Wikipedia

Gypsum

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

Gypsumis a softsulfate mineralcomposed ofcalcium sulfatedihydrate,with thechemical formulaCaSO4·2H2O.[4]It is widely mined and is used as afertilizerand as the main constituent in many forms ofplaster,drywallandblackboardorsidewalk chalk.[5][6][7][8]Gypsum alsocrystallizesas translucent crystals ofselenite.It forms as anevaporitemineral and as ahydrationproduct ofanhydrite.TheMohs scale of mineral hardnessdefines gypsum as hardness value 2 based onscratchhardness comparison.

Gypsum
General
CategorySulfate minerals
Formula
(repeating unit)		CaSO4·2H2O
IMA symbolGp[1]
Strunz classification7.CD.40
Crystal systemMonoclinic
Crystal classPrismatic (2/m)
H-M symbol:(2/m)
Space groupMonoclinic
Space group:I2/a
Unit cella = 5.679(5), b = 15.202(14)
c = 6.522(6) Å; β = 118.43°; Z = 4
Identification
ColorColorless (in transmitted light) to white; often tinged other hues due to impurities; may be yellow, tan, blue, pink, dark brown, reddish brown or gray
Crystal habitMassive, flat. Elongated and generally prismatic crystals
TwinningVery common on {110}
CleavagePerfect on {010}, distinct on {100}
FractureConchoidal on {100}, splintery parallel to [001]
TenacityFlexible, inelastic
Mohs scalehardness1.5–2 (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[2][3][4]
Major varieties
Satin sparPearly, fibrous masses
SeleniteTransparent and bladed crystals
AlabasterFine-grained, slightly colored

Fine-grained white or lightly tinted forms of gypsum known asalabasterhave been used forsculptureby many cultures includingAncient Egypt,Mesopotamia,Ancient Rome,theByzantine Empire,and theNottingham alabastersofMedieval England.

Etymology and history

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The wordgypsumis derived from theGreekwordγύψος(gypsos), "plaster".[9]Because thequarriesof theMontmartredistrict ofParishave long furnished burnt gypsum (calcinedgypsum) used for various purposes, this dehydrated gypsum became known asplaster of Paris.Upon adding water, after a few dozen 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.[10]

Gypsum was known in Old English asspærstān,"spear stone", referring to its crystalline projections. Thus, the wordsparin mineralogy, by comparison to gypsum, refers to any non-oremineral or crystal that forms in spearlike projections. In the mid-18th century, the German clergyman and agriculturalistJohann Friderich Mayerinvestigated and publicized gypsum's use as a fertilizer.[11]Gypsum may act as a source of sulfur for plant growth, 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.[12]

Physical properties

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Gypsum crystals are soft enough to bend under pressure of the hand. Sample on display at Musée cantonal de géologie de Lausanne.

Gypsum is moderately water-soluble (~2.0–2.5 g/L at 25 °C)[13]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 tocalcium sulfate hemihydrate(bassanite,often simply called "plaster" ) and, if heated further, to anhydrouscalcium sulfate(anhydrite). As withanhydrite,the solubility of gypsum in saline solutions and inbrinesis also strongly dependent onsodium chloride(common table salt) concentration.[13]

The structure of gypsum consists of layers of calcium (Ca2+) and sulfate (SO2−4) ions tightly bound together. These layers are bonded by sheets ofanion watermolecules via weakerhydrogen bonding,which gives the crystal perfect cleavage along the sheets (in the {010} plane).[4][14]

Crystal varieties

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Main article:Selenite (mineral)

Gypsum occurs in nature as flattened and oftentwinnedcrystals,and transparent, cleavable masses calledselenite.Selenite contains no significantselenium;rather, both substances were named for the ancient Greek word for theMoon.

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, calledalabaster,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 calleddesert rose.It also forms some of the largest crystals found in nature, up to 12 m (39 ft) long, in the form of selenite.[15]

Occurrence

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Gypsum is a common mineral, with thick and extensiveevaporitebeds in association withsedimentary rocks.Deposits are known to occur instratafrom as far back as theArchaeaneon.[16]Gypsum is deposited from lake and sea water, as well as inhot springs,fromvolcanicvapors, and sulfate solutions inveins.Hydrothermalanhydritein veins is commonly hydrated to gypsum by groundwater in near-surface exposures. It is often associated with the mineralshaliteandsulfur.Gypsum is the most common sulfate mineral.[17]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 theWhite Sands National Parkin the US state ofNew Mexicohave created a 710 km2(270 sq mi) expanse of white gypsum sand, enough to supply the US construction industry withdrywallfor 1,000 years.[18] Commercial exploitation of the area, strongly opposed by area residents, was permanently prevented in 1933 when PresidentHerbert Hooverdeclared the gypsumdunesa protectednational monument.

Gypsum is also formed as a by-product ofsulfideoxidation,amongst others bypyriteoxidation,when thesulfuric acidgenerated reacts withcalcium carbonate.Its presence indicates oxidizing conditions. Under reducing conditions, the sulfates it contains can be reduced back to sulfide bysulfate-reducing bacteria.This can lead to accumulation of elemental sulfur in oil-bearing formations,[19]such as salt domes,[20]where it can be mined using theFrasch process[21]Electric power stations burning coal withflue gas desulfurizationproduce large quantities of gypsum as a byproduct from the scrubbers.

Orbital pictures from theMars Reconnaissance Orbiter(MRO) have indicated the existence of gypsum dunes in the northern polar region of Mars,[22]which were later confirmed at ground level by theMars Exploration Rover(MER)Opportunity.[23]

Mining

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Estimated production of Gypsum in 2015
(thousand metric tons)[24]
Country Production Reserves
China 132,000 —
Iran 22,000 1,600
Thailand 12,500 —
United States 11,500 700,000
Turkey 10,000 —
Spain 6,400 —
Mexico 5,300 —
Japan 5,000 —
Russia 4,500 —
Italy 4,100 —
India 3,500 39,000
Australia 3,500 —
Oman 3,500 —
Brazil 3,300 290,000
France 3,300 —
Canada 2,700 450,000
Saudi Arabia 2,400 —
Algeria 2,200 —
Germany 1,800 450,000
Argentina 1,400 —
Pakistan 1,300 —
United Kingdom 1,200 55,000
Other countries 15,000 —
World total 258,000 —

Commercial quantities of gypsum are found in the cities ofAraripinaandGrajaúin Brazil; in Pakistan, Jamaica, Iran (world's second largest producer), Thailand, Spain (the main producer in Europe), Germany, Italy, England, Ireland, Canada[25]and the United States. Large open pit quarries are located in many places includingFort Dodge, Iowa,which sits on one of the largest deposits of gypsum in the world,[26]andPlaster City, California,United States, and EastKutai,Kalimantan,Indonesia. Several small mines also exist in places such asKalannieinWestern Australia,where gypsum is sold to private buyers for additions of calcium and sulfur as well as reduction of aluminum toxicities onsoilfor agricultural purposes.

Crystals of gypsum up to 11 m (36 ft) long have been found in the caves of theNaica MineofChihuahua,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 tonnes (61 short tons) and is around 500,000 years old.[27]

Synthesis

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Syntheticgypsum is produced as a waste product or by-product in a range of industrial processes.

Desulfurization

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Flue gas desulfurizationgypsum (FGDG) is recovered at some coal-fired power plants. The main contaminants are Mg, K, Cl, F, B, Al, Fe, Si, and Se. They come both from the limestone used in desulfurization and from the coal burned. This product is pure enough to replace natural gypsum in a wide variety of fields including drywalls, water treatment, and cement set retarder. Improvements in flue gas desulfurization have greatly reduced the amount of toxic elements present.[28]

Desalination

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Gypsum precipitates onto brackish watermembranes,a phenomenon known as mineral saltscaling,such as duringbrackishwaterdesalinationof water with high concentrations ofcalciumandsulfate.Scaling decreases membrane life and productivity.[29]This is one of the main obstacles in brackish water membrane desalination processes, such asreverse osmosisornanofiltration.Other forms of scaling, such ascalcitescaling, depending on the water source, can also be important considerations indistillation,as well as inheat exchangers,where either the saltsolubilityorconcentrationcan change rapidly.

A new study has suggested that the formation of gypsum starts as tiny crystals of a mineral calledbassanite(2CaSO4·H2O).[30]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.

Refinery waste

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The production ofphosphatefertilizers requires breaking down calcium-containingphosphate rockwith acid, producing calcium sulfate waste known asphosphogypsum(PG). This form of gypsum is contaminated by impurities found in the rock, namelyfluoride,silica,radioactive elements such asradium,and heavy metal elements such ascadmium.[31]Similarly, production oftitanium dioxideproduces titanium gypsum (TG) due to neutralization of excess acid withlime.The product is contaminated with silica, fluorides, organic matters, and alkalis.[32]

Impurities in refinery gypsum waste have, in many cases, prevented them from being used as normal gypsum in fields such as construction. As a result, waste gypsum is stored in stacks indefinitely, with significant risk of leaching their contaminants into water and soil.[31]To reduce the accumulation and ultimately clear out these stacks, research is underway to find more applications for such waste products.[32]

Occupational safety

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NFPA 704
safety square
[33]
Health 0: Exposure under fire conditions would offer no hazard beyond that of ordinary combustible material. E.g. sodium chlorideFlammability 0: Will not burn. E.g. waterInstability 0: Normally stable, even under fire exposure conditions, and is not reactive with water. E.g. liquid nitrogenSpecial hazards (white): no code
0
0
0
Gypsum

People can be exposed to gypsum in the workplace by breathing it in, skin contact, and eye contact. Calcium sulfateper seis nontoxic and is even approved as a food additive,[34]but as powdered gypsum, it can irritate skin and mucous membranes.[35]

United States

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TheOccupational Safety and Health Administration(OSHA) has set the legal limit (permissible exposure limit) for gypsum exposure in the workplace as TWA 15 mg/m3for total exposure and TWA 5 mg/m3for respiratory exposure over an eight-hour workday. TheNational Institute for Occupational Safety and Health(NIOSH) has set arecommended exposure limit(REL) of TWA 10 mg/m3for total exposure and TWA 5 mg/m3for respiratory exposure over an eight-hour workday.[35]

Uses

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Gypsum works,Valencian Museum of Ethnology
OldAlfarbkiln for making plaster as a construction material
British Gypsum,Kirkby Thore
Map of gypsum deposits in northern Ohio, black squares indicate the location of deposits, from"Geography of Ohio",1923

Gypsum is used in a wide variety of applications:

Construction industry

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  • Gypsum board[36]is primarily used as a finish for walls and ceilings, and is known in construction as plasterboard, "sheetrock", or drywall. Gypsum provides a degree of fire-resistance to these materials and glass fibers are added to their composition to accentuate this effect. Gypsum has little heat conductivity, giving its plaster some insulative properties.[37]
  • Gypsum blocksare used like concrete blocks in building construction.
  • Gypsum mortaris an ancient mortar used in building construction.
  • A component ofPortland cementused to prevent flash setting (too rapid hardening) ofconcrete.
  • A wood substitute in the ancient world: For example, when wood became scarce due to deforestation onBronze AgeCrete,gypsum was employed in building construction at locations where wood was previously used.[38]

Agriculture

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Modeling, sculpture and art

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  • Plasterfor casting moulds and modeling.
  • Asalabaster,a material for sculpture, it was used especially in the ancient world before steel was developed, when its relative softness made it much easier to carve.[45]During theMiddle AgesandRenaissance,it was preferred even tomarble.[46]
  • In the medieval period,scribesandilluminatorsused it as an ingredient ingesso,which was applied to illuminated letters and gilded with gold in illuminated manuscripts.[47]

Food and drink

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  • Atofu(soy bean curd) coagulant, making it ultimately a significant source of dietarycalcium.[48]
  • Addinghardnessto water used forbrewing.[49]
  • Used in baking as a dough conditioner, reducing stickiness, and as a baked-goods source of dietary calcium.[50]The primary component of mineral yeast food.[51]
  • Used in mushroom cultivation to stop grains from clumping together.

Medicine and cosmetics

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Other

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  • An alternative to iron oxide in some thermite mixes.[54]
  • Tests have shown that gypsum can be used to remove pollutants such aslead[55]orarsenic[56][57]from contaminated waters.
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  • Unusual gypsum specimens from around the world
  • Green gypsum crystals from Pernatty Lagoon, Mt Gunson,South Australia- its green color is due to presence ofcopperions.
  • Unusual selenite gypsum from theRed River,Winnipeg, Manitoba, Canada
  • Classic "ram's horn" gypsum fromSanta Eulalia, Chihuahua,Mexico, 7.5×4.3×3.8 cm
  • Desert rose,47 cm long
  • Gypsum from Pernatty Lagoon, Mt Gunson, Stuart Shelf area, Andamooka Ranges - Lake Torrens area, South Australia, Australia
  • Gypsum with crystallinenative copperinside
  • Gypsum from Swan Hill, Victoria, Australia. The coloring is due to the copper oxide
  • Waterclear twined crystal of the form known as "Roman sword". Fuentes de Ebro, Zaragoza (Spain)
  • Bright, cherry-red gypsum crystals 2.5 cm in height colored by rich inclusions of the rare mineralbotryogen
  • Gypsum from Naica, Mun. de Saucillo, Chihuahua, Mexico
  • Golden color gem, "fishtail" -twinned crystals of gypsum sitting atop a "ball" of gypsum which is composed of several single bladed crystals

See also

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References

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  2. ^Anthony, John W.; Bideaux, Richard A.; Bladh, Kenneth W.; Nichols, Monte C., eds. (2003)."Gypsum"(PDF).Handbook of Mineralogy.Vol. V (Borates, Carbonates, Sulfates). Chantilly, VA, US: Mineralogical Society of America.ISBN978-0962209703.Archived(PDF)from the original on 6 February 2006.
  3. ^Gypsum.Mindat
  4. ^abcKlein, Cornelis; Hurlbut, Cornelius S. Jr. (1985),Manual of Mineralogy(20th ed.), John Wiley, pp.352–353,ISBN978-0-471-80580-9
  5. ^Institute, Canadian Conservation (14 September 2017)."Care of Objects Made of Plaster of Paris – Canadian Conservation Institute (CCI) Notes 12/2".www.canada.ca.Retrieved20 January2023.
  6. ^Make your own sidewalk chalk. (1998, July 21). Christian Science Monitor. 13.
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  10. ^Szostakowski, B.; Smitham, P.; Khan, W.S. (17 April 2017)."Plaster of Paris–Short History of Casting and Injured Limb Immobilzation".The Open Orthopaedics Journal.11:291–296.doi:10.2174/1874325001711010291.ISSN1874-3250.PMC5420179.PMID28567158.
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