Anhidrit

Anhidrit, also known as anhydrite, is a mineral form of calcium sulfate with the chemical formula CaSO4. It is the anhydrous counterpart to gypsum (CaSO4·2H2O). The name derives from the Greek for “without water,” reflecting its lack of crystallized water compared with gypsum.

Occurrence and formation

Anhidrite forms in sedimentary evaporite deposits where seawater or saline lakes evaporate and concentrate sulfate. It can also develop from the dehydration of gypsum during burial or heating. It is commonly associated with other evaporite minerals such as gypsum, halite, sylvite, celestine, and dolomite and is found in evaporite basins worldwide, notably in Permian and Mesozoic sequences.

Crystal structure and appearance

Anhidrite crystallizes in the orthorhombic system and occurs as tabular to prismatic crystals, or in massive and fibrous aggregates. The color is typically colorless to white, but impurities can impart gray, brown, or blue tints. It has a vitreous to pearly luster on cleavage surfaces and ranges from transparent to translucent.

Properties

Hardness on the Mohs scale is about 3 to 3.5, and the specific gravity is around 3.0. It is relatively less soluble in water than gypsum and can persist as a dry mineral in exposed evaporite settings. Anhydrite can alter to gypsum upon hydration when exposed to water.

Uses and significance

Anhydrite is mined as a source of sulfate for chemical industries and, in cement production, may be added to regulate setting time. It also serves as a geological marker in evaporite-bearing basins and can indicate particular diagenetic conditions in sedimentary rocks.