What is "andalusite"

Andalusite \An`da*lu"site\, n. (Min.) A silicate of aluminium, occurring usually in thick rhombic prisms, nearly square, of a grayish or pale reddish tint. It was first discovered in Andalusia, Spain. [1913 Webster] ||

n. (context mineral English) An aluminium nesosilicate mineral, Al₂SiO₅.

Andalusite is an aluminium nesosilicate mineral with the chemical formula AlSiO. 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.