Sillimanite is a polymorph of andalusite and kyanite. These three minerals share the same formula, but are stable at different P/T conditions.
Physical Properties |
|
| Chemical formula | AlSi2O5 |
| Class | Nesosilicate Isolated tetrahedra |
| Crystal system | Orthorhombic |
| Habit | Acicular Long, slender prisms Fibrous Subparallel aggregates and splays Fine-grained fibrous mats |
| Color | White to brown |
| Hardness | 6 to 7 |
| Specific gravity | 3.23 |
| Cleavage | Perfect (010) (rare) |
| Fracture | Uneven |
| Luster | Vitreous |
| Transparency | Transparent to translucent |
| Streak | White |
Optical Properties |
|
| PPL | Colorless High relief |
| XPL | High 2nd order colors |
| δ | 0.022 |
| after Perkins, 345-346 |
Sillimanite in Hand Sample
Sillimanite (brown) in kaolinite
Sillimanite in Thin Section
Thin Section GigaPans
Sillimanite in sillimanite-garnet gneiss, plane polars
Sillimanite in sillimanite-garnet gneiss, crossed polars
Sillimanite, Plane Polars
Sillimanite in plane polarized light. Magnification: 100x Rock: Sillimanite-garnet gneiss Click here to watch this grain rotate in plane polars: https://yout…
Sillimanite, Crossed Polars
Sillimanite in cross polarized light. Magnification: 100x Rock: Sillimanite-garnet gneiss Click here to watch this grain rotate in plane polars: https://yout…