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{{short description|Nesosilicate mineral}}
{{Infobox mineral
| name = Sillimanite
| category = [[Nesosilicate]]
| boxwidth =
| image = Sillimanite-k302a.jpg
| alt =
| caption =
| formula = Al2SiO5
| IMAsymbol = Sil<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.AF.05
| dana = 52.02.02a.01
| system = [[Orthorhombic]]
| class = Dipyramidal (mmm) [[H-M symbol]]: (2/m 2/m 2/m)
| symmetry = ''Pbnm''
| unit cell = a = 7.47 [[Ångstrom|Å]], b = 7.66 Å c = 5.75 Å; Z = 4
| color = Colourless or white to grey, also brown, yellow, yellow-green, grey-green, blue-green, blue; colourless in thin section
| colour =
| habit = Prismatic crystals, fibrous, acicular
| twinning =
| cleavage = {010} perfect
| fracture = Splintery
| tenacity = Tough
| mohs = 7
| luster = Vitreous to subadamantine, silky
| streak = White
| diaphaneity = Transparent to translucent
| gravity = 3.24
| density =
| polish =
| opticalprop = Biaxial (+)
| refractive = nα = 1.653 - 1.661 nβ = 1.654 - 1.670 nγ = 1.669 - 1.684
| birefringence = δ = 0.020 - 0.022
| pleochroism = Colourless to pale brown to yellow
| 2V = 21 - 30°
| dispersion =
| extinction =
| length fast/slow =
| fluorescence=
| absorption =
| melt =
| fusibility =
| diagnostic =
| solubility =
| other =
| alteration =
| references = <ref>{{cite web|url=http://webmineral.com/data/Sillimanite.shtml|title=WebMineral entry|accessdate=2009-12-19}}</ref><ref name=Handbook>http://rruff.geo.arizona.edu/doclib/hom/sillimanite.pdf Handbook of Mineralogy</ref><ref name=Mindat>http://www.mindat.org/min-3662.html Mindat.org</ref><ref>{{cite book|last1=MacKenzie|first1=W. S.|last2=Guilford|first2=C.|title=Atlas of rock-forming minerals in thin section|url=https://archive.org/details/atlasrockforming00mack_408|url-access=limited|date=1980|publisher=Longman Scientific & Technical|location=Essex|isbn=0-582-45591-X|page=[https://archive.org/details/atlasrockforming00mack_408/page/n15 10]}}</ref>
}}
{{al2sio5 phase diagram}}
'''Sillimanite''' or '''fibrolite''' is an [[aluminosilicate]] [[mineral]] with the [[chemical formula]] Al2SiO5. Sillimanite is named after the American chemist [[Benjamin Silliman]] (1779–1864). It was first described in 1824 for an occurrence in [[Chester, Connecticut]].<ref name=Mindat/>

==Occurrence==
Sillimanite or is one of three [[aluminosilicate]] [[polymorphism (materials science)|polymorphs]], the other two being [[andalusite]] and [[kyanite]]. A common variety of sillimanite is known as ''fibrolite'', so named because the mineral appears like a bunch of fibres twisted together when viewed in [[thin section]] or even by the naked eye. Both the fibrous and traditional forms of sillimanite are common in [[metamorphism|metamorphosed]] [[sedimentary rock]]s. It is an [[index mineral]] indicating high temperature but variable pressure. Example rocks include [[gneiss]] and [[granulite]]. It occurs with andalusite, kyanite, [[potassium feldspar]], [[almandine]], [[cordierite]], [[biotite]] and [[quartz]] in [[schist]], gneiss, [[hornfels]] and also rarely in [[pegmatite]]s.<ref name=Handbook/> [[Dumortierite]] and [[mullite]] are similar mineral species found in [[porcelain]].<ref name=Klein>Klein, Cornelis and Cornelius S. Hurlbut, Jr., ''Manual of Mineralogy,'' 1985, Wiley 20th ed., p. 380 {{ISBN|0-471-80580-7}}</ref>

Sillimanite has been found in [[Brandywine Springs]], [[New Castle County, Delaware]]. It was named by the State Legislature in 1977 as the state mineral of Delaware by the suggestion of the [[Delaware Mineralogical Society]].<ref>{{cite web |url=http://www.dgs.udel.edu/Geology/Mineralogy/destatemineral.aspx |title=Delaware State Mineral - Delaware Geological Survey |accessdate=2009-12-19 |url-status=dead |archiveurl=https://web.archive.org/web/20100610070156/http://www.dgs.udel.edu/Geology/Mineralogy/destatemineral.aspx |archivedate=2010-06-10 }} Delaware State Mineral, Sillimanite, Delaware Geological Survey</ref>

==Uses==
Natural sillimanite is used in the manufacture of high [[alumina]] refractories or 55-60% alumina bricks. However, it has mostly been replaced by the other aluminosilicate polymorphs, andalusite and kyanite, for this purpose.<ref>{{cite journal |last1=Gilchrist |first1=J.D. |title=Alumino-Silicate Refractories |journal=Fuels, Furnaces and Refractories |date=1977 |pages=258–272 |doi=10.1016/B978-0-08-020430-7.50027-X|isbn=9780080204307 }}</ref> {{as of|1998}}, sillimanite was just 2% of all aluminosilicate mineral production in the western world.<ref>{{cite journal |last1=Ihlen |first1=Peter M. |title=Utilisation of sillimanite minerals, their geology, and potential occurrences in Norway – an overview |journal=Geological Survey of Norway Bulletin |date=2000 |volume=436 |page=113 |url=https://www.ngu.no/filearchive/102/Bulletin436_12.pdf |access-date=14 March 2022}}</ref>

==Images==
<gallery>
File:Sillimanite.jpg|Silimanite crystal from [[Sri Lanka]]
File:Fibrolite.JPG|Fibrolite micrograph
</gallery>

==See also==

*[[List of minerals]]
*[[List of minerals named after people]]

==References==
{{reflist}}

[[Category:Aluminium minerals]]
[[Category:Nesosilicates]]
[[Category:Orthorhombic minerals]]
[[Category:Minerals in space group 62]]
[[Category:Industrial minerals]]
[[Category:Symbols of Delaware]]

Sillimanite - Revision history

Ajay Kumar: Created a new article