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.

Related Terms:

water-base drilling fluid