A nesosilicate mineral of composition Al₂O(SiO₄), crystallizing in the orthorhombic system, space group Pbnm, a = 0.744, b = 0.760, and c = 0.575 nanometers, with four formula units in the cell. Sillimanite commonly occurs in slender crystals or parallel groups, and is frequently fibrous, hence the synonym fibrolite (see illus.). There is one perfect cleavage, luster is vitreous, color is brown, pale green, or white, hardness is 6–7 on Mohs scale, and the specific gravity is 3.23.