Andalusite

Andalusite is an aluminum silicate mineral and one of three Al₂SiO₅ polymorphs. It commonly forms in metamorphic rocks under relatively low pressure.

Al₂SiO₅ polymorphs: The three minerals andalusite, kyanite, and sillimanite share the chemical composition Al₂SiO₅ but have different crystal structures. Andalusite is one member of this compositionally identical mineral group.
Aluminum silicate: A mineral or material composed principally of aluminum, silicon, and oxygen. Andalusite belongs to this broad chemical family.
Refractory: A material that withstands high temperatures without losing its useful structure or function. Andalusite is used as a raw material for refractory products.
Contact metamorphism: Metamorphism caused mainly by heat from nearby magma intruding into surrounding rock. Low-pressure heating around intrusions can produce andalusite-bearing rocks.
Kyanite: Kyanite is a blue Al₂SiO₅ polymorph stable at relatively high pressures. Its stability at higher pressure contrasts with andalusite’s low-pressure stability.
Orthorhombic crystal system: A crystal system with three mutually perpendicular axes of unequal length. Andalusite’s crystal structure is orthorhombic.
Mullite: A stable aluminum silicate mineral valued for strength and resistance to high temperatures. Heating andalusite can produce mullite, the principal phase behind its refractory value.
Hornfels: A hard, fine-grained metamorphic rock commonly formed by contact metamorphism. Andalusite can grow in hornfels near igneous intrusions.
Sillimanite: Sillimanite is an Al₂SiO₅ polymorph stable at higher temperatures than andalusite under many conditions. It marks a higher-temperature stability field than andalusite.
Crystal twinning: A symmetrical intergrowth in which two or more parts of a crystal share a specific orientation relationship. Andalusite crystals can show characteristic twinning patterns.

Linked from 6 pages