Sillimanite

Sillimanite 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, Middlesex County, Connecticut, US.[3]

Sillimanite
General
Category	Nesosilicate
Formula (repeating unit)	Al2SiO5
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	[1][2][3][4]

Kyanite

Andalusite

Sillimanite

Phase diagram of Al₂SiO₅
(nesosilicates).[6]

Occurrence

Sillimanite 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.[2]

Natural sillimanite rocks cut into the required shape and size are used mainly in glass industries. Sillimanite is the best raw material for the manufacture of high alumina refractories or 55-60% alumina bricks. But its use on large scale is not possible due to its fine grading and high cost. Dumortierite and mullite are similar mineral species found in porcelain.[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.[8]

Images

• 
  Silimanite crystal from Sri Lanka
• 
  Fibrolite micrograph

See also

• List of minerals
• List of minerals named after people

References

1. "WebMineral entry". Retrieved 2009-12-19.
2. http://rruff.geo.arizona.edu/doclib/hom/sillimanite.pdf Handbook of Mineralogy
3. http://www.mindat.org/min-3662.html Mindat.org
4. 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.
5. 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.
6. 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.
7. Klein, Cornelis and Cornelius S. Hurlbut, Jr., Manual of Mineralogy, 1985, Wiley 20th ed., p. 380 ISBN 0-471-80580-7
8. "Archived copy". Archived from the original on 2010-06-10. Retrieved 2009-12-19.CS1 maint: archived copy as title (link) Delaware State Mineral, Sillimanite, Delaware Geological Survey