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(repeating unit)
Strunz classification9.AF.05
Dana classification52.02.02a.01
Crystal systemOrthorhombic
Crystal classDipyramidal (mmm)
H-M symbol: (2/m 2/m 2/m)
Space groupPbnm
Unit cella = 7.47 Å, b = 7.66 Å
c = 5.75 Å; Z = 4
ColorColourless or white to grey, also brown, yellow, yellow-green, grey-green, blue-green, blue; colourless in thin section
Crystal habitPrismatic crystals, fibrous, acicular
Cleavage{010} perfect
Mohs scale hardness7
LusterVitreous to subadamantine, silky
DiaphaneityTransparent to translucent
Specific gravity3.24
Optical propertiesBiaxial (+)
Refractive indexnα = 1.653 - 1.661 nβ = 1.654 - 1.670 nγ = 1.669 - 1.684
Birefringenceδ = 0.020 - 0.022
PleochroismColourless to pale brown to yellow
2V angle21 - 30°
Phase diagram of Al2SiO5

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, US.[3]


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.[citation needed] Dumortierite and mullite are similar mineral species found in porcelain.[6]

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.[7]


See also[edit]


  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. 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. ^ Klein, Cornelis and Cornelius S. Hurlbut, Jr., Manual of Mineralogy, 1985, Wiley 20th ed., p. 380 ISBN 0-471-80580-7
  7. ^ "Archived copy". Archived from the original on 2010-06-10. Retrieved 2009-12-19.{{cite web}}: CS1 maint: archived copy as title (link) Delaware State Mineral, Sillimanite, Delaware Geological Survey