Steven Dutch, Professor Emeritus, Natural and Applied Sciences, Universityof Wisconsin - Green Bay
| 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