Sillimanite
Sillimanite or fibrolite is an aluminosilicate mineral with the chemical formula Al2SiO5. Sillimanite is named after the American chemist Benjamin Silliman (1779–1864). It was first described in 1824 for an occurrence in Chester, Connecticut.
| Sillimanite | |
|---|---|
| General | |
| Category | Nesosilicate |
| Formula (repeating unit) | Al2SiO5 |
| IMA symbol | Sil |
| Strunz classification | 9.AF.05 |
| Dana classification | 52.02.02a.01 |
| Crystal system | Orthorhombic |
| Crystal class | Dipyramidal (mmm) H-M symbol: (2/m 2/m 2/m) |
| Space group | Pbnm |
| Unit cell | a = 7.47 Å, b = 7.66 Å c = 5.75 Å; Z = 4 |
| Identification | |
| Color | Colourless or white to grey, also brown, yellow, yellow-green, grey-green, blue-green, blue; colourless in thin section |
| Crystal habit | Prismatic crystals, fibrous, acicular |
| Cleavage | {010} perfect |
| Fracture | Splintery |
| Tenacity | Tough |
| Mohs scale hardness | 7 |
| Luster | Vitreous to subadamantine, silky |
| Streak | White |
| Diaphaneity | Transparent to translucent |
| Specific gravity | 3.24 |
| Optical properties | Biaxial (+) |
| Refractive index | nα = 1.653 – 1.661 nβ = 1.654 – 1.670 nγ = 1.669 – 1.684 |
| Birefringence | δ = 0.020 – 0.022 |
| Pleochroism | Colourless to pale brown to yellow |
| 2V angle | 21–30° |
| References | |
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