Editing Andalusite (section)

{{Short description|Aluminium nesosilicate mineral}}
{{Infobox mineral
| name        = Andalusite
| category    = [[Nesosilicates]] 
| image       = Andalusite - Malacheta, Minas Gerais, Brazil.jpg
| imagesize   = 260px
| alt         = 
| caption     = Andalusite
| formula     = Al<sub>2</sub>SiO<sub>5</sub>
| molweight   = 
|IMAsymbol=And| strunz      = 9.AF.10
| system      = [[Orthorhombic]]
| class       = Dipyramidal (mmm) <br/>[[H-M symbol]]: (2/m 2/m 2/m)
| symmetry    = ''Pnnm''
| unit cell   = a = 7.7980&nbsp;[[Ångstrom|Å]], b = 7.9031&nbsp;Å <br/>c = 5.5566&nbsp;Å; Z&nbsp;=&nbsp;4
| color       = Pink, violet, yellow, green, white, gray; in thin section, colorless to pink or green
| habit       = As euhedral crystals or columnar aggregates having nearly square cross sections; fibrous compact to massive
| twinning    = On {101}, rare
| cleavage    = Good on {110}, poor on {100}
| fracture    = uneven to subconchoidal
| mohs        = 6.5–7.5
| luster      = Vitreous
| polish      = 
| refractive  = n<sub>α</sub> = 1.629 – 1.640 n<sub>β</sub> = 1.633 – 1.644 n<sub>γ</sub> = 1.638 – 1.650
| opticalprop = double refractive, biaxial negative; ''chiastolite'' has anomalous aggregate reaction.<ref name=GIA/> 
| birefringence = δ = 0.009 – 0.010
| dispersion  = r < v strong
| pleochroism = strongly trichroic
| 2V          = 71–86°
| fluorescence= non-fluorescent 
| absorption  =
| streak      = White
| gravity     = 3.17 (± .04)
| density     = 
| melt        = 
| fusibility  = 
| diagnostic  = 
| solubility  = 
| diaphaneity = Transparent to nearly opaque with inclusions
| references  = <ref name=GIA>[[Gemological Institute of America]], ''GIA Gem Reference Guide'' 1995, {{ISBN|0-87311-019-6}}</ref><ref name=Handbook>http://rruff.geo.arizona.edu/doclib/hom/andalusite.pdf Handbook of Mineralogy</ref>
}}
'''Andalusite''' is an [[aluminium]] [[Silicate mineral|nesosilicate]] [[mineral]] with the chemical formula Al<sub>2</sub>SiO<sub>5</sub>. This mineral was called andalousite by Delamétherie,<ref>{{Cite journal|last=Delamétherie|first=Jean Claude|date=1798|title=Sur une pierre de l'Andalousie|journal=Journal de Physique, de Chimie d'Histoire Naturelle et des Arts|volume=46|pages=386–387}}</ref> who thought it came from [[Andalusia]], Spain. It soon became clear that it was a locality error, and that the specimens studied were actually from El Cardoso de la Sierra, in the Spanish province of [[Province of Guadalajara|Guadalajara]], not Andalusia.<ref>{{Cite book|title=Minerales y Minas de España. Vol. IX. Silicatos|last=Calvo|first=Miguel|publisher=Escuela Técnica Superior de Ingenieros de Minas de Madrid. Fundación Gómez Pardo|year=2018|isbn=978-84-8321-883-9|location=Madrid, Spain|pages=91–94}}</ref>

Andalusite is trimorphic with [[kyanite]] and [[sillimanite]], being the lower pressure mid temperature polymorph. At higher temperatures and pressures, andalusite may convert to sillimanite. Thus, as with its other polymorphs, andalusite is an [[aluminosilicate]] index mineral, providing clues to depth and pressures involved in producing the host rock.<ref>{{Cite journal|last=Whitney, D.L.|date=2002|title=Coexisting andalusite, kyanite, and sillimanite: Sequential formation of three Al2SiO5 polymorphs during progressive metamorphism near the triple point, Sivrihisar, Turkey|journal=American Mineralogist|volume=87|issue=4|pages=405–416|doi=10.2138/am-2002-0404|bibcode=2002AmMin..87..405W|s2cid=131616262}}</ref>
{{al2sio5 phase diagram}}
[[File:Round faceted andalusite 1.jpg|thumb|Round faceted andalusite]]