Editing Anorthoclase

{{short description|Alkali feldspar mineral}}
{{infobox mineral
| name        = Anorthoclase
| image       = Anorthoclase-tn23a.jpg
| alt         = 
| caption     = Doubly terminated anorthoclase crystal from [[Kinki Region]], [[Honshu]], Japan. Size: {{cvt|1.3|x|0.9|x|0.7|cm|1}}
| category    = [[Tectosilicate]] [[minerals]], [[feldspar]] group, [[Alkali feldspar|alkali]] series
| formula     = (Na,K)AlSi<sub>3</sub>O<sub>8</sub>
|IMAsymbol=Ano<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>
| molweight   = 
| strunz      = 9.FA.30
| dana        = 
| system      = [[Triclinic]]
| class       = Pinacoidal ({{overline|1}}) <br/><small>(same [[H–M symbol]])</small>
| symmetry    = ''C''{{overline|1}}
| unit cell   = ''a'' = 8.28, ''b'' = 12.97 <br/>''c'' = 7.15&nbsp;[Å]; α = 91.05° <br/>β = 116.26°, γ = 90.15°; ''Z''&nbsp;=&nbsp;4
| color       = Colorless, also white, pale creamy yellow, red, green
| colour      = 
| habit       = Short prismatic crystals; tabular, rhombic, flattened along [010]
| twinning    = Polysynthetic twinning produces a grid pattern on [100]
| cleavage    = Perfect on [001], other partings
| fracture    = Uneven
| tenacity    = Brittle
| mohs        = 6
| luster      = Vitreous to pearly on cleavage planes
| streak      = White
| diaphaneity = Transparent
| gravity     = 2.57–2.60
| density     = 
| polish      = 
| opticalprop = Biaxial (−)
| refractive  = ''n''<sub>α</sub> = 1.519–1.529<br />''n''<sub>β</sub> = 1.524–1.534<br />''n''<sub>γ</sub> = 1.527–1.536
| birefringence = ''δ'' = 0.008
| pleochroism = 
| 2V          = Measured: 34°–60°
| dispersion  = 
| extinction  = 
| length fast/slow =
| fluorescence = 
| absorption  = 
| melt        = 
| fusibility  = 
| diagnostic  = 
| solubility  = 
| impurities  = 
| alteration  = 
| references  = <ref name=HBM>{{cite web| url = http://www.handbookofmineralogy.com/pdfs/anorthoclase.pdf| title = Handbook of Mineralogy| access-date = 2011-12-04| archive-date = 2016-03-04| archive-url = https://web.archive.org/web/20160304062240/http://www.handbookofmineralogy.com/pdfs/anorthoclase.pdf}}</ref><ref name=Mindat>{{cite web| url = http://www.mindat.org/min-248.html| title = Mindat.org}}</ref><ref name=Webmin>{{cite web| url = http://webmineral.com/data/Anorthoclase.shtml| title = Webmineral data}}</ref>
}}

The mineral '''anorthoclase''' ((Na,K)AlSi<sub>3</sub>O<sub>8</sub>) is a crystalline [[solid solution]] in the [[alkali feldspar]] series, in which the sodium-aluminium silicate member exists in larger proportion. It typically consists of between 10 and 36 percent of KAlSi<sub>3</sub>O<sub>8</sub> and between 64 and 90 percent of NaAlSi<sub>3</sub>O<sub>8</sub>.<ref>{{cite book|last1=Deere |last2=Howie |last3=Zussman |series=Rock Forming Minerals |volume=4 |title=Framework Silicates |publisher=Wiley |pages=2–5, Fig. 1}}</ref>
[[File:Anorthoclase-219058.jpg|thumb|left|upright|An anorthoclase crystal from [[Mount Erebus]], Antarctica]]

==Structure and stability==
Anorthoclase is an intermediate member of the high [[albite]] – [[sanidine]] alkali feldspar [[solid solution]] series. Intermediate members of this series, high albite, anorthoclase and high sodium sanidine are stable at temperatures of {{convert|600|C|-2}} and above. Below {{convert|400|-|600|C|-2}} only very limited solution exists (less than about 5% on both the low albite and [[microcline]] ends).<ref name=Klein>{{cite book|last1=Klein |first1=Cornelis |first2=Cornelius S. Jr. |last2=Hurlbut |title=Handbook of Mineralogy |publisher=Wiley |pages=446–449, Fig. 11-95 |ISBN=0-471-80580-7}}</ref> Anorthoclase and high albite exhibit triclinic symmetry, whereas sanidine and the low temperature [[orthoclase]] have [[monoclinic]] symmetry.<ref name=Klein/> If the high temperature intermediate composition alkali feldspars are allowed to cool slowly, [[exsolution]] occurs and a [[perthite]] structure results.<ref name=Klein/>

==Occurrence==

Anorthoclase occurs in high temperature sodium rich [[volcanic]] and [[hypabyssal]] (shallow intrusive) rocks. The mineral is typically found as a constituent of the fine grained [[Matrix (geology)|matrix]] or as small phenocrysts which may occur as loose crystals in a weathered rock.<ref name=HBM/>

It was first described in 1885 for an occurrence on [[Pantelleria Island]], [[Trapani Province]], [[Sicily]]. The name is from the Greek αν-, ορθός and κλάσις (not cleaving at right-angles), for its oblique cleavage.<ref name=HBM/><ref name=Mindat/>

==See also==
*[[Erebus crystal]]

==References==
{{Reflist}}

[[Category:Feldspar]]
[[Category:Triclinic minerals]]
[[Category:Minerals in space group 2]]