gypsum

gypsum , mineral composed of calcium sulfate (calcium, sulfur, and oxygen) with two molecules of water, CaSO ₄ ·2H ₂ O. 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.

gypsum hydrated calcium sulphate, from which plaster of Paris is made. XVII. — L. — Gr. gúpsos, of Sem. orig.
So gypseous XVII. f. late L. gypseus.

gypsum (hydrated calcium sulphate, CaSO₄.2H₂O) Most common sulphate mineral. Huge beds of gypsum occur in sedimentary rocks, where it is associated with halite. It crystallizes in the monoclinic system. Varieties are alabaster, selenite (transparent and foliated) and satinspar (silky and fibrous). It is a source of plaster of Paris. Hardness 2; r.d. 2.3.