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.

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Sillimanite 

a mineral of the silicate class. Sillimanite is a high-temperature, polymorphic modification of the composition Al[AlSiO₅] and contains 1–1.5 percent Fe₂O₃ in admixture form. The sillimanite structure is made up of chains of alternating SiO₄ and AlO₄ tetrahedrons, connected by chains of Al octahedrons. The mineral crystallizes in the orthorhombic system, forming acicular crystals, compact radiated masses, or thin fibrous aggregates. It sometimes forms dispersed, filiform inclusions in other minerals (fibrolite variety). Sillimanite is gray, light brown, or pale green; it has a vitreous luster, a hardness of 6.5-7.5 on Mohs’ scale, and a density of 3,270 kg/m³. At high temperatures (approximately 1545°C), it decomposes into mullite and silica.

Sillimanite occurs in thermally and regionally metamorphosed clay rocks. It serves as a raw material in the production of refractories and acid-resistant products with a high alumina content. The largest known sillimanite deposits are in India (Khasi Hills and Pipri).

REFERENCE

Kostov, 1. Mineralogiia. Moscow, 1971. (Translated from English.)