Gypsum

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Gypsum
SeleniteGypsumUSGOV.jpg
Fibrous gypsum selenite showing its translucent property.
General
CategorySulfate minerals
Formula
(repeating unit)
CaSO4·2H2O
Strunz classification07.CD.40
Crystal symmetryMonoclinic 2/m
Unit cella = 5.679(5) Å, b = 15.202(14) Å, c = 6.522(6) Å; β = 118.43°; Z=4
Identification
ColorColorless to white; may be yellow, tan, blue, pink, brown, reddish brown or gray due to impurities
Crystal habitMassive, flat. Elongated and generally prismatic crystals
Crystal systemMonoclinic 2/m – Prismatic
TwinningVery common on {110}
CleavagePerfect on {010}, distinct on {100}
FractureConchoidal on {100}, splintery parallel to [001]
TenacityFlexible, inelastic.
Mohs scale hardness1.5–2 (defining mineral for 2)
LusterVitreous to silky, pearly, or waxy
StreakWhite
DiaphaneityTransparent to translucent
Specific gravity2.31–2.33
Optical propertiesBiaxial (+)
Refractive indexnα = 1.519–1.521
nβ = 1.522–1.523
nγ = 1.529–1.530
Birefringenceδ = 0.010
PleochroismNone
2V angle58°
Fusibility5
SolubilityHot, dilute HCl
References[1][2][3]
Major varieties
Satin sparPearly, fibrous masses
SeleniteTransparent and bladed crystals
AlabasterFine-grained, slightly colored
 
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This article is about the mineral. For other uses, see Gypsum (disambiguation).
Gypsum
SeleniteGypsumUSGOV.jpg
Fibrous gypsum selenite showing its translucent property.
General
CategorySulfate minerals
Formula
(repeating unit)
CaSO4·2H2O
Strunz classification07.CD.40
Crystal symmetryMonoclinic 2/m
Unit cella = 5.679(5) Å, b = 15.202(14) Å, c = 6.522(6) Å; β = 118.43°; Z=4
Identification
ColorColorless to white; may be yellow, tan, blue, pink, brown, reddish brown or gray due to impurities
Crystal habitMassive, flat. Elongated and generally prismatic crystals
Crystal systemMonoclinic 2/m – Prismatic
TwinningVery common on {110}
CleavagePerfect on {010}, distinct on {100}
FractureConchoidal on {100}, splintery parallel to [001]
TenacityFlexible, inelastic.
Mohs scale hardness1.5–2 (defining mineral for 2)
LusterVitreous to silky, pearly, or waxy
StreakWhite
DiaphaneityTransparent to translucent
Specific gravity2.31–2.33
Optical propertiesBiaxial (+)
Refractive indexnα = 1.519–1.521
nβ = 1.522–1.523
nγ = 1.529–1.530
Birefringenceδ = 0.010
PleochroismNone
2V angle58°
Fusibility5
SolubilityHot, dilute HCl
References[1][2][3]
Major varieties
Satin sparPearly, fibrous masses
SeleniteTransparent and bladed crystals
AlabasterFine-grained, slightly colored
Gypsum crystals in the Cave of the Crystals in Mexico. Note person for scale.

Gypsum is a soft sulfate mineral composed of calcium sulfate dihydrate, with the chemical formula CaSO4·2H2O.[3] It can be used as a fertilizer, is the main constituent in many forms of plaster and is widely mined. A massive fine-grained white or lightly tinted variety of gypsum, called alabaster, has been used for sculpture by many cultures including Ancient Egypt, Mesopotamia, Ancient Rome, Byzantine empire and the Nottingham alabasters of medieval England. It is the definition of a hardness of 2 on the Mohs scale of mineral hardness. It forms as an evaporite mineral and as a hydration product of anhydrite.

Etymology and history[edit]

The word gypsum is derived from the Greek word γύψος (gypsos), "chalk" or "plaster".[4] 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.) Gypsum may act as a source of sulfur for plant growth, which was discovered by J. M. Mayer, 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 1812.[5]

Physical properties[edit]

Gypsum is moderately water-soluble (~2.0–2.5 g/l at 25°C)[6] and, in contrast to most other salts, it exhibits a 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 concentration.[6]

Gypsum crystals are found to contain anion water and hydrogen bonding.[7]

Crystal varieties[edit]

Main article: Selenite (mineral)

Gypsum occurs in nature as flattened and often twinned crystals, 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 metres (39 ft) long, in the form of selenite.[8]

Occurrence[edit]

Veins of gypsum in the silts/marls of the Tea Green and Grey Marls, Blue Anchor, Somerset, UK.
Veins of gypsum in Caprock Canyons State Park, Texas.

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 Archaean eon.[9] Gypsum is deposited from 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. 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 Monument in the US state of New Mexico have created a 710 km2 (270 sq mi) expanse of white gypsum sand, enough to supply the construction industry with drywall for 1,000 years.[10] 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.

Gypsum is also formed as a by-product of sulfide oxidation, amongst others by pyrite oxidation, when the sulfuric acid generated reacts with calcium carbonate. Its presence indicates oxidizing conditions. Under reducing conditions, the sulfates it contains can be reduced back to sulfide by sulfate reducing bacteria. Electric power stations burning coal with flue gas desulfurization produce large quantities of gypsum as a byproduct from the scrubbers.

Orbital pictures from the Mars Reconnaissance Orbiter (MRO) have indicated the existence of gypsum dunes in the northern polar region of Mars,[11] which were later confirmed at ground level by the Mars Exploration Rover (MER) Opportunity.[12]

Mining[edit]

Golden gypsum crystals from Winnipeg.

Commercial quantities of gypsum are found in the cities of Araripina and Grajaú; in Brazil, Pakistan, Jamaica, Iran (world's second largest producer), Thailand, Spain (the main producer in Europe), Germany, Italy, England, Ireland, Canada[13] and the United States. There is also a large open pit quarry located at Plaster City, California, in Imperial County, and in East Kutai, Kalimantan. Several small mines also exist in places such as Kalannie in Western Australia, where gypsum is sold to private buyers for changing the pH levels of soil for agricultural purposes.

Crystals of gypsum up to 11 meters (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 short tons (50,000 kg) and is around 500,000 years old.[14]

Synthesis[edit]

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

Gypsum also precipitates onto brackish water membranes, a phenomenon known as mineral salt scaling, such as during brackish water desalination of water with high concentrations of calcium and sulfate. Scaling decreases membrane life and productivity. This is one of the main obstacles in brackish water membrane desalination processes, such as reverse osmosis or nanofiltration. Other forms of scaling, such as calcite scaling, depending on the water source, can also be important considerations in distillation, as well as in heat exchangers, where either the salt solubility or concentration can change rapidly.

A new study has suggested that the formation of gypsum starts as tiny crystals of a mineral called bassanite (CaSO4·0.5H2O).[15] This process occurs via a three-stage pathway: (1) homogeneous nucleation of nanocrystalline bassanite; (2) self-assembly of bassanite into aggregates, and (3) transformation of bassanite into gypsum.

Uses of gypsum[edit]

Gypsum is used in a wide variety of applications:

Gallery[edit]

See also[edit]

External links[edit]

References[edit]

  1. ^ Gypsum in Handbook of Mineralogy
  2. ^ Gypsum at Mindat
  3. ^ a b Cornelis Klein and Cornelius S. Hurlbut, Jr., 1985, Manual of Mineralogy, John Wiley, 20th ed., pp. 352–353, ISBN 0-471-80580-7
  4. ^ "Compact Oxford English Dictionary: gypsum". 
  5. ^ Smith, Joshua (2007). Borderland smuggling: Patriots, loyalists, and illicit trade in the Northeast, 1780-1820. Gainesville, FL: UPF. pp. passim. ISBN 0-8130-2986-4. 
  6. ^ a b Bock, E. (1961). "On the solubility of anhydrous calcium sulphate and of gypsum in concentrated solutions of sodium chloride at 25 °C, 30 °C, 40 °C, and 50 °C". Canadian Journal of Chemistry 39 (9): 1746–1751. doi:10.1139/v61-228. 
  7. ^ 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. 
  8. ^ 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. Bibcode:2007Geo....35..327G. doi:10.1130/G23393A.1. 
  9. ^ Cockell, C.S.; Raven J.A. (2007). "Ozone and life on the Archaean Earth". Philosophical Transactions of the Royal Society A 365 (1856): 1889–1901. Bibcode:2007RSPTA.365.1889C. doi:10.1098/rsta.2007.2049. Retrieved 16 February 2011. 
  10. ^ Abarr, James (1999-02-07). "Sea of sand". The Albuquerque Journal. Retrieved 2007-01-27. 
  11. ^ High-resolution Mars image gallery
  12. ^ NASA Mars Rover Finds Mineral Vein Deposited by Water
  13. ^ "Mines, mills and concentrators in Canada". Natural Resources Canada. 2005-10-24. Retrieved 2007-01-27. 
  14. ^ Alleyne, Richard (2008-10-27). "World's largest crystal discovered in Mexican cave". London: The Telegraph. Retrieved 2009-06-06. 
  15. ^ Van Driessche, A.E.S; L.G Benning, J.D Rodriguez-Blanco, M. Ossorio, P. Bots, J.M García-Ruiz (2012). "The role and implications of bassanite as a stable precursor phase to gypsum precipitation". Science, 6 April 2012 336 (6077): 69–72. Bibcode:2012Sci...336...69V. doi:10.1126/science.1215648. ISSN 0036-8075. Retrieved 2012-04-06. 
  16. ^ *Complimentary list of MasterFormat 2004 Edition numbers and titles (large PDF document)
  17. ^ Oster, J. D. & Frenkel, H. (1980). "The chemistry of the reclamation of sodic soils with gypsum and lime". Soil Science Society of America Journal 44 (1): 41–45. doi:10.2136/sssaj1980.03615995004400010010x. 
  18. ^ C. Michael Hogan, Knossos fieldnotes, Modern Antiquarian (2007)
  19. ^ "Water Chemistry Adjustment for Extract Brewing". How To Brew by John Palmer. Retrieved 2008-12-15. 
  20. ^ "Calcium sulphate for the baking industry" (pdf). Retrieved 2013-03-01. 
  21. ^ "Tech sheet for yeast food" (pdf). Retrieved 2013-03-01.