Class : Silicates
    Subclass : Nesosilicates
    Crystal system : Orthorhombic
    Chemistry : Al₂SiO₅
    Rarity : Common

Sillimanite is the polymorph of kyanite and andalusite. Very widespread, it is found mainly in clayey rocks that have undergone very strong thermal metamorphism (cordierite - sillimanite gneiss), more rarely in aluminous granites. It was named in honor of Benjamin Silliman, Professor of Chemistry and Mineralogy at Yale University (Connecticut). Sillimanite most often forms compact masses of very tight, extremely tenacious acicular crystals (fibrolite variety); the crystals are very rare, forming very elongated prisms striated longitudinally, with a square section, poorly terminated, barely exceeding 2 cm. Its color is white to gray, sometimes pinkish white, greenish, bluish or brownish. Sillimanite is incidentally used as an "ore" of mullite, a basic material in the manufacture of refractories. However, its exploitation is much more limited than that of kyanite or andalusite. Some specimens of sillimanite are cut as gemstone. Neolithic adzes made of sillimanite (of the very compact fibrolite variety), although rare, have been discovered during prehistoric excavations in Brittany (France).

Main photo : Sillimanite from Ratnapura, Sri Lanka © Aymeric Longi

No twinning known for this mineral species.

No fakes listed for this mineral species.