventing bubbles and cracks in casting. It is not as rare as some
other metals which have found a wider use.
Rutile
TiO₂
Pl. 28
Occurs in tetragonal crystals, and in grains; hardness 6.5; specific
gravity 4.2; color red to reddish-brown; streak yellowish-brown; luster
metallic to adamantine; translucent on thin edges.
Rutile usually occurs in crystals, which are either short and stout, or
in needle-like crystals. Twinning is common. In form and general
appearance it resembles cassiterite, but the reddish color, and the
yellowish-brown streak will distinguish the rutile. It is found in
similar rocks, granites, gneisses, syenites, and mica-schists, the two
minerals cassiterite and rutile often occurring together. This is also
true of the grains, which have been weathered out and are found in sands
and gravels of placer deposits. It is found in small quantities in all
the New England States, New York, and all down the Appalachian
Mountains, especially at Graves Mountain, Ga., and in Arkansas and
Alaska.
Ilmenite
FeTiO₃