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1. n. [Geology]

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 cap rock above salt domes.

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.