anhydrite

Anhydrite is a mineral consisting of calcium sulfate in its anhydrous form, with the chemical formula CaSO4. It is the dehydrated counterpart of gypsum (CaSO4·2H2O) and occurs in many evaporite and hydrothermal environments. Anhydrite forms by direct precipitation from saline waters or by dehydration of gypsum during burial in sedimentary basins. It is named from the Greek anhydros, meaning without water.

Physically, anhydrite is typically colorless or white, though it can display blue, violet, or gray tints from impurities. It has a vitreous to silky luster, a hardness of about 3.5 to 4 on the Mohs scale, and a relatively high specific gravity near 3.95. Crystals are commonly tabular to prismatic and belong to the orthorhombic crystal system.

Geologically, anhydrite is widespread in evaporite sequences and often occurs with gypsum, halite, and other soluble minerals. In arid to saline environments it can form extensive beds; burial and heating can convert gypsum to anhydrite, while hydration in the presence of fluids can revert it back to gypsum.

Uses of anhydrite are primarily industrial. It serves as a source of calcium sulfate for chemical and cement industries and can act as a set regulator in cement manufacture, similar to gypsum in some contexts. It is less commonly used directly as a decorative material than gypsum and is sometimes mined as part of evaporite ore bodies.