or 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, Al 2 OSiO 4 , it occurs in high-temperature regionally metamorphosed clay-rich rocks (e.g., schists and gneisses ). Sillimanite is found at many localities in France, Madagascar, and the eastern U.S.; a pale sapphire-blue gem variety occurs in the gravels of Sri Lanka.