ANHYDRITE

Anhydrite, known as anhydrous calcium sulfate, has the chemical formula CaSO4 and crystallizes in the orthorhombic system, exhibiting perfect cleavage along three symmetry planes. Unlike the orthorhombic sulfates of barium (baryte) and strontium (celestine), it is not isomorphous with them despite similar chemical formulas. Well-formed crystals are relatively uncommon, as the mineral typically appears in cleavage masses. It has a Mohs hardness of 3.5 and a specific gravity of 2.9, with colors ranging from white to shades of grey, blue, or purple. The most developed cleavage exhibits a pearly luster, while other surfaces appear glassy. Anhydrite readily converts to gypsum (CaSO4·2H2O) upon contact with water, and this process is reversible; heating gypsum or calcium sulfate hemihydrate to approximately 200 °C (400 °F) can regenerate anhydrite under normal atmospheric conditions. In vein deposits, anhydrite is often found alongside calcite, halite, and various sulfides, including galena, chalcopyrite, molybdenite, and pyrite.