Geologic Features
Gypsum is a hydrated calcium sulphate. It is frequently associated with minor quantities of anhydrite, which is calcium sulphate without water, and under the proper natural conditions either of these materials may be changed into the other.
Common impurities in gypsum deposits include clay and lime carbonate, and also magnesia, silica, and iron oxide. In the material as extracted, impurities may range from a trace to about 25 per cent. Gypsite, or gypsum dirt, is an impure mixture of gypsum with clay or sand found in Kansas and some of the western states; it is believed to have been produced in the soil or in shallow lakes, by spring waters carrying calcium sulphate which was leached from gypsum deposits or from other rocks.
Gypsum deposits, like deposits of common salt, occur in beds which are the result of evaporation of salt water. Calcium makes up a small percentage of the dissolved material in the sea, and when sea waters are about 37 per cent evaporated it begins to be precipitated as calcium sulphate. Conditions for precipitation are especially favorable in arid climates, in arms of the sea or in enclosed basins which may or may not once have been connected with the sea. Simultaneously with the deposition of gypsum, there may be occasional inwashings of clay and sand, and with slight changes of conditions organic materials of a limey nature may be deposited. Further evaporation of the waters may result in the deposition of common salt. Thus gypsum beds are found interbedded with shales, sandstones, and limestones, and frequently, but not always, they are associated with salt beds. The nature of these processes is further discussed under the heading of salt (pp. 295-298).
The anhydrite found in gypsum deposits is formed both by direct precipitation from salt water and by subsequent alteration of the gypsum. The latter process involves a reduction of volume, and consequently a shrinkage and settling of the sediments. The hydration of anhydrite to form gypsum, on the other hand, involves an increase of volume and may result in the doming up and shattering of the overlying sediments.
Gypsum is fairly soluble in ground-water, and sink-holes and solution cavities are often developed in gypsum deposits. These may allow the inwash of surface dirt and also may interfere with the mining.
All the important commercial gypsum deposits are believed to have been formed by evaporation of salt water in the manner indicated. Small quantities of gypsum are formed also when pyrite and other sulphides oxidize to sulphuric acid and this acid acts on limestone. Thus gypsum is found in the oxide zones of some ore bodies. These occurrences are of no commercial significance.
MICA
Economic Features
The principal use of sheet mica is for insulating purposes in the manufacture of a large variety of electrical equipment. The highest grades are employed particularly in making condensers for magnetos of automobile and airplane engines and for radio equipment, and in the manufacture of spark plugs for high tension gas engines. Sheet mica is also used in considerable amounts for glazing, for heat insulation, and as phonograph diaphragms. Ground mica is used in pipe and boiler coverings, as an insulator, in patent roofing, and for lubricating and decorative purposes.
India, Canada, and the United States are the important sheet mica-producing countries, before the war accounting for 98 per cent of the world's total. India has long dominated the sheet mica markets of the world, and will probably continue to supply the standard of quality for many years. The bulk of the Indian mica is consumed in the United States, Great Britain, and Germany. The mica of India and the United States is chiefly muscovite. Canada is the chief source of amber mica (phlogopite), though other deposits of potential importance are known in Ceylon and South Africa. Canadian mica is produced chiefly in Quebec and Ontario, and is exported principally to the United States.
Important deposits of mica (principally muscovite) are also known in Brazil, Argentina, and German East Africa. Large shipments were made from the two former countries during the war, both to Europe and the United States, and Brazil particularly should become of increasing importance as a producer of mica. The deposits in German East Africa were being quite extensively developed immediately before the war and large shipments were made to Germany in 1913.
The United States is the largest consumer of sheet mica and mica splittings, absorbing normally nearly one-half of the world's production. Approximately three-fifths of this consumption is in the form of mica splittings, most of which are made from muscovite in India and part from amber mica in Canada. Due to the cheapness of labor in India and the amenability of Indian mica to the splitting process, India splittings should continue to dominate the market in this country. Amber mica is a variety peculiarly adapted to certain electrical uses. There are no known commercial deposits of this mica in the United States, but American interests own the largest producing mines in Canada. Shipments of Brazilian mica are not of such uniformly high quality as the Indian material, but promise to become of increasing importance in American markets.
Of the sheet mica consumed annually, the United States normally produces about one-third. War conditions, although stimulating the production of domestic mica very considerably, did not materially change the situation in this country as regards the dependence of the United States on foreign supplies for sheet mica.
About 70 per cent of the domestic mica comes from North Carolina and 25 per cent from New Hampshire. The deposits are small and irregular, and mining operations are small and scattered. These conditions are largely responsible for the heterogeneous nature of the American product. It is hardly possible for any one mine to standardize and classify its product, although progress was made in this direction during the war by the organization of associations of mica producers. This lack of standardization and classification is a serious handicap in competition with the standard grades and sizes which are available in any desired amounts from foreign sources.
For ground mica, the domestic production exceeds in tonnage the total world production of sheet mica, and is adequate for all demands.