gypsum, mineral composed of calcium sulfate (calcium, sulfur, and oxygen) with two molecules of water, CaSO₄·2H₂O. It is the most common sulfate mineral, occurring in many places in a variety of forms. A transparent crystalline variety is selenite. A massive gypsum of delicate color and texture, readily worked into ornamental vases, boxes, and the like, is called alabaster. A lustrous gypsum with fibrous structure, called satin spar, is used in jewelry and for other ornaments, but it is soft and easily marred. Plaster of Paris, a fine white powder, is produced by heating gypsum to expel the water. If this powder is moistened and then allowed to dry, it becomes hard, or sets. Its major use is in the manufacture of gypsum lath and wall board, and for casts and molds. It is widely used for staff, the material of which temporary exposition buildings are made. Uncalcined gypsum is added to Portland cement as a retarder.

Common sulfate mineral, hydrated calcium sulfate (CaSO₄·2H₂O), of great commercial importance. Deposits occur in many countries, but the U.S., Canada, France, Italy, and Britain are among the leading producers. Crude gypsum is used as a fluxing agent, soil conditioner, filler in paper and textiles, and retarder in portland cement. About three-fourths of the total production is calcined for use as plaster of paris and as building materials in plaster, board products, and tiles and blocks.

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