alusite

Andalusite, also spelled alusite, is an aluminum silicate mineral with the chemical composition Al2SiO5. It belongs to the aluminosilicate group that includes kyanite and sillimanite, which are polymorphs of Al2SiO5 formed under different pressure–temperature conditions. Andalusite forms as a mineral in metamorphic rocks, especially under relatively low-grade regional metamorphism and in contact metamorphic aureoles. The name derives from Andalusia in southern Spain, where the mineral was first described.

Andalusite crystallizes in the orthorhombic system and typically occurs as tabular or prismatic crystals, or as porous, blocky aggregates known as porphyroblasts, within schists, gneisses, quartzites, and granulites. It often co-occurs with minerals such as cordierite, sillimanite, quartz, and muscovite. A characteristic variety, chiastolite, contains a distinctive cross-shaped pattern formed by carbonaceous or ferruginous inclusions, making it recognizable to collectors and in jewelry.

In terms of appearance, andalusite ranges from colorless or white to gray, brown, or pink, and it exhibits pleochroism in polarized light. It is relatively hard and chemically stable, with good resistance to high temperatures, properties that underpin its use as a refractory material in the ceramic and metallurgical industries, including firebricks and kiln linings. It is also cut as a gemstone for some jewelry, though it is less common than other metamorphic aluminosilicates.

Geologically, andalusite forms at lower temperatures and pressures than kyanite or sillimanite and helps indicate specific metamorphic conditions during rock formation. Notable localities include Andalusia in Spain (the namesake), Brazil, Sri Lanka, Madagascar, and parts of the United States such as California and North Carolina.