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 a high degree of metamorphism.

  • 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.