Andalusite

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  1. A silicate of aluminum built around independent tetrahedra, Al2SiO5. Characteristic of middle-grade metamorphism. Compare with kyanite, which has same composition and forms under similar conditions but which has different crystal habit. Contrast with sillimanite, which has same composition but different crystal habit and forms at highest metamorphic temperatures.
    Source: Leet, L. Don. 1982. Physical Geology, 6th Edition. Englewood Cliffs, NJ: Prentice-Hall
  2. An orthorhombic mineral, Al2 SiO5 ; trimorphous with kyanite and sillimanite; Mohs hardness, 7-1/2; in aluminous shales and slates subjected to high-temperature, low-stress metamorphism; transparent green varieties used as gems. Syn: cross-stone
    Source: Dictionary of Mining, Mineral, and Related Terms


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