Anhydrite is an anhydrous calcium sulfate with a composition of CaSO₄. It is closely related to gypsum, which has a chemical composition of CaSO₄.2H₂O. The worldwide abundance of gypsum greatly exceeds the abundance of anhydrite.

Anhydrite is an evaporite mineral that occurs in extensive layered deposits in sedimentary basins where large volumes of sea water have been evaporated. It is typically interbedded with halite, gypsum, and limestone in accumulations that can be up to hundreds of feet thick. On a much smaller scale, anhydrite can form in shoreline or tidal flat sediments from the evaporation of sea water.

Physical properties

One of the most distinctive properties of anhydrite is its cubic cleavage. It cleaves in three directions at right angles. This can easily be seen in coarsely crystalline specimens or with a hand lens in fine-grained specimens. This distinctive cleavage has earned anhydrite the nickname of "cube spar."

Synthesis

Hydrofluoric acid is produced using fluorite and sulfuric acid. For every ton of hydrofluoric acid produced, about 3 1/2 tons of synthetic anhydrite is produced. For decades this synthetic anhydrite was considered to be a nuisance product that had a disposal expense. However, much of it is now dried in a kiln and used as a raw material for producing cement, plaster, and flooring. It is also used as a filler in the production of plastics and paper products.

Uses

Anhydrite can be substituted for gypsum in some of its uses. Both minerals are crushed for use as a soil treatment, and in this purpose anhydrite is superior. One ton of anhydrite has more calcium than one ton of gypsum - because gypsum is about 21% water by weight. This yields more calcium per ton in a soil application. Anhydrite also has a higher solubility, which helps it benefit the soil quickly.

Small amounts of anhydrite are used as drying agents in plaster, paint, and varnish. It is also used along with gypsum to produce plaster, joint compound, wallboard, and other products for the construction industry. Anhydrite has also been used as a source of sulfur in the production of sulfuric acid.