Anhydrite is calcium sulfate stripped of water, and that absence is the whole point. It forms in evaporite basins where saline waters concentrate enough dissolved ions for CaSO4 to precipitate, or by dehydration of gypsum at elevated temperature and burial conditions. Most people meet gypsum first and think of sulfate minerals as soft, hydrated, and almost provisional. Anhydrite is what remains when the system dries further or is driven harder, a drier architecture that can later take water back and expand into gypsum again.
The mineral carries reversibility inside its name.
In sedimentary settings, the parent environment is an evaporating restricted basin, sabkha, or saline lagoon where calcium and sulfate activities rise as water is removed. As burial proceeds, gypsum can dehydrate to anhydrite, especially where temperature rises and pore fluids change. In hydrothermal veins, anhydrite can precipitate directly from sulfate rich fluids at elevated temperatures.
Pressure matters mostly because it accompanies burial and compaction, but fluid activity and water availability are decisive. This is a mineral that tells a hydrologic story as much as a thermal one.
Anhydrite is orthorhombic. The sulfate tetrahedra are isolated, coordinated with calcium in a framework that is more compact than gypsum because it lacks interlayer water. That seemingly modest structural difference changes everything. The crystal is harder, denser, and less obviously yielding than its hydrated counterpart. It often occurs in massive beds, granular aggregates, fibrous habits, or thick tabular crystals, and those forms reflect a lattice built to hold together under drier conditions.
Because it can revert to gypsum on hydration, many exposures of anhydrite at the water table interface are unstable over geologic time, swelling, cracking, or transforming as water returns.
So anhydrite is not simply dry gypsum. It is a record of reduction, of a system becoming more concentrated and less forgiving. Yet what remains is not emptiness. Dehydration gives the mineral a more skeletal precision, a sulfate framework tightened by loss. In bodily terms that geology reads as subtraction becoming structure: not everything essential is added. Sometimes a mineral becomes more exact when water leaves, and what remains is a firmer outline, a quieter but more load bearing form built from what the rock could no longer afford to keep.
