Gypsum

Gypsum, common mineral consisting of hydrated calcium sulphate (CaSO₄·2H₂O). It is a widely distributed form of sedimentary rock, formed by the precipitation of calcium sulphate 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 sulphuric acid on calcium-containing minerals; it is also found in most clays as a product of the action of sulphuric acid on limestone. It occurs in all parts of the world; some of the best workable deposits are in France, Switzerland, the United States, and Mexico. 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. Rock phosphate, the essential constituent of which is tricalcium phosphate, is treated with sulphuric acid, producing phosphoric acid and gypsum. The gypsum is compacted into blocks and used for the construction of non-supporting walls in buildings. By properly controlling the concentration and temperature of sulphuric acid added to phosphate rock, a mixture of monocalcium phosphate, dicalcium phosphate, and gypsum may be obtained. This mixture is a valuable fertilizer, known as superphosphate.

Gypsum crystallizes in the monoclinic system in white or colourless crystals, which are massive or foliated in formation. Many specimens are coloured 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 relative density of 2.3. When heated to 128° C (262.4° F), it loses part of its water of crystallization and is converted into plaster of Paris, CaSO₄ ·yH₂O. Finely ground plaster of Paris, when mixed to a paste 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 small spaces on 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 for dry, 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.