Sillimanite is an alumino-sillicate mineral with the chemical formula Al2SiO5. Sillimanite is named after the American chemist B. Silliman (1779-1824).
Sillimanite is one of three alumino-sillicate 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.