The common name for the naturally occurring calcium sulfate. Anhydrite is calcium sulfate (CaSO<sub>4</sub> ) existing in the orthorhombic crystalline form. The anhydrite crystal cannot become a gypsum crystal (monoclinic form) in a single [[hydration]] step. Anhydrite must first become dissociated in solution before it can crystallize as gypsum (or recrystallize as anhydrite). Compare [[gypsum]].