anhidrite

Anhidrite is a mineral species with the chemical formula CaSO4, representing the anhydrous form of calcium sulfate. It occurs in evaporite sequences and in arid-region sedimentary rocks, forming as clear to white crystals or as coarse granular masses. It is typically colorless to white, but can show pale yellow, gray, brown, or blue tints in some specimens. The mineral has a vitreous to resinous luster, a hardness of 3.5 to 4 on the Mohs scale, and a specific gravity of about 3.0. Anhidrite crystallizes in the orthorhombic system and is commonly found as tabular or prismatic crystals, though it is frequently encountered in massive or nodular form.

Formation and occurrence: Anhidrite forms by the dehydration of gypsum (CaSO4·2H2O) when subjected to heating or burial, and can also precipitate directly from sulfate-rich solutions in evaporitic basins. It hydrates to gypsum when exposed to water. The mineral is commonly associated with gypsum, halite, and other evaporites and occurs in sedimentary basins around the world, with notable deposits in Europe, Africa, the Middle East, the Americas, and Australia.

Uses and distinguishing features: In the laboratory and industry, heating gypsum yields anhydrite, while hydration of anhydrite forms gypsum. Anhidrite is used as a source of calcium sulfate in some cement and plaster formulations, though gypsum is more widely employed for consumer plaster products. Its different hydration behavior means it can influence diagenesis and rock properties where it occurs, making proper identification important in geological and engineering contexts.