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An important metamorphic index mineral and one of the three Al2SiO5 mineral polymorphs, the other two being sillimanite and andalusite; sp. gr. 3.5–3.6; hardness 4–7; triclinic; light to darker blue, sometimes green or yellow, occasionally black; vitreous lustre; crystals usually long, columnar, often flattened and bladed, with occasional stellate intergrowths; cleavage pinacoidal, good {100}, {010}, parting {001}; occurs in regional metamorphic rocks that have been subjected to high pressures and low to moderate temperatures, and may be found in schists or gneisses associated with staurolite and garnet.

Subjects: Earth Sciences and Geography.

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