Gypsum is a hydrated calcium sulfate with composition CaSO₄·2H₂O. It commonly occurs as a chalky powder or occasionally as small crystals. In gypsum, the structure is layered, parallel to the b direction. Tetrahedral SO₄ anions are linked by Ca cations as chains parallel to c, which form as double sheets interlayered with sheets of water molecules.

Dehydrated forms of gypsum range isostructurally from CaSO₄·2H₂O to CaSO₄. The hemihydrate CaSO₄·0.5H₂O is a stable form known commercially as plaster of Paris and in nature as bassanite.

Distribution

Gypsum is an important rock-forming mineral in evaporite deposits of chemical sedimentary rocks, where they may be associated with halite, and in carbonates in association with barite. Gypsum deposition may form massive and stratified beds that are several meters thick, usually in association with beds of limestone, red clays, halite and other evaporite minerals. Large crystals have been found in several localities in the USA (New York, Utah, Ohio, Oklahoma), Australia and Mexico. Extensive beds occur in the USA, Saharan Africa, and Mexico. Land formations containing high quantities of gypsum or anhydrite should be avoided for large-scale construction projects involving water such as dams or irrigation canals because the rapid dissolution of these minerals may cause the development of pseudo-karst landscapes and failure of the structures.

Gypsum is a common primary and secondary mineral of arid and semi-arid region soils. Gypsum is by far the dominant sulfate mineral in soils. A saturated gypsum solution at 25C contains ~15 mM CaSO₄, or 2.63 g/L. It is approximately 100 times less soluble than other common sulfate minerals. Gypsum is not common in soils of more humid regions due to its relatively high solubility in water and its rapid removal in leachate. It may also be a major constituent of sedimentary rocks. Gypsum amendments to soils are sometimes recommended to improve soil structure and supply Ca and S to plants; such treatments do not persist in humid environments because of dissolution and leaching of gypsum.

Gypsum is widely used as an agricultural amendment for saline soils, where it is used as a flocculating agent. Its relatively high solubility allows for sufficient Ca2+ concentrations in solution to keep clays from dispersing, a major problem in sodic and saline soils. It is also the major component of gypsum wall board.