Atomic Positional Parameters
Ca 4c 0.7500 0.0000 0.3476
S  4c 0.2500 0.0000 0.1556
O1 8g 0.2500 0.1699 0.0162
O2 8f 0.0819 0.0000 0.2975

Reference
FC Hawthorne and RB Ferguson (1975) Canadian Mineralogist, 13, 289-292

Mineral Chemistry
Anhydrite is found as a constituent of evaporite deposits, typically associated with gypsum, alkali halides and carbonates. This sulphate mineral is also observed in submarine hydrothermal systems, and in some igneous rocks. The structure of anhydrite comprises sulphate (SO₄) tetrahedra and calcium coordinated by eight oxygens. The name anhydrite derives from the composition of this mineral - there is no water of crystallisation, which distinguishes this phase from gypsum (CaSO₄.2H₂O).

Anhydrite cannot be hydrated easily. To form gypsum from anhydrite at low temperatures, dissolution and reprecipitation may be necessary. Direct heating of gypsum to about 360^oC is required to form anhydrite; at lower temperatures, gypsum dehydrates to g-CaSO₄. Anhydrite has retrograde solubility in water with temperature, but increases in solubility with pressure. Experimental formation of anhydrite under aqueous conditions which simulate evaporitic fluids has been explored by a number of researchers. Increased salinity decreases the temperature at which gypsum can transform to anhydrite, thus appearing to favour anhydrite formation, although the exact mechanism(s) of anhydrite formation in evaporites may still require further investigation.

The CHIME figure shows calcium ions as large spheres with the bonds from oxygens to calciums shown as thin red lines. This should make the sulphate tetrahedra easier to identify.