The word gypsum is derived from the Greek word γύψος (gypsos), "plaster". 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. Upon addition of water, after a few tens of 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.
Gypsum was known in Old English as spærstān, "spear stone", referring to its crystalline projections. (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). In the mid-18th century, the German clergyman and agriculturalist Johann Friderich Mayer investigated and publicized gypsum's use as a fertilizer. 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. 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. In the 19th century, it was also known as lime sulfate or sulfate of lime.
Gypsum crystals are soft enough to bend under pressure of the hand. Sample on display at Musée cantonal de géologie de Lausanne.
Gypsum is moderately water-soluble (~2.0–2.5 g/l at 25 °C) and, in contrast to most other salts, it exhibits retrograde solubility, becoming less soluble at higher temperatures. When gypsum is heated in air it loses water and converts first to calcium sulfate hemihydrate, (bassanite, often simply called "plaster") and, if heated further, to anhydrous calcium sulfate (anhydrite). As for anhydrite, its solubility in saline solutions and in brines is also strongly dependent on NaCl (common table salt) concentration.
Gypsum occurs in nature as flattened and often twinnedcrystals, and transparent, cleavable masses called selenite. Selenite contains no significant selenium; rather, both substances were named for the ancient Greek word for the Moon.
Selenite 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, called alabaster, 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. It also forms some of the largest crystals found in nature, up to 12 m (39 ft) long, in the form of selenite.
Gypsum is a common mineral, with thick and extensive evaporite beds in association with sedimentary rocks. Deposits are known to occur in strata from as far back as the Archaeaneon. Gypsum is deposited from lake and sea water, as well as in hot springs, from volcanic vapors, and sulfate solutions in veins. Hydrothermalanhydrite in veins is commonly hydrated to gypsum by groundwater in near-surface exposures. It is often associated with the minerals halite and sulfur. Gypsum is the most common sulfate mineral. Pure gypsum is white, but other substances found as impurities may give a wide range of colors to local deposits.
Because gypsum dissolves over time in water, gypsum is rarely found in the form of sand. However, the unique conditions of the White Sands National Park in the US state of New Mexico have created a 710 km2 (270 sq mi) expanse of white gypsum sand, enough to supply the US construction industry with drywall for 1,000 years.
Commercial exploitation of the area, strongly opposed by area residents, was permanently prevented in 1933 when President Herbert Hoover declared the gypsum dunes a protected national monument.
Commercial quantities of gypsum are found in the cities of Araripina and Grajaú in Brazil; in Pakistan, Jamaica, Iran (world's second largest producer), Thailand, Spain (the main producer in Europe), Germany, Italy, England, Ireland and Canada and the United States. Large open pit quarries are located in many places including Fort Dodge, Iowa, which sits on one of the largest deposits of gypsum in the world, and Plaster City, California, United States, and East Kutai, Kalimantan, Indonesia. Several small mines also exist in places such as Kalannie in Western Australia, where gypsum is sold to private buyers for additions of calcium and sulfur as well as reduction of aluminum toxicities on soil for agricultural purposes.
Crystals of gypsum up to 11 m (36 ft) long have been found in the caves of the Naica Mine of Chihuahua, Mexico. The crystals thrived in the cave's extremely rare and stable natural environment. Temperatures stayed at 58 °C (136 °F), and the cave was filled with mineral-rich water that drove the crystals' growth. The largest of those crystals weighs 55 tonnes (61 short tons) and is around 500,000 years old.
As alabaster, a material for sculpture, it was used especially in the ancient world before steel was developed, when its relative softness made it much easier to carve.
A wood substitute in the ancient world: For example, when wood became scarce due to deforestation on Bronze AgeCrete, gypsum was employed in building construction at locations where wood was previously used.
A tofu (soy bean curd) coagulant, making it ultimately a major source of dietary calcium, especially in Asian cultures which traditionally use few dairy products
^Anthony, John W.; Bideaux, Richard A.; Bladh, Kenneth W.; Nichols, Monte C., eds. (2003). "Gypsum"(PDF). Handbook of Mineralogy. V (Borates, Carbonates, Sulfates). Chantilly, VA, US: Mineralogical Society of America. ISBN978-0962209703.
Thaer, Albrecht Daniel; Shaw, William, trans.; Johnson, Cuthbert W., trans. (1844). The Principles of Agriculture. vol. 1. London, England: Ridgway. pp. 519–520.
Klaus Herrmann (1990), "Mayer, Johann Friedrich", Neue Deutsche Biographie (NDB) (in German), 16, Berlin: Duncker & Humblot, pp. 544–545; (full text online) From p. 544: " … er bewirtschaftete nebenbei ein Pfarrgüttchen, … für die Düngung der Felder mit dem in den nahen Waldenburger Bergen gefundenen Gips einsetzte." ( … he also managed a small parson's estate, on which he repeatedly conducted agricultural experiments. In 1768, he first published the fruits of his experiences during this time as "Instruction about Gypsum", in which he espoused the fertilizing of fields with the gypsum that was found in the nearby Waldenburg mountains.)
Beckmann, Johann (1775). Grundsätze der deutschen Landwirthschaft [Fundamentals of German Agriculture] (in German) (2nd ed.). Göttingen, (Germany): Johann Christian Dieterich. p. 60. From p. 60: "Schon seit undenklichen Zeiten … ein Gewinn zu erhalten seyn wird." (Since times immemorial, in our vicinity, in the ministry of Niedeck [a village southeast of Göttingen], one has already made this use of gypsum; but Mr. Mayer has the merit to have made it generally known. In the History of Farming in Kupferzell, he had depicted a crushing mill (p. 74), in order to pulverize gypsum, from which a profit has been obtained, albeit with difficulty.)
^Smith, Joshua (2007). Borderland smuggling: Patriots, loyalists, and illicit trade in the Northeast, 1780–1820. Gainesville, FL: UPF. pp. passim. ISBN978-0-8130-2986-3.
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^Mandal, Pradip K; Mandal, Tanuj K (2002). "Anion water in gypsum (CaSO4·2H2O) and hemihydrate (CaSO4·1/2H2O)". Cement and Concrete Research. 32 (2): 313. doi:10.1016/S0008-8846(01)00675-5.
^Rodriguez, J. D.; Jimenez, A.; Prieto, M.; Torre, L.; Garcia-Granda, S. (2008). "Interaction of gypsum with As(V)-bearing aqueous solutions: Surface precipitation of guerinite, sainfeldite, and Ca2NaH(AsO4)2⋅6H2O, a synthetic arsenate". American Mineralogist. 93 (5–6): 928. Bibcode:2008AmMin..93..928R. doi:10.2138/am.2008.2750.
^Rodríguez-Blanco, Juan Diego; Jiménez, Amalia; Prieto, Manuel (2007). "Oriented Overgrowth of Pharmacolite (CaHAsO4⋅2H2O) on Gypsum (CaSO4⋅2H2O)". Cryst. Growth Des. 7 (12): 2756–2763. doi:10.1021/cg070222+.