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 61 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 ") . Pre-historic implements made of compact sillimanite are found in western Europe, and have a certain resemblance to jade implements .

(L . J .