Anhydrite

Anhydrite, or Karstenite, the anhydrous sulphate of lime, so called in contradistinction to gypsum, the hydrous sulphate. Like most anhydrous sulphates, anhydrite crystallises in the prismatic system. It is much harder and slightly heavier than gypsum, is white and translucent, and occurs in beds associated with gypsum and rock salt.