Anhydrite is a mineral—anhydrous
calcium sulfate, CaSO
4. It is in the
orthorhombic crystal system, with three directions of perfect
cleavage parallel to the three planes of
symmetry. It is not
isomorphous with the orthorhombic
barium (
baryte) and
strontium (
celestine) sulfates, as might be expected from the chemical formulas. Distinctly developed
crystals are somewhat rare, the mineral usually presenting the form of cleavage masses. The hardness is 3.5 and the specific gravity 2.9. The color is white, sometimes greyish, bluish, or purple. On the best developed of the three cleavages, the lustre is pearly; on other surfaces it is vitreous. When exposed to water, anhydrite readily transforms to the more commonly occurring
gypsum, (CaSO
4·2H
2O) by the absorption of water. This transformation is reversible, with gypsum or
calcium sulfate hemihydrate forming anhydrite by heating to ~200°C under normal atmospheric conditions. Anhydrite is commonly associated with
calcite,
halite, and
sulfides such as
galena,
chalcopyrite,
molybdenite, and
pyrite in vein deposits.