Anhydrite

Lore & Background

Anhydrite is a mineral composed of anhydrous calcium sulfate, with a chemical formula of CaSO₄. It crystallizes in the orthorhombic system and exhibits three directions of perfect cleavage, each parallel to a plane of symmetry. Unlike the orthorhombic sulfates of barium and strontium, it is not isomorphous with them.

Well-formed crystals are uncommon, and the mineral typically appears as cleavage masses. The color is generally white, sometimes greyish, bluish, or purple. On the most developed cleavage surface, the luster is pearly, while on other surfaces it is glassy. When exposed to water, anhydrite readily absorbs moisture and transforms into gypsum, a process that can be reversed by heating gypsum or calcium sulfate hemihydrate under normal atmospheric conditions.

Anhydrite is frequently associated with calcite, halite, and sulfides such as galena, chalcopyrite, molybdenite, and pyrite in vein deposits. It is most often found in evaporite deposits alongside gypsum, first discovered in 1794 in a salt mine near Hall in Tirol. Depth is critical in such occurrences, as near the surface anhydrite alters to gypsum through groundwater absorption.

In aqueous solutions, calcium sulfate deposits as gypsum, but if the solution contains excess sodium or potassium chloride, anhydrite forms at elevated temperatures, a process that mirrors its natural origin in salt basins. In tidal flat environments, such as the Persian Gulf sabkhas, anhydrite occurs as massive diagenetic replacement nodules with a netted, chicken-wire texture. It also forms in salt dome caprocks, where it constitutes 1–3% of the minerals, left as a cap after halite is removed by pore waters. In oil fields, high-temperature interaction with hydrocarbons can reduce sulfate to hydrogen sulfide, precipitating calcite in a process called thermochemical sulfate reduction.

Igneous rocks

Anhydrite has also been found in igneous rocks, including the intrusive dioritic pluton of El Teniente, Chile, and in trachyandesite pumice from El Chichón volcano, Mexico. The name was given by A. G. Werner in 1804, referring to the absence of water of crystallization. Obsolete names include muriacite and karstenite.

Reader's Guide

Anhydrite is significant primarily as a mineral in evaporite deposits and salt dome cap rocks, where it plays a role in the geological cycle of calcium sulfate. Its transformation to gypsum upon water absorption is a key process in subsurface environments, as demonstrated by the extensive structural damage in Staufen im Breisgau, Germany, where geothermal drilling allowed water to invade an anhydrite layer, causing ground swelling.

The mineral also participates in thermochemical sulfate reduction in oil fields, reducing sulfate to hydrogen sulfide and precipitating calcite. Industrially, anhydrite has been used in ornamental carving, as seen in a relief carving of an anhydrite kiln at the Catalyst Science Discovery Centre in Widnes, England. Its naming history reflects early confusion with chlorides, and its varieties like angelite are used for decorative purposes.