Gypsum

This article is about the mineral. For other uses, see Gypsum (disambiguation).
Gypsum

Fibrous gypsum selenite showing its translucent property
General
Category Sulfate minerals
Formula
(repeating unit)		 CaSO4·2H2O
Strunz classification 07.CD.40
Crystal system Monoclinic 2/m – Prismatic
Unit cell a = 5.679(5) Å, b = 15.202(14) Å, c = 6.522(6) Å; β = 118.43°; Z=4
Identification
Color Colorless to white; may be yellow, tan, blue, pink, brown, reddish brown or gray due to impurities
Crystal habit Massive, flat. Elongated and generally prismatic crystals
Crystal symmetry Monoclinic 2/m
Twinning Very common on {110}
Cleavage Perfect on {010}, distinct on {100}
Fracture Conchoidal on {100}, splintery parallel to [001]
Tenacity Flexible, inelastic.
Mohs scale hardness 1.5–2 (defining mineral for 2)
Luster Vitreous to silky, pearly, or waxy
Streak White
Diaphaneity Transparent to translucent
Specific gravity 2.31–2.33
Optical properties Biaxial (+)
Refractive index nα = 1.519–1.521
nβ = 1.522–1.523
nγ = 1.529–1.530
Birefringence δ = 0.010
Pleochroism None
2V angle 58°
Fusibility 5
Solubility Hot, dilute HCl
References [1][2][3]
Major varieties
Satin spar Pearly, fibrous masses
Selenite Transparent and bladed crystals
Alabaster Fine-grained, slightly colored

Gypsum is a soft sulfate mineral composed of calcium sulfate dihydrate, with the chemical formula CaSO4·2H2O.[3] It is widely mined and is used as a fertilizer, and as the main constituent in many forms of plaster, blackboard chalk and wallboard. 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

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 1820.[5] In the 19th century, it was also known as lime sulphate or sulphate of lime.

Physical properties

Gypsum is moderately water-soluble (~2.0–2.5 g/l at 25 °C)[6] 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 concentration.[6]

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

Crystal varieties

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

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

Estimated production of Gypsum in 2014
(thousand metric tons)[13]
Country Production Reserves
 China132,000N/A
 United States17,100700,000
 Iran13,000N/A
 Turkey8,300N/A
 Spain6,400N/A
 Thailand6,300N/A
 Japan5,500N/A
 Russia5,300N/A
 Mexico5,000N/A
 Italy4,100N/A
 Brazil3,700230,000
 Australia3,500N/A
 India3,50069,000
 Oman3,000N/A
 Saudi Arabia2,400N/A
 France2,300N/A
 Algeria2,100N/A
 Germany1,900N/A
 Canada1,800450,000
 United Kingdom1,700N/A
 Argentina1,400N/A
 Poland1,30055,000
Other countries14,500N/A
World total246,000N/A

Commercial quantities of gypsum are found in the cities of Araripina and Grajaú in Brazil; in Pakistan, Jamaica, Iran (world's third largest producer), Thailand, Spain (the main producer in Europe), Germany, Italy, England, Ireland, Canada[14] and the United States. Large open pit quarries are located in many places including Plaster City, California, USA, 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 changing the pH levels of 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 tons and is around 500,000 years old.[15]

Synthesis

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).[16] 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.

Occupational safety

People can be exposed to gypsum in the workplace by breathing it in, skin contact, and eye contact.

United States

The Occupational Safety and Health Administration (OSHA) has set the legal limit (permissible exposure limit) for gypsum exposure in the workplace as TWA 15 mg/m3 for total exposure and TWA 5 mg/m3 for respiratory exposure over an 8-hour workday. The National Institute for Occupational Safety and Health (NIOSH) has set a recommended exposure limit (REL) of TWA 10 mg/m3 for total exposure and TWA 5 mg/m3 for respiratory exposure over an 8-hour workday.[17]

Uses

Gypsum is used in a wide variety of applications:

Gallery

See also

External links

Wikimedia Commons has media related to Gypsum.

References

  1. ↑ Anthony, John W.; Bideaux, Richard A.; Bladh, Kenneth W. and Nichols, Monte C., ed. (2003). "Gypsum". Handbook of Mineralogy (PDF). V (Borates, Carbonates, Sulfates). Chantilly, VA, US: Mineralogical Society of America. ISBN 0962209708.
  2. ↑ Gypsum. Mindat
  3. 1 2 Klein, Cornelis; Hurlbut, Cornelius S., Jr. (1985), Manual of Mineralogy (20th ed.), John Wiley, 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. 1 2 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. ↑ García-Ruiz, Juan Manuel; Villasuso, Roberto; Ayora, Carlos; Canals, Angels; Otálora, Fermín (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.
  10. ↑ Abarr, James (7 February 1999). "Sea of sand". The Albuquerque Journal. Retrieved 27 January 2007.
  11. ↑ High-resolution Mars image gallery. University of Arizona
  12. ↑ NASA Mars Rover Finds Mineral Vein Deposited by Water, NASA, 7 December 2011.
  13. ↑ "GYPSUM" (PDF). U.S. Geological Survey.
  14. ↑ "Mines, mills and concentrators in Canada". Natural Resources Canada. 24 October 2005. Retrieved 27 January 2007.
  15. ↑ Alleyne, Richard (27 October 2008). "World's largest crystal discovered in Mexican cave". London: The Telegraph. Retrieved 6 June 2009.
  16. ↑ Van Driessche, A.E.S.; Benning, L. G.; Rodriguez-Blanco, J. D.; Ossorio, M.; Bots, P.; García-Ruiz, J. M. (2012). "The role and implications of bassanite as a stable precursor phase to gypsum precipitation". Science 336 (6077): 69–72. Bibcode:2012Sci...336...69V. doi:10.1126/science.1215648.
  17. ↑ "CDC - NIOSH Pocket Guide to Chemical Hazards - Gypsum". www.cdc.gov. Retrieved 2015-11-03.
  18. ↑
  19. ↑ 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.
  20. ↑ Hogan, C. Michael (2007). "Knossos fieldnotes". Modern Antiquarian.
  21. ↑ Palmer, John. "Water Chemistry Adjustment for Extract Brewing". HowToBrew.com. Retrieved 15 December 2008.
  22. ↑ "Calcium sulphate for the baking industry" (pdf). United States Gypsum Company. Retrieved 1 March 2013.
  23. ↑ "Tech sheet for yeast food" (pdf). Lesaffre Yeast Corporation. Retrieved 1 March 2013.
  24. ↑ Astilleros, J.M., Godelitsas, A., Rodriguez-Blanco, J.D., Fernandez-Diaz, L., Prieto, M., Lagoyannis, A., Harissopulos, S. (2010) Interaction of gypsum with lead in aqueous solutions. Applied Geochemistry, 25, 1008-1016. doi: 10.1016/j.apgeochem.2010.04. 007.
  25. ↑ Rodríguez, J.D., Jiménez, A., Prieto, M., Torre, L., and García-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, 928-939. doi: 10.2138/am.2008.2750.
  26. ↑ Rodríguez-Blanco, J.D., Jiménez, A., and Prieto. M. (2007) Oriented Overgrowth of Pharmacolite (CaHAsO4⋅2H2O) on Gypsum (CaSO4⋅2H2O). Crystal Growth & Design, 12, 2756-2763. doi: 10.1021/cg070222+
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