Andalusite

Andalusite is an aluminum nesosilicate mineral with the chemical formula Al2SiO5. A trimorph with kyanite and sillimanite, andalusite occurs as the polymorph at lower pressure and moderate temperatures. At higher temperatures and pressures, andalusite can transform into sillimanite. Like its other polymorphs, andalusite is therefore an aluminosilicate index mineral that provides information on the depth and pressure at which the host rock was formed.