Gypsum

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Gypsum
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Desert rose, 10 cm long
General
CategoryMineral
Chemical formulaCalcium Sulfate CaSO4·2H2O
Identification
ColorWhite to grey, pinkish-red
Crystal habitMassive, flat. Elongated and generally prismatic crystals
Crystal systemMonoclinic 2/m
Twinningcommon {110}
Cleavagegood (66° and 114°)
FractureConchoidal, sometimes fibrous
Mohs Scale hardness1.5-2
LusterVitreous to silky, pearly, or waxy
Refractive indexα=1.520, β=1.523, γ=1.530
Optical Properties2V = 58° +
PleochroismNone
StreakWhite
Specific gravity2.31 - 2.33
Fusibility3
Solubilityhot, dilute HCl
Diaphaneitytransparent to translucent
Major varieties
Satin SparPearly, fibrous masses
SeleniteTransparent and bladed crystals
AlabasterFine-grained, slightly colored

Gypsum is a very soft mineral composed of calcium sulfate dihydrate, with the chemical formula CaSO4·2H2O.[1]

Crystal varieties

File:Gypsum Australia.jpg
Gypsum from New South Wales, Australia

Gypsum occurs in nature as flattened and often twinned crystals and 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, which 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. The most visually striking variety, however, is the giant crystals from Naica Mine. Up to the size of 11m long, these megacrystals are among the largest crystals found in nature. A recent publication shows that these crystals are grown under constant temperature such that large crystals can grow slowly but steadily without excessive nucleation.[2]

Occurrence

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 early as the Permian age.[3] Gypsum is deposited in lake and sea water, as well as in hot springs, from volcanic vapors, and sulfate solutions in veins. Hydrothermal anhydrite in veins is commonly hydrated to gypsum by groundwater in near surface exposures. It is often associated with the minerals halite and sulfur.

File:GipsitaEZ.jpg
Fibrous Gypsum from Brazil

The word gypsum is derived from the aorist form of the Greek verb μαγειρεύω, "to cook", referring 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. It is also used in foot creams, shampoos and many other hair products. It is water-soluble.

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 Monument in the US state of New Mexico have created a 710 km² (275 sq mile) expanse of white gypsum sand, enough to supply the construction industry with drywall for 1,000 years.[4] 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 Jamaica, Iran, Thailand, Spain (the main producer in Europe), Germany, Italy, England, Ireland, in British Columbia, Manitoba, Ontario, Nova Scotia and Newfoundland in Canada,[5] and in New York, Michigan, Indiana[5],Texas(in the Palo Duro Canyon),Iowa, Kansas, Oklahoma, Arizona, New Mexico, Colorado, Utah and Nevada in the United States. There is also a large mine located at Plaster City, California in Imperial County, and in East Kutai, Kalimantan.

Vast crystals of gypsum, up to 10 metres in length have been found in the "Cueva de los Crystales" in Naica, Chihuahua, Mexico.[6]

Uses of Gypsum

There are a large number of uses for gypsum throughout prehistory and history. Some of these uses are:

File:Gypsum cones.jpg
Cones of gypsum which formed on the sea floor during the Messinian salinity crisis

References

  1. Cornelis Klein and Cornelius S. Hurlbut, Jr., 1985, Manual of Mineralogy, John Wiley, 20th ed., pp. 352-353, ISBN 0-471-80580-7
  2. Juan Manuel García-Ruiz, Roberto Villasuso, Carlos Ayora, Angels Canals, and Fermín Otálora (2007). "Formation of natural gypsum megacrystals in Naica, Mexico". Geology. 35 (4): 327–330. doi:10.1130/G23393A.1.
  3. Barry F. Beck, Felicity M. Pearson, P.E. LaMoreaux & Associates, National Groundwater Association (U.S.), Karst Geohazards: Engineering and Environmental Problems in Karst Terrane, 1995, Taylor & Francis, 581 pages ISBN:9054105356
  4. Abarr, James (1999-02-07). "Sea of Sand". The Albuquerque Journal. Retrieved 2007-01-27. Check date values in: |date= (help)
  5. 5.0 5.1 "Mines, Mills and Concentrators in Canada". Natural Resources Canada. 2005-10-24. Retrieved 2007-01-27.
  6. NAICA, Cueva de los Cristales, Cave of the Crystals, in the Naica-Peñoles mine, cave of largest selenite (gypsum) crystals in Naica | La Venta Exploring Team
  7. C. Michael Hogan, Knossos fieldnotes, Modern Antiquarian (2007)

External links

ar:جص bg:Гипс ca:Guix cs:Sádrovec da:Gips de:Gips et:Kips el:Γύψος eo:Gipsoŝtono ko:석고 hr:Gips it:Gesso (mineralogia) he:גבס la:Gypsum lv:Ģipsis lt:Gipsas hu:Gipsz nl:Gips no:Gips sk:Sadrovec sl:Sadra sr:Гипс sv:Gips th:ยิปซัม zh-yue:石膏


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