Editing Anhydrite (section)

{{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.