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calcium sulfate[′kal·se·əm ′səl‚fāt]
CaSO4, a salt, existing in nature as the dihydrate CaSO4-2H2O (gypsum, selenite) and in an anhydrous state (anhydrite). Anhydrous calcium sulfate occurs in the form of colorless crystals with a density of 2.96 g/cm3 and a melting point of 1450°C. It combines very slowly with water, exhibiting a poor solubility of 0.2036 g per 100 g H2O at 20°C and 0.067 g at 100°C. The half-hydrate CaSO4-1/2 H2O is known; when mixed with water, it hardens rapidly, converting into CaSO4««2H2O. Calcium sulfate is used in the manufacture of figures and casts, as a construction material, and in medicine.