Sillimanite

Sillimanite also called Bucholzite is an alumino-silicate mineral with the chemical formula Al₂SiO₅. Sillimanite is named after the American chemist Benjamin Silliman (1779-1864).

Occurrence

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 under 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 metamorphism.

Sillimanite has been found in Brandywine Springs, New Castle County, Delaware, USA. By suggestion of the members of the Delaware Mineralogical Society, it was named by the State Legislature in 1977 as the Official Mineral of Delaware

Details

Crystallographic system	Orthorhombic
Colours	White, brown and green
Lustre	Silky if in fibrous form, else vitreous
Hardness	7.5
Refractive index	1.641-1.648
Cleavage	1
Twinning	n/a
Pleochroism	Colourless to pale brown to yellow
Type locality	Vltava, Susice, Czech Republic.

See also

• Andalusite
• Kyanite
• List of minerals
• List of minerals named after people

External links

• WebMineral entry