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Sillimanite

Sillimanite is an alumino-sillicate mineral with the chemical formula Al2SiO5. Sillimanite is named after the American chemist B. Silliman (1779-1824).

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Occurrence Sillimanite is one of three alumino-sillicate polymorphs[?], the other two being andalusite and kyanite. A common variety of sillimanite is known as fibrolite, so named because the mineral appears like a bunch of fibres twisted together when viewed under thin section or even by the naked eye. Both the fibrous and traditional forms of sillimanite are common in metamorphosed sedimentary rocks.

Details

Crystallographic systemOrthorhombic
ColoursWhite, brown and green
LustreSilky if in fibrous form, else vitreous
Hardness7.5
Refractive index 1.641-1.648
Cleavage1
Twinningn/a
PleochroismColourless to pale brown to yellow
Type localityVltava, Susice[?], Czech Republic.

Related minerals

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