sillimanite

{{Infobox mineral

| name = Sillimanite

| boxwidth =

| image = Sillimanite-k302a.jpg

| alt =

| caption =

| formula = Al₂SiO₅

| IMAsymbol = Sil{{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}}

| 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 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 = n_α = 1.653 – 1.661 n_β = 1.654 – 1.670 n_γ = 1.669 – 1.684

| birefringence = δ = 0.020 – 0.022

| pleochroism = Colourless to pale brown to yellow

| 2V = 21–30°

| dispersion =

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| references = {{cite web|url=http://webmineral.com/data/Sillimanite.shtml|title=WebMineral entry|accessdate=2009-12-19}}http://rruff.geo.arizona.edu/doclib/hom/sillimanite.pdf Handbook of Mineralogyhttp://www.mindat.org/min-3662.html Mindat.org{{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]}}

}}

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.

Occurrence

Sillimanite or fibrolite is one of three aluminosilicate 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 metamorphosed sedimentary rocks. 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 pegmatites. Dumortierite and mullite are similar mineral species found in porcelain.Klein, Cornelis and Cornelius S. Hurlbut, Jr., Manual of Mineralogy, 1985, Wiley 20th ed., p. 380 {{ISBN|0-471-80580-7}}

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.{{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

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.{{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 }} {{as of|1998}}, sillimanite was just 2% of all aluminosilicate mineral production in the western world.{{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}}