Anhydrite's a mineral - anhydrous calcium sulfate, CaSO4. It's in the orthorhombic crystal system, with three directions of perfect cleavage parallel to the three planes of symmetry. It isn't 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's 3.5 and the specific gravity 2.9. The color's w… (More on Anhydrite)