1. n. [Geology]

[CaSO4]

A member of the evaporite group of minerals and the soft rock comprising anhydrite formed by precipitation of calcium sulfate from evaporation of seawater. Anhydrite can also form through the dehydration of gypsum, another sulfate mineral found in evaporites. Anhydrite may occur as a caprock above salt domes.

See: caprock,  evaporite,  gypsum,  mineral,  salt dome

2. n. [Drilling Fluids]

The anhydrous mineral form of calcium sulfate, CaSO₄. (gypsum, CaSO₄·2H₂O, is the hydrated form.) The presence of anhydrite or gypsum in rock will influence the type of mud selected for drilling the rock because when CaSO₄ dissolves in a water mud, Ca⁺² and SO₄^–2 ions are formed. Although both ions are detrimental to freshwater mud properties, Ca⁺² is the more harmful of the two.

See: calcium contamination,  calcium mud,  gyp mud,  water-based drilling fluid