Formula | Al2SiO5 | Most andalusite contains relatively pure Al, however, Mn and Fe may substitute Al in octahedral sites, andalusite with substantial Fe and Mn is called viridine |
Crystal System | orthorhombic | 2/m 2/m 2/m |
Crystal Habit | anhedral grains and irregular masses | |
Cleavage | Good, 2 at nearly right angles | |
Color/Pleochroism | Pink, white, pinkish brown, violet or gray; darker colors associated with high Mn and Fe Thin section: usually colorless in PPL, less common pink, reddish-pink or slightly green; colored show weak pleochroism usually pink to colorless, others green and yellow |
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Optic Sign | Biaxial (-) | |
2V | 71-88o | 2VX |
Optic Orientation | X = c |
Length fast; samples with > 6.5 mole % Mn3+ + Fe 3+ substituting for Al are length slow and are oriented X = a, Y = b, Z = c, optic plane = (010), and biaxial (+), with 2 VZ between 65 and 85o |
Refractive Indices |
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They vary with composition: the higher mole % of Fe + Mn, the higher the n |
Max Birefringence | 0.013 | Standard thin sections yield a 1st order gray or white interference color |
Elongation | Parallel to c-axis; length fast | |
Extinction | Parallel; in basal sections it is symmetrical | |
Dispersion | Weak with v > r, occasionally r > v | |
Distinguishing Features | Hand sample: H=6.5-7.5; specific gravity=3.13-3.16 Thin section: moderately high relief, large 2V, parallel extinction, length fast |
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Occurrence | Common in regional and contact metamorphic rocks (hornfels, schists); may be associated with cordierite, staurolite, garnet, sillimanite, kyanite, chlorite, muscovite, biotite, and plagioclase feldspar; less common in granitic pegmatites and granite | |
Editors | Alicia Simonti ('05), Jennifer Woolley ('08) |