Thermodynamic and experimental data on stability of gypsum,hennihydrate, and anhydrite under hydrothermal conditions

Experimentally, gypsum remains stable at temperatures up to 110°C, whereupon it is replaced by hemihydrate [plaster of Paris], which in turn is replaced by anhydrite at 140°C and over. However, the hemihydrate proves to be metastable by thermodynamic calculations. Anhydrite, by the calculations 6 is a highly stable mineral; its decomposition begins at temperatures exceeding 1385°C. — Author.