Gypsum is deposited from lake and sea water, as well as in hot springs, from volcanic vapors, and sulfate solutions in veins. Deposits are known to occur in strata from as far back as the Archaean eon. Gypsum is a common mineral, with thick and extensive evaporite beds in association with sedimentary rocks. It also forms some of the largest crystals found in nature, up to 12 m (39 ft) long, in the form of selenite. In arid areas, gypsum can occur in a flower-like form, typically opaque, with embedded sand grains called desert rose. A very fine-grained white or lightly tinted variety of gypsum, called alabaster, is prized for ornamental work of various sorts. In hand-sized samples, it can be anywhere from transparent to opaque. Finally, it may also be granular or quite compact. Selenite may also occur in a silky, fibrous form, in which case it is commonly called 'satin spar'. Selenite contains no significant selenium rather, both substances were named for the ancient Greek word for the Moon. Gypsum occurs in nature as flattened and often twinned crystals, and transparent, cleavable masses called selenite. American farmers were so anxious to acquire it that a lively smuggling trade with Nova Scotia evolved, resulting in the so-called 'Plaster War' of 1820. Gypsum may act as a source of sulfur for plant growth, and in the early 19th century, it was regarded as an almost miraculous fertilizer. In the mid-18th century, the German clergyman and agriculturalist Johann Friderich Mayer investigated and publicized gypsum's use as a fertilizer. (Thus, the word spar in mineralogy is by way of comparison to gypsum, referring to any non-ore mineral or crystal that forms in spearlike projections). Gypsum was known in Old English as spærstān, 'spear stone', referring to its crystalline projections. Upon adding water, after a few dozen minutes, plaster of Paris becomes regular gypsum (dihydrate) again, causing the material to harden or 'set' in ways that are useful for casting and construction. Because the quarries of the Montmartre district of Paris have long furnished burnt gypsum ( calcined gypsum) used for various purposes, this dehydrated gypsum became known as plaster of Paris. The word gypsum is derived from the Greek word γύψος ( gypsos), 'plaster'. The Mohs scale of mineral hardness defines gypsum as hardness value 2 based on scratch hardness comparison. It forms as an evaporite mineral and as a hydration product of anhydrite. Gypsum also crystallizes as translucent crystals of selenite. Alabaster, a fine-grained white or lightly tinted variety of gypsum, has been used for sculpture by many cultures including Ancient Egypt, Mesopotamia, Ancient Rome, the Byzantine Empire, and the Nottingham alabasters of Medieval England. It is widely mined and is used as a fertilizer and as the main constituent in many forms of plaster, drywall and blackboard or sidewalk chalk. Gypsum is a soft sulfate mineral composed of calcium sulfate dihydrate, with the chemical formula CaSO 4♲H 2O. Elongated and generally prismatic crystalsĬonchoidal on, splintery parallel to

For other uses, see Gypsum (disambiguation).Ĭolorless (in transmitted light) to white often tinged other hues due to impurities may be yellow, tan, blue, pink, dark brown, reddish brown or gray

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