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Gypsum (hydrous-calcium sulfate) is the hydrated form of anhydrite. It may occur as finely-granular, thick- bedded, or massive rock. Because the formation of karst is controlled by the rate of physical solution, the karstic features may not develop as well in humid climates because the mechanical weathering of the material is great. However, if the gypsum is located below a layer of another rock type, karst features also can develop in the humid climates.
Gypsum occurs in many different mineralogical forms depending on the degree of hydration. The water content of gypsum minerals varies from 2 molecules to 1/2 molecule of water per calcium sulphate molecule. Above 95 C, gypsum no longer retains any water and forms polymorphs of anhydrite. Satin spar is a fibrous form of gypsum with a silky luster. Alabaster is the fine-grained massive variety. Selenite is a variety which yields with broad colorless and transparent cleavage folia. In drier caves where the deposits are preserved, very unique speleothems (cave deposits) will form.
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