Distinguishing Andalusite, Kyanite and Sillimanite in Thin Section

Steven Dutch, Natural and Applied Sciences, University of Wisconsin - Green Bay
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  Andalusite Kyanite Sillimanite
Color Clear to pleochroic in patchy shades of pink Colorless to grayish Colorless, clear
Crystal Habit Square prisms. Chiastolite cross-markings in cross-section are distinctive but not invariably present Bladed grains Needles or slender prisms. Masses of fine needles common.
Twinning Lamellar twinning common    
Cleavage Three sets nearly perpendicular Prismatic cleavage, two sets Pinacoidal cleavage, transverse cracks characteristic.
Refractive Index High Highest of the three High
Birefringence Low (.011) Moderate (.014) Moderately high (.023)
Extinction Parallel Inclined Parallel
Elongation Negative Generally Positive Positive
Optic Sign Negative, 2V=86 degrees Negative, 2V=83 degrees Positive, low 2V (24 degrees)

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Created 17 December, 1999, Last Update 14 Dec 2009

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