Andalusite

Andalusite is an aluminium silicate mineral with the chemical formula Al2SiO5. It is one of the three polymorphs of Al2SiO5, together with kyanite and sillimanite. The mineral crystallizes in the orthorhombic system and commonly forms slender, prismatic crystals or is found in massive aggregates. Andalusite is named after Andalusia, Spain, where it was first described.

Physical properties include a color range from colorless to gray, greenish, or brown, a vitreous luster, and a hardness of about 7 to 7.5 on the Mohs scale. Its specific gravity is approximately 3.15–3.23. Andalusite exhibits strong pleochroism, with X and Y axes often producing different colors. Cleavage is poor, and fracture is uneven to splintery.

Andalusite forms during low- to medium-grade regional metamorphism and in contact metamorphic rocks rich in alumina. It is commonly associated with quartz, mica, garnet, cordierite, and sometimes sillimanite or kyanite. The stability field places andalusite at relatively low pressure; with increasing temperature, sillimanite becomes stable, and with high pressures, kyanite becomes stable.

A well-known variety is chiastolite, which contains cross-shaped inclusions of graphite and is valued as a gemstone. Industrially, andalusite is used as an alumina-bearing raw material for refractories and ceramics, prized for resistance to thermal shock; it can be processed into bricks and kiln furniture.

Notable localities include Andalusia in Spain; other sources include Sri Lanka, Brazil, Madagascar, Finland, and the United States.