Gypsum is a very soft sulfate mineral composed of calcium sulfatedihydrate, with the chemical formula CaSO4·2H2O.[3] It is found in alabaster, a decorative stone used in Ancient Egypt. It is the second softest mineral on the Mohs Hardness Scale. It forms as an evaporite mineral and as a hydration product of anhydrite.

Selenite, a Gypsum crystal. This image is a work of the

Fibrous Gypsum Selenite showing its translucent property

Etymology and history

The word gypsum is derived from the Greek word γύψοςgypsos, "chalk" or "plaster".[4] 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.

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 early nineteenth 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

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 the crystal lattice is heated it loses liquid water molecules to evaporation and thus gains solidity. 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

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 from 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 is called alabaster, and 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. Gypsum forms some of the largest crystals found in nature, up to 11 metres long, in the form of selenite.[8]


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.[9] 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.

Veins of gypsum in the silts/marls of the Tea Green and Grey Marls, Blue Anchor, Somerset, UK. Author: Ashley Dace

Veins of gypsum in the silts/marls of the Tea Green and Grey Marls, Blue Anchor, Somerset, UK.

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 sulfideoxidation, amongst others by pyriteoxidation, 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.

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


Commercial quantities of gypsum are found in the cities of Araripina and Grajaú, Brazil, Pakistan, Jamaica, Iran (world's second largest producer), Thailand, Spain (the main producer in Europe), Germany, Italy, England, Ireland, in British Columbia, Manitoba, Ontario, Nova Scotia[12] and Newfoundland in Canada,[13] and in New York, Michigan, Indiana,[13]Texas (in the Palo Duro Canyon), Iowa, Kansas, Oklahoma, Arizona, New Mexico, Colorado, Utah, Arkansas 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. Several small mines also exist in places like Kalannie in Western Australia where gypsum is sold to private buyers for changing the pH levels of soil for agricultural purposes.

Gypsum veins in the Chugwater Formation. Outcrop near Thermopolis. Rock hammer at right for scale. Author: Jstuby

Veins of gypsum in the Chugwater Group, Wyoming

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][15]


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.

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 calcitescaling, depending on the water source, can also be important considerations in distillation as well as in heat exchangers where either the salt solubility or salt concentration can change rapidly.
Locality: Red River Floodway, Winnipeg, Manitoba, Canada. Author: Rob Lavinsky /

Golden Gypsum crystals from Winnipeg.

Uses of gypsum

Gypsum is used in a wide variety of applications:
  • Gypsum Board[16] primarily used as a finish for walls and ceilings; known in construction as drywall.
  • Plaster ingredient.
  • Fertilizer and soil conditioner. In the late 18th and early 19th century, Nova Scotia gypsum, often referred to as plaster, was a highly sought fertilizer for wheat fields in the United States. It is also used in ameliorating sodic soils.[17]
  • 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 AgeCrete, gypsum was employed in building construction at locations where wood was previously used.[18]
  • 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.[19]
  • A component of Portland cement used to prevent flash setting of concrete.
  • Soil/water potential monitoring (soil moisture tension).
  • A common ingredient in making mead.
  • In the medieval period it was mixed, by scribes and illuminators, with lead carbonate (powdered white lead) to make gesso which was applied to illuminated letters and gilded with gold in illuminated manuscripts.
  • In foot creams, shampoos and many other hair products.
  • A medicinal agent in traditional Chinese medicine called Shi Gao.
  • Impression plasters in dentistry

Gypsum sand from White Sands National Monument, New Mexico. Photograph by Mark A. Wilson (Department of Geology, The College of Wooster).

Gypsum sand from White Sands National Monument, New Mexico.


Desert rose, 47 cm long
Gypsum with inclusions of green atacamite from Peru
Bright, cherry-red gypsum crystals 2.5 cm in height colored by rich inclusions of the rare mineral botryogen
Gypsum with crystalline native copper inside
Classic "ramshorn" gypsum from Santa Eulalia, Chihuahua, Mexico. Size 7.5×4.3×3.8 cm
Green gypsum crystals from Pernatty Lagoon, Mt Gunson, South Australia. Its green color is due to presence of copper ions
Selenite from the Houston Museum of Natural Science

See also
External links
  1. ^Gypsum in Handbook of Mineralogy
  2. ^Gypsum at Mindat
  3. ^ ab 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. ^ abBock, 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 Chemistry39 (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 Research32 (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". Geology35 (4): 327–330. doi:10.1130/G23393A.1.
  9. ^Cockell, C.S.; Raven J.A. (2007). "Ozone and life on the Archaean Earth". Philisophical Transactions of the Royal Society A365 (1856): 1889–1901. 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. ^Ducklow, Stu (February 9, 2010). "Gypsum: long history in Nova Scotia". The Hants Journal (Windsor, Nova Scotia). Retrieved March 31, 2011.
  13. ^ ab"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. ^Electric Caverns – picture from Peñoles Mine – article also includes a link to a picture of a spectacular gypsum flower at Lechuguilla Cave
  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 Sci Soc Am J44 (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.

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