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2 Details 3 Related minerals 4 External links |
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.
| 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. |