Gypsum

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Gypsum is a very soft mineral composed of calcium sulfate dihydrate, with the chemical formula CaSO4·2H2O.[3]

Contents

Etymology

The word gypsum is derived from the Greek word γύψος gypsos, "chalk" or "plaster".[4] Because the gypsum from the quarries of the Montmartre district of Paris has long furnished burnt gypsum used for various purposes, this material has been called plaster of Paris.

Physical properties

Gypsum is moderately water-soluble (~2.0–2.5 g/L at 25 °C)[5] and, in contrast to most other salts, it exhibits a retrograde solubility, becoming less soluble at higher temperatures. As for anhydrite, its solubility in saline solutions and in brines is also strongly dependent on NaCl concentration.[5]

Gypsum crystals are found to contain anion water and hydrogen bonding.[6]

Crystal varieties

Gypsum occurs in nature as flattened and often twinned crystals and transparent cleavable masses called selenite. It may also occur in a silky, fibrous form, in which case it is commonly called satin spar. Finally it may also be granular or quite compact. In hand-sized samples, it can be anywhere from transparent to opaque. A very fine-grained white or lightly tinted variety of gypsum is called alabaster, and is prized for ornamental work of various sorts. In arid areas, gypsum can occur in a flower-like form typically opaque with embedded sand grains called desert rose. Up to the size of 11 m long, gypsum forms some of the largest crystals found in nature, in the form of selenite.[7]

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