1911 Encyclopædia Britannica/Sillimanite

by Leonard James Spencer

SILLIMANITE, a rock-forming mineral consisting of aluminium silicate, Al₂SiO₅. It has the same percentage chemical composition as cyanite (q.v.) and andalusite (q.v.), but differs from these in crystalline form and physical characters. It crystallizes in the orthorhombic system and has the form of long, slender needles without terminal planes, which are often aggregated together to form fibrous and compact masses; hence the name fibrolite, which is often employed for this species. The name sillimanite is after Benjamin Silliman the elder. There is a perfect cleavage in one direction parallel to the length of the needles. The colour is greyish-white or brownish, and the lustre vitreous. The hardness is 6½ and the specific gravity 3.23. Sillimanite is a characteristic mineral of gneisses and crystalline schists, and it is sometimes a product of contact-metamorphism. It has been observed at many localities; e.g. in Bohemia (the Faserkiesel of Lindacker, 1792), with corundum in the Carnatic (fibrolite of comte de Bournon, 1802), Chester in Connecticut (sillimanite of G. T. Bowen, 1824), Monroe in New York (" monrolite "), Bamle near Brevik in Norway (" bamlite "). Prehistoric implements made of compact sillimanite are found in western Europe, and have a certain resemblance to jade implements.  (L. J. S.)