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gypsum


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Evaporite mineral, CaSO4.2H2O; sp. gr. 2.3; hardness 1.5–2; monoclinic; clear white, but sometimes shades of yellow, grey, red, and brown; white streak; vitreous lustre; crystals usually tabular, often with curved faces, but can occur massive or granular; cleavage perfect {010}, good {100}, {011}; occurs in bedded deposits in association with halite and anhydrite. It is very insoluble and therefore the first mineral to precipitate from evaporating sea water; usually succeeded by anhydrite and halite. Occasionally it results from the reaction of sulphuric acid on limestone in volcanic areas. It can also result from the secondary hydration of anhydrite. Selenite is the colourless, transparent form; satin spar is the fibrous variety; alabaster is the fine-grained variety and can be carved.

Subjects: Earth Sciences and Geography.


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