Which compound is the main component of the mineral gypsum, with the formula CaSO₄·2H₂O?

The compound that forms the mineral gypsum and has the formula CaSO₄·2H₂O is calcium sulfate dihydrate.

Calcium sulfate dihydrate contains calcium sulfate plus two waters of crystallization for each formula unit. Those water molecules are part of the crystal structure, not simply moisture trapped between grains. Gypsum is usually colorless, white, or pale-colored and has a relatively low hardness.

When gypsum is heated, it loses part of its crystal water and becomes calcium sulfate hemihydrate, commonly called plaster of Paris. Adding water to plaster of Paris allows it to set back into a hydrated calcium sulfate material. This reversible transformation explains gypsum’s importance in plaster, wallboard, casts, and molds.

Anhydrite is calcium sulfate without the two waters of crystallization. Limestone is mainly calcium carbonate, so it is chemically different even though both minerals can occur in building materials.