Kyanite |
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Formula | Al2SiO5 | Fe+3, Ti+4, or Cr+3 may be present. |
Crystal System | Triclinic | |
Crystal Habit | Elongate or columnar crystals in bladed aggregates | |
Cleavage | Perfect cleavage on (100) and good cleavage on (010) intersect at 79° | |
Color/Pleochroism | Pale blue in hand samples. Colorless to light patchy blue in thin section. Weak pleochroism in thin section where X= colorless, Y= light violet blue, and Z= light cobalt blue | Color may be streaky in hand samples. |
Optic Sign | Biaxial (-) | |
2V | 78°-84° |
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Optic Orientation | Z: inclined 27° - 32° to the c axis Y: inclined 27° - 32° to the b axis X: inclined a few degrees to the a axis |
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Refractive Indices
alpha = beta = gamma = delta = |
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Elongation | Prismatic crystals and cleavage fragments are length slow | |
Extinction | Inclined (see optic orientation). | Bladed crystals display 0° - 30° extinction. |
Dispersion | Weak r > v | |
Distinguishing Features | Colorless and dark in thin section with high positive relief! Second-order interference colors. Two prominent, high angle cleavages occur parallel and perpendicular to the length of the crystal blades. Hardness = 4-5 parallel to c and 7.5 at right angles to c. G = 3.53 to 3.67. Streak is white. Luster is vitreous. | |
Occurrence | Commonly forms in rock types that have undergone high pressures. Kyanite is usually found in metamorphic rocks such as pelitic schists and gneisses. Infrequently found in kimberlite, granitic pegmatite, and eclogite. Associated minerals include staurolite, garnet, sillimanite, and andalusite. | |
References | Nesse, William D. (2000) Introduction to mineralogy. New York: Oxford University Press. |
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Editors | Wendy Kelly ('05), Rhiannon Nolan ('19) |