- 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
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
- 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
Dictionary of Mining, Mineral, and Related Terms