Anhydrite

Anhydrite is a mineral from the mineral class of sulfates and belongs to the chemical group of calcium sulfates. It has the chemical formula Ca[SO4] and is chemically calcium sulfate. The name "anhydrite" comes from the Greek and means "stone without water". Unlike the chemically related gypsum, anhydrite does not contain water.

Anhydrite usually occurs in coarse-grained, massive aggregates, but can also form cubic and prismatic crystals up to about 20 cm in size. In its pure state, anhydrite is transparent and colorless. However, due to lattice defects or polycrystalline structures, it can be translucent white and, depending on impurities, can take on a bluish, reddish, violet or brown color. The line color of anhydrite is always white. The surfaces of anhydrite crystals exhibit a vitreous luster, while lamellar or granular aggregates tend to have a pearlescent to greasy luster.

Anhydrite has a Mohs hardness of 3 to 3.5, which means it can be scratched with a copper coin. The density is about 2.98 g/cm³. The mineral has perfect cleavage to {010}, near perfect cleavage to {100}, and good cleavage to {001}. The fracture is uneven to splintery, and anhydrite is brittle.

In terms of optical properties, anhydrite is biaxially positive and exhibits pleochroism. In purple material, anhydrite may exhibit pleochroism where the color varies in different directions. Anhydrite is sparingly soluble in water and has a solubility of about 2 g/l at 25 °C.

The formation of anhydrite occurs primarily through the evaporation of water containing dissolved salts. Optimal conditions for the formation of anhydrite are found in lagoons or lakes where water evaporates and high temperatures and low humidity prevail. Primarily formed anhydrite can also occur in hydrothermal veins or in the vicinity of magmatic intrusions.

In geologic history, anhydrite can convert to gypsum by absorbing water and changing its crystal structure. This process is called hydration. Anhydrite can also be converted to gypsum or lime by pressure and temperature.

In terms of use, anhydrite has several applications. For example, it is used in the building materials industry to produce building materials such as cement and gypsum board. Anhydrite can also be used as a fertilizer and is used in some industrial processes.