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).
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 a high degree of metamorphism.
Crystallographic system Orthorhombic
Colours White, brown and green
Lustre Silky if in fibrous form, else vitreous
Refractive index 1.641-1.648
Pleochroism Colourless to pale brown to yellow
Type locality Vltava, Susice, Czech Republic.