GYPSUM (also known as GYP), a native hydrated sulphate of calcium, having the formula CaSO4+2}120; the water of crystal lization being the only thing that differentiates it, chemically, from the orthorhombic mineral anhydrite. Gypsum is usually colorless or white. It crystallizes in the monoclinic system, contact twins and penetration twins being very common; and it also occurs in massive forms. The pure crystals have a hardness of from 1.5 to 2.0, and- a specific gravity of about 2.32. Gypsum is an exceedingly abundant substance, and is met with in many parts of the earth, and in a variety of forms. When found in the form of clear, transparent crystals, it is known as selenite; when the mineral is finely fibrous, and the fibres are parallel to one another so as to form a mass with a pearly opalescence, the mineral is called satin spar; when it occurs in uniform, fine-grained, translucent masses, it is known as alabaster; and when it occurs in large beds of massive rock, often mixed with clay, calcium carbonate and other impurities, it constitutes the earthy gypsum, or rock gypsum, of commerce. The mineral is often found as sociated with salt (q.v.) and is believed to have been formed in the same manner by evaporation of lakes in arid regions. Extensive beds of the mineral are known in western United States in the so-called red beds (q.v.) of Permian and Triassic (qq.v.) Age. It also occurs in the salina (q.v.) beds of New York. Gypsum is
soluble in from 400 to 500 parts of water at ordinary temperatures, but it dissolves more freely in hydrochloric acid. When heated, it loses part of its water of crystallization, though it retains the power of recombining with water to form a hard, non-crystalline mass, if the temperature to which it is exposed does not ex ceed 500° F. It is this property of recombining with water which gives to dehydrated gypsum much of its industrial value. (See PLASTER OF PARIS). When heated with charcoal, gypsum is converted into calcium sulphide, which dis solves readily in dilute acids, with evolution of sulphuretted hydrogen gas.. In this way the sparingly soluble sulphate of calcium may be converted into the soluble chloride or nitrate of calcium. Gypsum, when pulverized, is used as a fertilizer, its efficiency in this respect being apparently due in large measure to the fact that it facilitates the decomposition of rocks con taining alkaline silicates. The production of gypsum in the United States in 1914 was 2, 476,465 short tons, valued at $6,895,9,39. New York, Iowa, Michigan, Ohio, Texas and Okla homa were the principal producing States, ranking in the order given. The United States ranks second in the world's productionof gypsum, France being first, and Canada third. See MINERAL PRODUCTION IN THE UNITED STATES.