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Graphite
Mineral Facts:
Chemical
Formula: C
Sometimes
specimens are impure, containing associated iron oxides, clay, etc. Known to
the ancients, the mineral name is derived from the Greek word meaning "to
write."
Colors:
Black color
with brownish tinge, black streak.
Hardness:
1 to 2
Hardness is such that the material
easily writes on paper and soils fingers handling the specimen, giving a
greasy feel to the touch.
Density:
2.2
Cleavage:
One perfect cleavage on 0001.
Crystallography:
Hexagonal
Principally in amorphous
masses of a black, clayey appearance, in radiated masses, in brilliant lead
black scales or plates, and occasionally in crystals with a rhombohedral
habit.
Luster:.
Metallic luster and the mineral is opaque even in
the thinnest flakes.
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Composition, Structure and
Associated Minerals:
Graphite most commonly occurs in metamorphic rocks, such as crystalline
limestones, schists and gneisses. It may occur as large crystalline plates
enclosed in the rock or disseminated in small flakes in sufficient amount to
form a considerable proportion of the rock. In these cases, it has probably
been derived from carbon material of organic origin which has been converted
into graphite during the metamorphism of the rock. Instances are known in
which coal beds, under influence of strong metamorphic action, such as the
intrusion into them of an igneous rock, have in a greater or less degree
been converted into graphite. Examples of such an occurrence are to be found
in the graphitic coals of Rhode Island, and in the coal fields of Sonora,
Mexico. Graphite also occurs in fissure veins associated with
calcite,
quartz, orthoclase, pyroxene, etc. An example of such veins is to be found
in the deposits at Ticonderoga, New York. Here the veins traverse a gneiss
and
besides the graphite contain
quartz,
biotite, orthoclase,
tourmaline,
apatite,
pyrite,
titanite, etc. The graphite may have been formed
in these veins from hydrocarbons introduced into them during the
metamorphism of the region and derived from the surrounding
carbon-bearing rocks. Graphite occurs occasionally as an original
constituent in igneous rocks. It has been observed in the basalts
of Ovifak, Greenland, in an elasolite syenite from India, in a granite
pegmatite from Maine, in meteorites, etc.
Identification
and Diagnostics
Infusible and very refractory
in its chemical nature. Graphite is recognized by its color, foliated
structure and softness.
Its color, softness and infusibility serve to distinguish graphite from all
other minerals but
molybdenite. It is distinguished
from molybdenite by the brownish tinge to it's black color (molybdenite has
a secondary blue tone) and the lack of chemical positive tests for sulfur or
molybdenum.
Graphite is noncombustible even at moderately high temperatures. Like
diamond, however, it may be burned under certain conditions at very high
temperatures. It is unaffected by the common acids and is not acted upon by
the atmosphere.
Localities
The principal source of supply for commercial natural graphite is Sri Lanka
where it occurs in coarsely foliated masses in veins
in gneiss. It occurs in large amounts in various localities in
Austria, Italy, India, Mexico, and other countries. In the United States the
mineral has been mined on the southeast side of the Adirondacks in New York;
in Chester County, Pennsylvania; near Dillon, Montana; at several points in
Arkansas, Georgia, Alabama and North Carolina; in Wyoming; in Baraga County,
Michigan, and to a small extent in Colorado, Nevada, and Wisconsin. It
occurs also abundantly at many other places. Its chief source in the United
States is Graphite, near Lake George, New York.
Industrial Uses of
Graphite
Graphite is an important industrial
material and used in many applications.
Graphite products are made from
both natural graphite as well as material made synthetically from coke or
anthracite coal. Because it is both electrically conductive and strongly
corrosion resistant, it has many applications. It is used
in the manufacture of refractory crucibles for the steel, brass and bronze
industries. The purified mineral is
mixed with clay and / or other materials and made into crucibles for use at
high temperatures. Most of the graphite
used in this way is imported from Ceylon. Natural graphite is added to iron
to increase its carbon content for steel manufacture. It is used widely when
mixed with various organic liquids, as a lubricant,
polish, and black paint for metal surfaces, in all of which it is especially
valuable on account of its non-corroding properties.
Mixed with fine clay, it forms the "lead" of
common pencils. Much of the graphite used in the United States for this
purpose comes from Sonora, Mexico. Used in the manufacture of a protective
paint for structural iron and steel works. Used in the coating of foundry
facings, for electrodes, stove polishes, in brake liners for heavy equipment
(non automotive), high performance gaskets and heat insulators, etc.
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