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Title: Sillimanite  
Author: World Heritage Encyclopedia
Language: English
Subject: Andalusite, Kyanite, Metamorphic facies, Metamorphic rock, Physical geography of Assam
Collection: Aluminium Minerals, Nesosilicates, Orthorhombic Minerals
Publisher: World Heritage Encyclopedia


Lustrous crystals of sillimanite (to 3 cm) embedded in schist matrix from Norwich, New London County, Connecticut
Category Nesosilicate
(repeating unit)
Strunz classification 09.AF.05
Dana classification 52.02.02a.01
Crystal symmetry Orthorhombic (2/m 2/m 2/m)
Unit cell a = 7.47 Å, b = 7.66 Å, c = 5.75 Å; Z=4
Color Colorless or white to gray, also brown, yellow, yellow-green, gray-green, blue-green, blue; colorless in thin section
Crystal habit Prismatic crystals, fibrous, acicular
Crystal system Orthorhombic - Dipyramidal
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 Colorless to pale brown to yellow
2V angle 21 - 30°
References [1][2][3][4]
Phase diagram of Al2SiO5

Sillimanite is an alumino-silicate 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, USA.[3]


Silimanite crystal from Sri Lanka

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]

Sillimanite has been found in Brandywine Springs, New Castle County, Delaware, USA. It was named by the State Legislature in 1977 as the state mineral of Delaware by suggestion of the Delaware Mineralogical Society, Inc.[7]

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 used in quality porcelain.[8]

See also


  1. ^ "WebMineral entry". Retrieved 2009-12-19. 
  2. ^ a b Handbook of Mineralogy
  3. ^ a b
  4. ^ MacKenzie, W. S.; Guilford, C. (1980). Atlas of rock-forming minerals in thin section. Essex: Longman Scientific & Technical. p. 10.  
  5. ^ Whitney, D.L. (2002), polymorphs during progressive metamorphism near the triple point, Sivrihisar, Turkey"5SiO2"Coexisting andalusite, kyanite, and sillimanite: Sequential formation of three Al, American Mineralogist 87 (4): 405–416 
  6. ^ Whitney, D.L. (2002), polymorphs during progressive metamorphism near the triple point, Sivrihisar, Turkey"5SiO2"Coexisting andalusite, kyanite, and sillimanite: Sequential formation of three Al, American Mineralogist 87 (4): 405–416 
  7. ^ Delaware State Mineral, Sillimanite, Delaware Geological Survey
  8. ^ Klein, Cornelis and Cornelius S. Hurlbut, Jr., Manual of Mineralogy, 1985, Wiley 20th ed., p. 380 ISBN 0-471-80580-7
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