Sillimanite
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Sillimanite also called Bucholzite is an alumino-silicate mineral with the chemical formula Al2SiO5. Sillimanite is named after the American chemist Benjamin Silliman (1779-1864).
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[edit] Occurrence
Sillimanite is one of three alumino-silicate 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. It is an index mineral indicating a high degree of metamorphism.
Sillimanite has been found in Brandywine Springs, New Castle County, Delaware, USA. By suggestion of the members of the Delaware Mineralogical Society, it was named by the State Legislature in 1977 as the Official Mineral of Delaware. It is also known from the Rock Creek Shear Zone within the Piedmont physiographic province of the District of Columbia, USA.Sillimanite is one of the index minerals that are used to estimate the temperature, depth, and pressure at which a rock undergoes metamorphism.
[edit] 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. |
