In geology, gypsum is the mineral with the chemical formula CaSO₄·2H₂O.
The formula shows that gypsum is hydrated calcium sulfate: two water molecules are incorporated into its crystal structure. It commonly forms when saline water evaporates, making it a characteristic mineral of evaporite deposits. It can also occur in sedimentary environments and as a product of mineral alteration.
Gypsum is soft, ranking 2 on the Mohs hardness scale, and can be scratched with a fingernail. Transparent gypsum crystals are sometimes called selenite, while fine-grained massive varieties are known as alabaster.
Anhydrite has the same calcium sulfate composition without the water molecules. That distinction is a common source of confusion, but gypsum and anhydrite are separate minerals.