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 |