gypsum

Gypsum is a soft sulfate mineral with the chemical formula CaSO4·2H2O. It crystallizes in the monoclinic system and has a Mohs hardness of about 2. Its color is typically white or gray, and it commonly forms tabular or needle-like crystals, as well as massive beds with a pearly luster.

Gypsum forms primarily in sedimentary environments through the evaporation and precipitation of sulfate-rich waters. It often occurs with halite and anhydrite and can also develop by alteration of anhydrite or as a low-temperature hydrothermal product. It readily rehydrates and can be found in evaporite sequences worldwide.

Commercially, gypsum is mined from natural deposits and is also produced as synthetic gypsum from flue-gas desulfurization in power plants. The mineral is calcined to produce calcium sulfate hemihydrate, known as plaster of Paris, which rehydrates to gypsum when mixed with water. Gypsum is widely processed into plaster, plasterboard (drywall), and other building materials, and it is used as a cement additive to influence setting time and workability.

Uses extend beyond construction: gypsum is employed in sculpting and medical casts, as a soil conditioner to improve structure and supply calcium and sulfur, and in various industrial applications. It is chemically inert under normal conditions and is non-flammable, though dust can irritate the respiratory system if inhaled.

The name derives from the Greek gypsos, meaning gypsum. Gypsum has been utilized since antiquity for plaster and construction, with the term plaster of Paris originating from early plaster quarries near Paris.