Sillimanite
Lustrous crystals of sillimanite (to 3 cm) embedded in schist matrix from Norwich, New London County, Connecticut
General
Category	Silicate mineral
Chemical formula	Al2SiO5
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
Identification
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
Pleochroism	Colorless to pale brown to yellow
2V angle	21 - 30°
References	[1][2][3]

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]

Occurrence[link]

Silimanite crystal from Sri Lanka

Sillimanite is one of three alumino-silicate 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.^[4]

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 used in quality porcelain.^[5]

See also[link]

• List of minerals
• List of minerals named after people

References[link]

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