A mineral consisting of anhydrous calcium sulfate: CaSO4. It represents gypsum without its water of crystallization, and it alters readily to gypsum from which it differs in crystal form (anhydrite is orthorhombic) and in being harder and slightly less soluble. Anhydrite usually occurs in white or slightly colored, granular to compact masses, forming large beds or seams in sedimentary rocks [sedimentary rock] or associated with gypsum and halite in evaporites [evaporite]. Syn: cube spar.