From Wikipedia, the free encyclopedia.
Gypsum is a very soft mineral composed of hydrous calcium sulfate, with the formula CaSO4·2H2O. The anhydrous form is called anhydrite.
desert rose, 10 cm long|
It occurs as flattened and often twinned crystals in transparent cleavable masses called selenite, it may also occur silky and fibrous, 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 often called desert rose.
Gypsum is a very common mineral, with thick and extensive beds association with sedimentary rocks. The largest deposits known occur in strata from the Permian age. Gypsum is deposited in lake and sea water, as well as in hot springs and from volcanic vapors, and sulfate solutions in veins.
The word gypsum is derived from the Greek meaning 'to cook', in reference to the burnt or calcined mineral. 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.
|Colours||Colourless to white and grey though it may be tinted red, yellow, blue, brown, etc. by impurities.|
|Lustre||Vitreous to silky or pearly lustre|
|Refractive index||1.522( average)|
|Cleavage||2 good (66° and 114°)|