sillimanite

sillimanite

 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, Al₂OSiO₄, 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.

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sillimanite [′sil·ə·mə‚nīt]

(mineralogy)

Al₂SiO₅A brown, pale-green, or white neosilicate mineral with vitreous luster crystallizing in the orthorhombic system; commonly occurs in slender crystals, often in fibrous aggregates; hardness is 6-7 on Mohs scale, and specific gravity is 3.23. Also known as fibrolite.