Anhydrite is a mineral - anhydrous
calcium sulfate, CaSO4. 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, (CaSO4·2H2O) 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