What is 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.
Anhydrite also occurs as a vein-filling mineral in hydrothermal deposits. It is deposited from solution, often along with calcite and halite, as gangue in sulfide mineral deposits. Anhydrite is also found in the cap rock of salt domes and in cavities of trap rock.
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 receives its name from the Greek "anhydrous" which means "without water." It readily converts to gypsum under humid conditions or in contact with groundwater. This transition involves the absorption of water and a significant change in volume. That expansion can cause deformation in the rock units. If gypsum is heated to about 200 degrees Celsius, it will yield water and be converted to anhydrite. This reaction occurs much less often.