Examples from the Web for anhydrite
Anhydrite (CaSO4) also occurs in rock masses, and is often associated with rock salt.A Textbook of Assaying: For the Use of Those Connected with Mines.|Cornelius Beringer and John Jacob Beringer
Anhydrite, anhydrous sulphate of calcium, a mineral presenting several varieties of structure and colour.
Conversely gypsum may, under certain conditions, be dehydrated or reduced to anhydrite.
These observations appear to show that the change to anhydrite is a very gradual process at temperatures below a red heat.