Andalusite
Andalusite is an alumino-silicate mineral with the chemical formula Al2SiO5.
Occurrence
Andalusite is a common metamorphic mineral which forms under high pressure and/or temperature. The minerals kyanite and sillimanite are polymorphs of andalusite, each occurring under different temperature-pressure regimes and are therefore rarely found together in the same rock. Because of this the three minerals are a useful tool to help identify the pressure-temperature paths of the host rock in which they are found.
| Crystallographic system | Orthorhombic |
| Colours | Brown, green, pink, violet and red varieties |
| Lustre | |
| Hardness | 7.5 |
| Refractive index | 1.641-1.648 |
| Cleavage | 3 |
| Twinning | Lamellar |
| Pleochroism | Clear to patchy pink |