Anhydrit

Anhydrit, also known as anhydrite, is a mineral consisting of calcium sulfate without water of hydration, CaSO4. It is the anhydrous counterpart of gypsum (CaSO4·2H2O) and forms in evaporitic settings and hydrothermal environments. Crystals belong to the orthorhombic system and commonly occur as colorless to white to gray crystals, or in massive granular forms. It has a vitreous to pearly luster, a hardness of 3–3.5 on the Mohs scale, and a specific gravity around 3.9–4.0. It is transparent to opaque and may show color zoning due to impurities.

Formation and dehydration: Anhydrit forms by dehydration of gypsum under evaporitic and arid conditions at temperatures roughly above 40–60°C and low humidity. It can also crystallize directly from sulfate-rich waters or replace gypsum in sedimentary sequences as dehydration proceeds. Rehydration occurs when exposed to water at lower temperatures, converting it back to gypsum.

Occurrence and uses: Anhydrit is widely distributed in evaporite basins worldwide, including Europe and North America, and occurs with gypsum, halite, and other sulfate minerals. Industrially, it is mined as a source of calcium sulfate for plaster, cement, and chemical industries. In cement production, anhydrite can act as a set-regulating additive by hydration to gypsum, influencing curing behavior.

Etymology: The name derives from Greek for "without water."