This paper reviews the work leading up to the statement by Linck and Jung that only three crystal forms of calcium sulphate exist. It describes some new X-ray analyses of gypsum dehydrated to less than 0.3 per cent H2O which show that this material, commonly called “soluble anhydrite,” has a crystal structure identical with that of the hemihydrate, and supports the hypothesis of Linck and Jung that the hemihydrate is zeolitic in nature, losing and regaining its water of hydration without change in crystal structure.
An equilibrium system for the three forms of calcium sulphate in contact with water is described, and some original experiments are recorded which show that gypsum is converted into anhydrite when in contact with water at 100°C, probably passing through the hemihydrate as an intermediate stage. Similar experiments conducted with anhydrite in contact with water at 11°–15°C show that it is slowly converted into gypsum. Photomicrographs of both transitions are included.