SILLIMANITE

also called Fibrolite, brown, pale green, or white glassy silicate mineral that often occurs in long, slender, needlelike crystals frequently found in fibrous aggregates. An aluminum silicate, Al2OSiO4, it occurs in high-temperature regionally metamorphosed clay-rich rocks (e.g., schists and gneisses). Sillimanite is found at many points in France, Madagascar, and the eastern United States; a pale sapphire-blue gem variety occurs in the gravels of Sri Lanka. For detailed physical properties, see silicate mineral (table).