Andalusite is an aluminium mineral with the chemical formula Al2SiO5. This mineral was called andalousite by Delamétehrie, who thought it came from Andalusia. It soon became clear that it was a locality error, and that the specimens studied were actually from El Cardoso de la Sierra, in the province of Guadalajara, not Andalusia.
Andalusite is trimorphic with kyanite and sillimanite, being the lower pressure mid temperature polymorph. At higher temperatures and pressures, andalusite may convert to sillimanite. Thus, as with its other polymorphs, andalusite is an aluminosilicate index mineral, providing clues to depth and pressures involved in producing the host rock. Kyanite Andalusite Sillimanite ./File:Al2SiO5_phase_diagram.svg Phase diagram of Al2SiO5 (nesosilicates