Sillimanite (S)

One of three polymorphs of Al2SiO5. Polymorphs are minerals which have the same chemical composition and formula, but very different atomic lattices. The three pseudomorphs of Al2SiO5 are Sillimanite, Andalusite and Kyanite. They form within different distinct temperature and pressure ranges and can be used to gauge the P-T history of metamorphic rocks.