Geologic Features
The mineral graphite is a soft, steel-gray, crystalline form of carbon.
Ceylon graphite occurs in veins and lenses cutting gneisses and limestones. Usually the veins consist almost entirely of graphite, but sometimes other minerals occur in important amounts, especially pyrite and quartz. The association of graphite with these minerals, and also with feldspar, pyroxene, apatite, and other minerals, suggests that the veins are of igneous origin, like some of the pegmatite veins in the Adirondacks of New York. The graphite is mined from open pits and shafts, and sorted by hand and mechanically. The product consists of angular lumps or chips with a relatively small amount of surface in proportion to their volume.
In Madagascar the graphite is mainly disseminated in a graphitic schist, though to some extent it is present in the form of veins and in gneiss. Most of the graphite is mined from a weathered zone near the surface, and the material is therefore soft and easily concentrated. The product is made up of flakes or scales, and in the making of crucibles requires the use of larger amounts of clay binder than the Ceylon graphite.
The flake graphite of the United States, principally in the Appalachian region, occurs in crystalline graphitic schists, resulting from the anamorphism of sedimentary rocks containing organic matter. Certain beds or zones of comparatively narrow width carry from 3 to 10 per cent of disseminated graphite. The graphite is recovered by mechanical processes of sorting. The graphite is believed to be of organic origin, the change from organic carbon to graphite having been effected by heat and pressure accompanying mountain-building stresses. Some of the graphite also occurs in pegmatite intrusives and adjacent wall rocks. This graphite is considered to be of inorganic origin, formed by the breaking up of gaseous oxides of carbon in the original magma of the pegmatites. The Montana graphite is similar in origin. This inorganic graphite in pegmatite veins resembles Ceylon graphite, in breaking into large lumps and chips, but supplies are very limited.
Amorphous graphite is formed in many places where coal and other carbonaceous materials have undergone extreme metamorphism. It represents simply a continuation in the processes by which high grade coals are formed from plant matter (pp. 123-127). The Mexican deposits are of this type, and occur in beds up to 24 feet in thickness interbedded with metamorphosed sandstones.
In general, graphite is primarily concentrated both by igneous processes in dikes, and by sedimentary processes in beds. In the latter case anamorphism is necessary to recrystallize the carbon into the form of graphite.
GYPSUM
Economic Features
The principal use of gypsum is in structural materials. About two-thirds of the gypsum produced in the United States is used in the manufacture of various plasters—wall plaster, plaster of Paris, and Keene's cement (for statuary and decorative purposes),—and about a fifth is used as a retarder in Portland cement. Another important structural use is in the manufacture of plaster boards, blocks, and tile for interior construction. Gypsum is used as a fertilizer under the name of "land plaster," and with the growing recognition of the lack of sulphur in various soils an extension of its application is not unlikely. Minor uses are in the polishing of plate glass, in the manufacture of dental plaster, in white pigments, in steampipe coverings, and as a filler in cotton goods.
The world's gypsum deposits are widely distributed. Of foreign countries, France, Canada, and the United Kingdom are the principal producers. Germany, Algeria, and India produce comparatively meager amounts. The United States is the largest producer of gypsum in the world. In spite of its large production, the United States normally imports quantities equivalent to between one-fifteenth and one-tenth of the domestic production, mainly in the crude form from Nova Scotia and New Brunswick for consumption by the mills in the vicinity of New York. This material is of a better grade than the eastern domestic supply, and is cheaper than the western supply for eastern consumption. During the war this importation was practically stopped because of governmental requisition of the carrying barges for the coal-carrying trade, but with the return of normal conditions it was resumed. There is no prospect of importation of any considerable amount from any other sources. The domestic supply is ample for all demands.
Production of gypsum in the United States comes from eighteen states. Four-fifths of the total comes from New York, Iowa, Michigan, Ohio, Texas, and Oklahoma. There are extensive deposits in some of the western states, the known reserves in Wyoming alone being sufficient for the entire world demands for many decades.
The United States exports a small amount of crude gypsum to Canada, principally for use in Portland cement manufacture. This exportation is due to geographic location. The United States is the largest manufacturer of plaster boards, insulating materials, and tile, and exports large quantities of these products to Cuba, Australia, Japan, and South America.