gypsum

[Middle English gipsum, from Latin gypsum, from Greek gupsos, probably of Semitic origin, akin to Arabic jibs, jiṣṣ, jaṣṣ, from Akkadian gaṣṣu.]

The most common sulfate mineral, characterized by the chemical formula CaSO₄ · 2H₂O; it shows little variation from this composition.

Gypsum is one of the several evaporite minerals. This mineral group includes chlorides, carbonates, borates, nitrates, and sulfates. These minerals precipitate in seas, lakes, caves, and salt flats due to concentration of ions by evaporation. When heated or subjected to solutions with very large salinities, gypsum converts to bassanite (CaSO₄·H₂O) or anhydrite (CaSO₄). Under equilibrium conditions, this conversion to anhydrite is direct. The conversion occurs above 108°F (42°C) in pure water. The presence of halite (NaCl) or other sulfates in the solution lowers this temperature, although metastable gypsum exists at higher temperatures. See also Anhydrite; Halite.

Crystals of gypsum are commonly tabular, diamond-shaped, or lenticular; swallow-tailed twinsare also common. The mineral is monoclinic with symmetry 2/m. The common colors displayed are white, gray, brown, yellow, and clear. Cleavage surfaces show a pearly to vitreous luster. Gypsum is the index mineral chosen for hardness 2 on Mohs scale with a specific gravity of 2.32. In addition to free crystals, the common forms of gypsum are satin spar (fibrous), alabaster (finely crystalline), and selenite (massive crystalline).

Gypsum is used for a variety of purposes, but chiefly in the manufacture of plaster of paris, in the production of wallboard, in agriculture to loosen clay-rich soils, and in the manufacture of fertilizer. Plaster of paris is made by heating gypsum to 392°F (200°C) in air. A hemihydrate is formed as part of the water of crystallization is driven off. Later, when water is added, rehydration occurs. The interlocking, finely crystalline texture that results forms a uniform hardened mass. The slightly increased volume of the set plaster serves to fill the mold into which it has been poured. See also Plaster of paris.

Gypsum deposits are mined throughout the world, with the United States being a world leader in gypsum production. The majority of United States gypsum is mined in Michigan, Iowa, Texas, California, and Oklahoma. Canada is the world's second largest producer. Most Canadian production is in the province of Nova Scotia. Among the other leading producers are France, Japan, Iran, Russia, Italy, Spain, and the United Kingdom.

The dihydrate of calcium sulfate (CaSO₄-2H₂O). α Hemihydrate and β hemihydrate are derived from gypsum. See also plaster of paris.

For more information on gypsum, visit Britannica.com.

A soft mineral consisting of a hydrated calcium sulfate from which gypsum plaster is made (by heating); colorless when pure; used as a retarder in portland cement.

Environment

Sedimentary rocks as massive beds, in free crystals in clay beds, alkaline lake muds, and crystallized in cavities in limestone. Often in opaque, sand-filled crystal clusters. Crystals constantly grow and clog various contrivances of human making: water pipes, mines, and dumps.

Crystals are common, often assuming a tabular habit: model-like, backward-slanting, monoclinic plates, with the horizontal axis the shortest. "Fishtail" twins are frequent. The most common crystals--which may be large ill-formed sheets--are found loose and free-growing in clay beds and cover outcrops in mica-like sheets; glassy gypsum known as selenite. Stony bands of massive Italian gypsum, known as alabaster, are carved and dyed in Florence; fibrous warm-hued chatoyant veins known as satin spar are Russian sculptors' grist.

Colorless, white, and pale tints. Luster glassy, pearly (on cleavage face), and silky (avoid washing very lustrous surfaces, for even water has been found to dull them); hardness 2; specific gravity 2.3; fracture conchoidal and splintery; cleavage 2, perfect and micaceous. Sectile; often fluoresces yellow in an hourglass pattern within crystal; also phosphorescent.

Hydrous calcium sulfate (32.6% CaO, 46.5% SO ₃ , 20.9% H ₂ O).

Soluble in hot dilute hydrochloric acid; the addition of barium chloride solution makes a white precipitate. After firing, fluorescent and phosphorescent in longwave ultraviolet light.

With its low hardness, and flakes that are easily scratched by a fingernail, no other test is needed. The clear plates bend but lack the elastic rebound of mica; they are softer than uncommon brucite. Massive alabaster is softer than anhydrite or marble, and gypsum will not bubble in acid like the latter.

A widespread, commercially important mineral. The massive material is quarried, or mined, for the manufacture of plaster of Paris and various plaster products. The most abundant deposits are the sedimentary beds, some of which have formed from the alteration of the water-free variety, anhydrite. It is such beds that are mined for economic applications in New York, Michigan, Texas, Iowa, and California. Nova Scotia has great beds of altered anhydrite that show interesting crumpling of the layers as they swelled with the addition of water--metamorphic structures on a very small scale.

Good crystals are found in clay beds of Ohio and Maryland, and interesting cave rosettes of spreading fibers (gypsum flowers) come from Kentucky. The most beautiful gypsum (selenite) crystals are foreign in origin, though the largest probably came from a cave in Utah. The large water-clear crystals from the Sicilian sulfur mines, often with inclusions of sulfur, are classics of all collections. In Naica, Chihuahua, Mexico, a cavern in the mine (Cave of the Swords) contains meter-long, slender, slightly milky needles with tubular water-filled cavities and movable bubbles.

The name plaster of Paris comes from its early source in the Montmartre quarries of Paris. The name gypsum comes from the Greek word for the calcined (or "burned") material. Selenite comes from a Greek comparison of the pearly luster of a cleavage plate to moonlight. Decomposed gypsum is considered a source of sulfur in the Sicilian mines.

Native calcium sulfate, which, when calcined, becomes plaster of Paris; used in making plaster casts for fractures.

A mineral, calcium sulfate, that is used to add calcium to the soil or to improve the structure of clay soils without affecting the pH. See also flocculate.

It has been suggested that Selenite (mineral) be merged into this article or section. (Discuss)

For other uses, see Gypsum (disambiguation).

Gypsum
Desert rose, 10 cm long
General
Category	Sulfate mineral
Chemical formula	Calcium sulfate CaSO4·2H2O
Identification
Color	Colorless to white; with impurities may be yellow, tan, blue, pink, brown, reddish brown or gray
Crystal habit	Massive, flat. Elongated and generally prismatic crystals
Crystal system	Monoclinic 2/m - Prismatic
Twinning	Very common on {110}
Cleavage	Perfect on {010}, distinct on {100}
Fracture	Conchoidal on {100}, splintery parallel to [001]
Mohs scale hardness	1.5-2
Luster	Vitreous to silky, pearly, or waxy
Streak	White
Diaphaneity	transparent to translucent
Specific gravity	2.31 - 2.33
Optical properties	2V = 58° Biaxial (+)
Refractive index	nα = 1.519 - 1.521 nβ = 1.522 - 1.523 nγ = 1.529 - 1.530
Birefringence	δ = 0.010
Pleochroism	None
Fusibility	5
Solubility	hot, dilute HCl
References	[1][2]
Major varieties
Satin spar	Pearly, fibrous masses
Selenite	Transparent and bladed crystals
Alabaster	Fine-grained, slightly colored

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

Contents 1 Crystal varieties 2 Occurrence 3 Uses of Gypsum 4 References 5 External links

Crystal varieties

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 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, 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. Up to the size of 11m long, gypsum forms some of the largest crystals found in nature, in the form of selenite.^[4]

Occurrence

Gypsum var. selenite from Andamooka Ranges - Lake Torrens area, South Australia

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.^[5] 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.

Fibrous Gypsum from Brazil

The word gypsum is derived from the Greek word γύψος, "chalk" or "plaster"^[6]. 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 square mile) expanse of white gypsum sand, enough to supply the construction industry with drywall for 1,000 years.^[7] 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, Brazil, Pakistan, Jamaica, Iran, Thailand, Spain (the main producer in Europe), Germany, Italy, England, Ireland, in British Columbia, Manitoba, Ontario, Nova Scotia and Newfoundland in Canada,^[8] and in New York, Michigan, Indiana^[8],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 open pit quarry located at Plaster City, California in Imperial County, and in East Kutai, Kalimantan.

Crystals of gypsum up to 11 meters 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, and the cave was filled with mineral-rich water that drove the crystals' growth. The largest of those crystals weighs 55 tons and is around 500,000 years old.^[9][10]

Synthetic gypsum is recovered via flue gas desulfurization at some coal-fired electric power plants. It can be used interchangeably with natural gypsum in some applications.

Orbital pictures from the Mars Reconnaissance Orbiter indicate the existence of gypsum dunes in the northern polar region of Mars.^[11]

Uses of Gypsum

The lists in this article may contain items that are not notable, encyclopedic, or helpful. Please help out by removing such elements and incorporating appropriate items into the main body of the article. (April 2008)

Gypsum is used in a wide variety of applications:

Cones of gypsum which formed on the sea floor during the Messinian salinity crisis

• Gypsum Board^[12] primarily used as a finish for walls and ceilings; known in construction slang as Drywall
• Plaster ingredient.
• Fertilizer and soil conditioner. In the late eighteenth and early nineteenth century, Nova Scotia gypsum, often referred to as plaister, was a highly sought fertilizer for wheat fields in the United States. It is also used in ameliorating sodic soils^[13].
• A Binder in Fast-Dry tennis court clay.
• Plaster of Paris (surgical splints; casting moulds; modeling).
• A wood substitute in the ancient world; for example, when wood became scarce due to deforestation on Bronze Age Crete, gypsum was employed in building construction at locations where wood was previously used.^[14]
• 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.
• Adding hardness to water used for homebrewing.^[15]
• Blackboard chalk.
• A component of Portland cement used to prevent flash setting of concrete.
• Soil/water potential monitoring (soil moisture tension)
• Has a common use as an ingredient in making mead.
• A medicinal agent in traditional Chinese medicine called Shi Gao.

References

1. ^ http://rruff.geo.arizona.edu/doclib/hom/gypsum.pdf Handbook of Mineralogy
2. ^ http://www.mindat.org/min-1784.html Mindat
3. ^ Cornelis Klein and Cornelius S. Hurlbut, Jr., 1985, Manual of Mineralogy, John Wiley, 20th ed., pp. 352-353, ISBN 0-471-80580-7
4. ^ 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. 
5. ^ 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
6. ^ "Compact Oxford English Dictionary: gypsum". http://www.askoxford.com/concise_oed/gypsum. 
7. ^ Abarr, James (1999-02-07). "Sea of Sand". The Albuquerque Journal. http://www.abqjournal.com/venue/travel/tourism/heritage_whitesands.htm. Retrieved 2007-01-27. 
8. ^ ^{a b} "Mines, Mills and Concentrators in Canada". Natural Resources Canada. 2005-10-24. http://mmsd1.mms.nrcan.gc.ca/mmsd/producers/commodityCompany_e.asp?nId=51&mineType=nonMetal. Retrieved 2007-01-27. 
9. ^ "World's largest crystal discovered in Mexican cave". The Telegraph. http://www.telegraph.co.uk/news/newstopics/howaboutthat/3269047/Worlds-largest-crystal-discovered-in-Mexican-cave.html. Retrieved 2009-06-06. 
10. ^ [1] Electric Caverns - picture from Peñoles Mine - article also includes a link to a picture of a spectacular gypsum flower at Lechuguilla Cave
11. ^ http://hirise.lpl.arizona.edu/nea.php
12. ^ *Complimentary list of MasterFormat 2004 Edition Numbers and Titles (large PDF document)
13. ^ Oster and Frenkel. 1980. The Chemistry of the Reclamation of Sodic Soils with Gypsum and Lime. SSSAJ. 44:41-45
14. ^ C. Michael Hogan, Knossos fieldnotes, Modern Antiquarian (2007)
15. ^ "Water Chemistry Adjustment for Extract Brewing". How To Brew by John Palmer. http://www.howtobrew.com/section1/chapter4-2.html. Retrieved 2008-12-15. 

External links

• WebMineral data
• Mineral Galleries- Gypsum
• U.S. and National Gypsum Co. mines, Shoals, Martin Co., Indiana, USA

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Dansk (Danish)
n. - gips, gipssten

Nederlands (Dutch)
gips

Français (French)
n. - (Minér, Géol) gypse

Deutsch (German)
n. - Gips

Ελληνική (Greek)
n. - (ορυκτολ.) γύψος, θειικό ασβέστιο

Italiano (Italian)
ingessatura, pietra da gesso

Português (Portuguese)
n. - gesso (m) (Quím.)

Русский (Russian)
гипс

Español (Spanish)
n. - yeso

Svenska (Swedish)
n. - gips

中文（简体）(Chinese (Simplified))
石膏

中文（繁體）(Chinese (Traditional))
n. - 石膏

한국어 (Korean)
n. - 기브스, 석고

日本語 (Japanese)
n. - 石膏, ギプス
v. - 石膏で処理する

العربيه (Arabic)
‏(الاسم) جص, جبص‏

עברית (Hebrew)
n. - ‮גבס‬

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