WHAT IS CALCIUM SULFATE?
Calcium sulfate is a chemical compound made of one molecule of calcium, one molecule of sulfur, and four molecules of oxygen: CaSO4. Its purest form, straight CaSO4, is called anhydrite gypsum, with anhydrite meaning ‘no water’. More commonly, it occurs with two water molecules attached to it in crystalline form, CaSO4.2 H2O, widely known as dihydrate gypsum, with dihydrate meaning ‘two waters’.

Both minerals are found naturally in various crystalline forms, soft-to-medium hard rocks that include some exotic-looking forms such as desert rose. Calcium sulfate is found in ancient sea beds, where the different minerals collected in layers on the sea bottom may be hundreds of feet thick. Anhydrite gypsum and dihydrate gypsum often occur close together, and the mined minerals may contain both. Other minerals such as limestone may be beneath the anhydrite. Natural deposits are never completely pure. Even a very high-quality natural source could only be expected to be about 95-98% of the pure mineral.

Both dihydrate gypsum and anhydrite gypsum deposits are widely distributed over the earth, although in North America, dihydrate gypsum is far more common. There are major anhydrite gypsum deposits on the North American continent, located in Nevada.

Dihydrate gypsum is a soft rock. On the MOHS hardness scale, it is the defining material for Hardness 2, soft enough to be scratched by a fingernail. (The MOHS scale, where 1 is the softest and 10 the hardest, is based on the idea that a harder material can scratch a softer material.) anhydrite gypsum is harder, with a MOHS hardness of 3.5, making it about as scratchable as a copper penny.