Andalusite (Al₂SiO₅) is the low-pressure aluminosilicate polymorph (with kyanite high-P and sillimanite high-T). Its name comes from Andalusia (Spain). The “chiastolite” variety contains carbonaceous cross-shaped inclusions producing iconic “fairy cross” patterns on cut sections — a longtime collector favorite from Madera (California) and Lancaster (Massachusetts).
Key Facts
- Mohs hardness 6.5–7.5.
- Orthorhombic; one of three Al₂SiO₅ polymorphs.
- Strong pleochroism in colored varieties (green / yellow / red).
- Chiastolite variety: cross-shaped carbonaceous inclusions on cleaved cross-sections.
- Diagnostic indicator of low-pressure metamorphism.
Notable Localities
Andalusia (Spain) and Madera (California) yield chiastolite. Minas Gerais (Brazil) supplies gem material. Lancaster (Massachusetts) is a classic American source.
Found at these Localities
- Swiss Alps (Alpine-Cleft Province) (瑞士阿尔卑斯高山裂隙矿物产地)
- Eastern Brazilian Pegmatite Province (巴西东部伟晶岩省)
