Anhydrite specimen from Chihuahua, Mexico, in the Harvard Museum of Natural History. Source: Wikipedia.
Anhydrite is a mineral form of anhydrous calcium sulfate (CaSO4, see calcium), occurring worldwide as white-gray masses of orthorhombic crystals. It is an evaporite that is typically interbedded with halite, gypsum, and limestone. Thick beds of anhydrite were formed where old seawater lagoons evaporated. It also occurs as a vein-filling mineral in hydrothermal deposits.
Anhydrite crystallizes in the orthorhomic system and cleaves in three directions at right angles to form cubic fragments. Its cleavage gives rise to its nickname, 'cube spar'.
Anhydrite is used in producing sulfuric acid and ammonium sulfate and as a drying agent. Hardness 3–3.5, specific gravity 2.9–3.