Sillimanite

Sillimanite
Lustrous crystals of sillimanite (to 3 cm) embedded in schist matrix from Norwich, New London County, Connecticut
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	n_α = 1.653 - 1.661 n_β = 1.654 - 1.670 n_γ = 1.669 - 1.684
Birefringence	δ = 0.020 - 0.022
Pleochroism	Colourless to pale brown to yellow
2V angle	21 - 30°
References

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, US.

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.

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