Sillimanite

Sillimanite
Sillimanite
General
Category	Nesosilicate
Formula; (repeating unit)	Al2SiO5
IMA symbol	Sil
Strunz classification	9.AF.05
Dana classification	52.02.02a.01
Crystal system	Orthorhombic
Crystal class	Dipyramidal (mmm) ; H-M symbol: (2/m 2/m 2/m)
Space group	Pbnm
Unit cell	a = 7.47 Å, b = 7.66 Å ; c = 5.75 Å; Z = 4
Identification
Color	Colourless or white to grey, also brown, yellow, yellow-green, grey-green, blue-green, blue; colourless in thin section
Crystal habit	Prismatic crystals, fibrous, acicular
Cleavage	{010} perfect
Fracture	Splintery
Tenacity	Tough
Mohs scale hardness	7
Luster	Vitreous to subadamantine, silky
Streak	White
Diaphaneity	Transparent to translucent
Specific gravity	3.24
Optical properties	Biaxial (+)
Refractive index	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 angle	21–30°
References

Phase diagram of Al₂SiO₅
(aluminosilicates).^[6]

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.^[4]

^ Warr, L.N. (2021). "IMA–CNMNC approved mineral symbols". Mineralogical Magazine. 85 (3): 291–320. Bibcode:2021MinM...85..291W. doi:10.1180/mgm.2021.43. S2CID 235729616.
^ "WebMineral entry". Retrieved 2009-12-19.
^ http://rruff.geo.arizona.edu/doclib/hom/sillimanite.pdf Handbook of Mineralogy
^ ^a ^b http://www.mindat.org/min-3662.html Mindat.org
^ MacKenzie, W. S.; Guilford, C. (1980). Atlas of rock-forming minerals in thin section. Essex: Longman Scientific & Technical. p. 10. ISBN 0-582-45591-X.
^ Whitney, D.L. (2002). "Coexisting andalusite, kyanite, and sillimanite: Sequential formation of three Al₂SiO₅ polymorphs during progressive metamorphism near the triple point, Sivrihisar, Turkey". American Mineralogist. 87 (4): 405–416. doi:10.2138/am-2002-0404.