Editing Anhydrite

{{Short description|Mineral, anhydrous calcium sulfate}}
{{Redirect|Angelite|the musical ensemble|Angelite (choir)}}
{{Distinguish|anhydride}}
{{Infobox mineral
| name        = Anhydrite
| category    = [[Sulfate mineral]]
| boxwidth    = 
| boxbgcolor  =#b7dfda 
| image       = Anhydrite_HMNH1.jpg
| imagesize   = 260px
| alt         = 
| caption     = Anhydrite, from Chihuahua, Mexico
| formula     = CaSO<sub>4</sub>
|IMAsymbol=Anh<ref>{{Cite journal|last=Warr|first=L.N.|date=2021|title=IMA–CNMNC approved mineral symbols|journal=Mineralogical Magazine|volume=85|issue=3|pages=291–320|doi=10.1180/mgm.2021.43|bibcode=2021MinM...85..291W|s2cid=235729616|doi-access=free}}</ref>
| strunz      = 7.AD.30
| dana        = 28.3.2.1
| system      = [[Orthorhombic]]
| class       = Dipyramidal (mmm) <br/>[[H–M symbol]]: ({{sfrac|2|m}} {{sfrac|2|m}} {{sfrac|2|m}})
| symmetry    = ''Amma''
| unit cell   = ''a'' = 6.245(1)&nbsp;Å, ''b'' = 6.995(2)&nbsp;Å<br/>''c'' = 6.993(2)&nbsp;Å; ''Z''&nbsp;=&nbsp;4
| color       = Colorless to pale blue or violet if transparent; white, mauve, rose, pale brown or gray from included impurities
| habit       = Rare tabular and prismatic crystals. Usually occurs as fibrous, parallel veins that break off into cleavage fragments. Also occurs as grainy, massive, or nodular masses
| twinning    = Simple or repeatedly on {011} common; contact twins rare on {120}
| cleavage    = [010] perfect<br/> [100] perfect<br/> [001] good, resulting in pseudocubic fragments
| fracture    = Conchoidal
| tenacity    = Brittle
| mohs        = 3.5
| luster      = Pearly on {010}<br/> vitreous to greasy on {001}<br/> vitreous on {100}
| refractive  = ''n''<sub>α</sub> = 1.567–1.574<br/> ''n''<sub>β</sub> = 1.574–1.579<br/> ''n''<sub>γ</sub> = 1.609–1.618
| opticalprop = Biaxial (+)
| birefringence = ''δ'' = 0.042–0.044
| pleochroism = For violet varieties <br/>''X'' = colorless to pale yellow or rose<br/> ''Y'' = pale violet or rose<br/> ''Z'' = violet.
| 2V          = 56–84°
| streak      = White
| gravity     = 2.97
| melt        = 
| fusibility  = 2
| diagnostic  = 
| solubility  = 
| diaphaneity = Transparent to translucent
| other       = Some specimens fluoresce; many more fluoresce after heating
| references  = <ref>{{cite book|last1=Klein|first1=Cornelis|first2=Cornelius S.|last2=Hurlbut|date=1985|title=Manual of Mineralogy|edition=20th|publisher=John Wiley and Sons|location=New York|isbn=978-0-471-80580-9|url-access=registration |url=https://archive.org/details/manualofmineralo00klei}}</ref><ref>{{cite web|url=http://webmineral.com/data/Anhydrite.shtml|title=Anhydrite|website=Webmineral}}</ref><ref>{{cite web|url=http://www.mindat.org/min-234.html|title=Anhydrite|website=Mindat.org}}</ref><ref>{{cite web|url=http://rruff.geo.arizona.edu/doclib/hom/anhydrite.pdf|title=Anhydrite|website=Handbook of Mineralogy}}</ref>
}}
'''Anhydrite''', or anhydrous [[calcium sulfate]], is a [[mineral]] with the [[chemical formula]] CaSO<sub>4</sub>. It is in the [[orthorhombic]] crystal system, with three directions of perfect [[Cleavage (crystal)|cleavage]] parallel to the three planes of [[symmetry]]. It is not [[Isomorphism (crystallography)|isomorphous]] with the orthorhombic [[barium]] ([[baryte]]) and [[strontium]] ([[Celestine (mineral)|celestine]]) sulfates, as might be expected from the chemical formulas. Distinctly developed [[crystal]]s are somewhat rare, the mineral usually presenting the form of cleavage masses. The [[Mohs hardness]] is 3.5, and the [[specific gravity]] is 2.9. The color is white, sometimes greyish, bluish, or purple. On the best developed of the three cleavages, the [[lustre (mineralogy)|lustre]] is pearly; on other surfaces it is glassy. When exposed to water, anhydrite readily transforms to the more commonly occurring [[gypsum]], (CaSO<sub>4</sub>·2H<sub>2</sub>O) by the absorption of water. This transformation is reversible, with gypsum or [[Bassanite|calcium sulfate hemihydrate]] forming anhydrite by heating to around {{convert|200|°C|-2}} under normal atmospheric conditions.<ref>{{cite book|last1=Deer|last2=Howie|last3=Zussman|title=An Introduction to the Rock=Forming Minerals|publisher=Pearson Education|location=England|edition=2nd|date=1992|page=614|isbn=978-0-582-30094-1}}</ref> Anhydrite is commonly associated with [[calcite]], [[halite]], and [[sulfide]]s such as [[galena]], [[chalcopyrite]], [[molybdenite]], and [[pyrite]] in vein deposits.

==Occurrence==
[[Image:Anhydrite.png|left|thumb|Crystal structure of anhydrite]]
Anhydrite is most frequently found in [[evaporite]] deposits with gypsum; it was, for instance, first discovered in 1794 in a salt mine near [[Hall in Tirol]]. In this occurrence, depth is critical since nearer the surface anhydrite has been altered to gypsum by absorption of circulating ground water.

From an [[aqueous solution]], calcium sulfate is deposited as crystals of gypsum, but when the solution contains an excess of [[sodium]] or [[potassium chloride]], anhydrite is deposited if the temperature is above {{convert|40|°C}}. This is one method by which the mineral has been prepared artificially and is identical with its mode of origin in nature. The mineral is common in salt [[Depression (geology)|basin]]s.

===Tidal flat nodules===
Anhydrite occurs in a [[tidal flat]] environment in the [[Persian Gulf]] [[sabkha]]s as massive [[Diagenesis|diagenetic]] replacement [[nodule (geology)|nodule]]s. Cross sections of these nodular masses have a netted appearance and have been referred to as ''chicken-wire anhydrite''. Nodular anhydrite occurs as replacement of gypsum in a variety of sedimentary depositional environments.<ref>{{cite book|last=Michael A.|first=Church|title=Encyclopedia of Sediments & Sedimentary Rocks|publisher=Springer|date=2003|pages=17–18|isbn=978-1-4020-0872-6}}</ref>

===Salt dome cap rocks===
Massive amounts of anhydrite occur when [[salt dome]]s form a [[caprock]]. Anhydrite is 1–3% of the minerals in salt domes and is generally left as a cap at the top of the salt when the [[halite]] is removed by pore waters. The typical cap rock is a salt, topped by a layer of anhydrite, topped by patches of gypsum, topped by a layer of calcite.<ref>{{cite journal|first=C. W.|last=Walker|title=Origin of Gulf Coast salt-dome cap rock|journal=AAPG Bulletin|date=December 1976|volume=60|issue=12|pages=2162–2166|doi=10.1306/c1ea3aa0-16c9-11d7-8645000102c1865d}}</ref> Interaction of anhydrite with [[hydrocarbon]]s at high temperature in [[oil field]]s can reduce [[sulfate]] ({{chem|SO|4|2–}}) into [[hydrogen sulfide]] (H<sub>2</sub>S) with a concomitant precipitation of [[calcite]].<ref>{{cite journal|title=Origin of 'exotic' minerals in Mississippi salt dome cap rocks: results of reaction-path modeling|journal=Applied Geochemistry|volume=11|issue=5|date=September 1996|pages=667–676|doi=10.1016/S0883-2927(96)00032-7|first1=James A.|last1=Saunders|first2=Robert C.|last2=Thomas|bibcode=1996ApGC...11..667S}}</ref> The process is known as [[thermochemical sulfate reduction]] (TSR).
[[Image:Chickenwire Gypsum anhydrite.jpg|left|thumb|Hand sample of gypsum and anhydrite from diapir caprock showing "chicken wire" texture.]]

===Igneous rocks===
Anhydrite has been found in some [[igneous rock]]s, for example in the [[Intrusive rock|intrusive]] [[Diorite|dioritic]] [[pluton]] of El Teniente, Chile and in [[trachyandesite]] [[pumice]] erupted by [[El Chichón]] volcano, Mexico.<ref>{{cite journal|title=Primary igneous anhydrite: Progress since its recognition in the 1982 El Chichón trachyandesite|last=Luhr|first=James F.|journal=Journal of Volcanology and Geothermal Research|year=2008|volume=175|issue=4|pages=394–407|doi=10.1016/j.jvolgeores.2008.02.016|bibcode=2008JVGR..175..394L}}</ref>

==Naming history==
The name anhydrite was given by [[A. G. Werner]] in 1804, because of the absence of water of crystallization, as contrasted with the presence of water in gypsum. Some obsolete names for the species are muriacite and karstenite; the former, an earlier name, being given under the impression that the substance was a chloride (muriate). A peculiar variety occurring as contorted concretionary masses is known as tripe-stone, and a scaly granular variety, from [[Costa Volpino|Volpino]], near [[Bergamo]], in [[Lombardy]], as vulpinite; the latter is cut and polished for ornamental purposes.

A semi-transparent light blue-grey variety from [[Peru]] is referred to by the trade name ''angelite''.<ref>{{cite web|url=http://www.mindat.org/min-232.html|title=Angelite|website=Mindat.org}}</ref>

==Other uses==
[[File:Anhydrite Kiln by Ophelia Gordon Bell.jpg|thumb|left|Relief carving of an anhydrite kiln, made from a piece of anhydrite, by [[Ophelia Gordon Bell]] ]]
The [[Catalyst Science Discovery Centre]] in [[Widnes|Widnes, England]], has a relief carving of an anhydrite kiln, made from a piece of anhydrite, for the [[United Sulphuric Acid Corporation]].

[[Staufen im Breisgau#Geothermal drilling controversy|Extensive structural damage in the German city of Staufen im Breisgau]] has occurred since a 2007 geothermal drilling project allowed subsurface water to invade a layer of anhydrite below the city, causing extensive but uneven ground swelling as pockets of the anhydrite converted to gypsum.
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== References ==
{{Reflist}}

==Further reading==
* Spencer, Leonard James. Anhydrite. ''[[1911 Encyclopædia Britannica]]''
* [https://web.archive.org/web/20041019083222/http://mineral.galleries.com/minerals/sulfates/anhydrit/anhydrit.htm Mineralgalleries.com]
* [http://www.minerals.net/mineral/sulfates/anhydrit/anhydrit.htm Minerals.net]

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[[Category:Calcium minerals]]
[[Category:Sulfate minerals]]
[[Category:Evaporite]]
[[Category:Salt domes]]
[[Category:Baryte group]]
[[Category:Luminescent minerals]]
[[Category:Orthorhombic minerals]]
[[Category:Minerals in space group 63]]