Gypsum is a very soft sulphate mineral composed of calcium sulphate dihydrate, with the chemical formula CaSO4⋅2H2O. On heating, gypsum loses water and gives the hemihydrate (CaSO4⋅
1
2
H2O) or the anhydrite.
2CaSO4⋅2H2O
gypsum
120∘C
──────▶
(CuSO4)2⋅H2O+3H2O
plasterofParis(Calciumsulphatehemihydrate)
The hemihydrate is known as calcined gypsum or plaster of Paris.