Chemically known as “calcium sulfate dihydrate” (CaSO4·2H2O), gypsum contains calcium, sulfur bound to oxygen, and water. The majority of gypsum (white rock) produced in North America is used to manufacture gypsum board, building plasters, and cement production. Gypsum is a non-toxic mineral that can be helpful to humans, animals, plant life, and the environment and is widely used as a fertilizer.
Anhydrite (blue-grey rock) is an anhydrous calcium sulfate with a composition of CaSO4. It is closely related to gypsum, which has a chemical composition of CaSO4. Anhydrite is an evaporite mineral with a worldwide abundance much less than gypsum. Anhydrite can be substituted for gypsum in some of its uses such as agriculture and cement production. Both minerals are crushed for use as a soil treatment, and in this purpose anhydrite is superior. One tonne of anhydrite has more calcium than one ton of gypsum – because gypsum is about 21% water by weight and therefore, yields more calcium per tonne in a soil application. Anhydrite also has a higher solubility, which helps it benefit the soil quickly. It is also used as drying agents in plaster, paint, and varnish; used along with gypsum to produce plaster, joint compound, wallboard, and other products for the construction industry; and it has also been used as a source of sulfur in the production of sulfuric acid.
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