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.

Details

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

Related minerals

1. Kyanite
2. Sillimanite
3. Chiastolite[?]