Graphite Composition, Crystallization, Structure and Occurrence
Graphite Composition. Carbon, like the diamond. Sometimes impure with iron oxide, clay , etc.
Crystallization. Hexagonal-rhombohedral. In tabular crystals with hexagonal outline. Prominent basal plane. Distinct planes of other forms very rare. Rhombohedral symmetry sometimes shown by triangular markings on base
Structure. In foliate masses; scaly; granular to compact; earthly. Sometimes in globular forms with radiated structure.
Physical Properties. Perfect basal cleavage. H.=1-2 (readily marks paper and soils the fingers). G.= 2.2. Luster metallic sometimes dull earthy, Black color with brownish tinge. Black streak. Greasy feel. Folia flexible but not elastic.
Tests. Infusible. Very refractory in its chemical nature. Recognized by its color, foliated structure and softness. Distinguished from molybdenite by the brownish tinge to its black color (molybdenite has a blue one and the lack of chemical tests.
Occurrence. Graphite most commonly occurs in metamorphic rocks, such as crystalline limestones, schists and gneisses. It may occur as large crystalline plates inclosed in the rock or disseminated in small flakes in sufficient amount to form a considerable proportion of the rock. The 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 or strong metamorphic action, such as the intrusion into them of an igneous rock, have in a greater or less degree been converted into 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, biotitic, 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. It has been observed in the basalts of Ovifak, Greenland, in a nepheline syenite from India, in a granite pegmatite from Maine, in meteorites, etc.
The most productive deposits of graphite at present are on the island of Ceylon, where it occurs in coarsely foliated masses in veins in gneiss. It occurs in large amounts in various localities in Austria, Italy, India, Mexico; Etc. Graphite is found in quantity with schistose rocks in Madagascar. The chief deposits in the United States are in the Adirondack region of New York, in Essex, Warren and Washington counties, particularly at Ticonderoga.
Artificial. Artificial graphite is manufactured on a scale in the electrical furnaces at Niagara Falls. Anthracite coal or petroleum coke with a small amount of evenly distributed ash is subjected to the intense heat of the electrical current and converted into graphite. The output of artificial graphite is considerably in excess of that of the natural mineral.
Name. Derived from the Greek work “to write”
Use. Used in the manufacture of refractory crucibles for the steel, brass and bronze industries. Most of the graphite used in this way is imported from Ceylon. Used widely, when mixed with oil, as a lubricant. Mixed with fine clay, it forms the “lead” of 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 electrotyping, etc.
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