Gypsum

Gypsum, common mineral consisting of hydrated calcium sulfate (CaSO₄·2H₂O). It is a widely distributed form of sedimentary rock, formed by the precipitation of calcium sulfate from seawater, and is frequently associated with other saline deposits, such as halite and anhydrite, as well as with limestone and shale. Gypsum is produced in volcanic regions by the action of sulfuric acid on calcium-containing minerals; it is also found in most clays as a product of the action of sulfuric acid on limestone. It occurs in all parts of the world; some of the best workable deposits are in France, Switzerland, and Mexico, as well as in California, Ohio, Michigan, and Utah in the United States. Alabaster, selenite, and satin spar are varieties of gypsum.

Artificial gypsum is obtained as a by-product in an old method for the manufacture of phosphoric acid. Phosphate rock, the essential constituent of which is tricalcium phosphate, is treated with sulfuric acid, producing phosphoric acid and gypsum. The gypsum is compacted into blocks and used for the construction of nonsupporting walls in buildings. By properly controlling the concentration and temperature of sulfuric acid added to phosphate rock, a mixture of monocalcium phosphate, dicalcium phosphate, and gypsum may be obtained. This mixture is a valuable fertilizer, superphosphate.

Gypsum crystallizes in the monoclinic system in white or colorless crystals, massive or foliated in formation. Many specimens are colored green, yellow, or black by impurities. With a hardness ranging from 1.5 to 2, it is soft enough to scratch with a fingernail and has a specific gravity of 2.3. When heated to 128° C (262.4° F), it loses part of its water of hydration and is converted into plaster of Paris, CaSO₄·yH₂O. Finely ground plaster of Paris, when moistened with water, sets in a short time into a hard mass of gypsum, the rehydrated crystals forming and interlocking in such a way as to cause expansion in volume.

Because of its property of swelling and filling all interstices upon drying, plaster of Paris is used extensively in making casts for statuary, ceramics, dental plates, fine metal parts for precision instruments, and surgical splints. Uncalcined gypsum is used as a fertilizer in the form of land plaster for arid, alkaline soil. It is also used as a bed for polishing plate glass and as a basis for paint pigments. Large amounts of gypsum are used as a retarder in portland cement.