Sillimanite

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

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 system	Orthorhombic
Colours	White, brown and green
Lustre	Silky if in fibrous form, else vitreous
Hardness	7.5
Refractive index	1.641-1.648
Cleavage	1
Twinning	n/a
Pleochroism	Colourless to pale brown to yellow
Type locality	Vltava, Susice, Czech Republic.

Related minerals

Andalusite

Kyanite

External links

WebMineral entry

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