A white solid compound, CaSO4; r.d. 2.96; 1450°C. It occurs naturally as the mineral anhydrite, which has a rhombic structure, transforming to a monoclinic form at 200°C. More commonly, it is found as the dihydrate, gypsum, CaSO4.2H2O (monoclinic; r.d. 2.32). When heated, gypsum loses water at 128°C to give the hemihydrate, 2CaSO4.H2O, better known as plaster of Paris. Calcium sulphate is sparingly soluble in water and is a cause of permanent hardness of water. It is used in the manufacture of certain paints, ceramics, and paper. The naturally occurring forms are used in the manufacture of sulphuric acid.