Smith College - Geology 222b - Petrology
Petrographic Data File

Andalusite
Property
Value
Comments
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

Optic Sign Biaxial (-)  
2V 71-88o

2VX
Optic Orientation

X = c
Y = b
Z = a
optic plane = (010)

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
alpha = 
beta = 
gamma = 
delta = 


1.629-1.640
1.634-1.645
1.638-1.650
0.009-0.013

 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

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)

 

Photomicrograph of andalusite in thin section in crossed polarized light. Click on the image for a larger rollover version in xpl and xpl with the quartz plate. Note the very clear dark inclusions that form a cross and the low 1st order gray of the interference color. Also, the fact that the color changes from 1st order gray to 2nd order blue shows that andalusite is length fast.

Photomicrograph of andalusite in thin section in plain polarized light. Click on the image for a larger rollover version with both ppl and xpl. Notice the dark inclusions in the andalusite that form a cross through the basal section, marking it as a type of andalusite called chiastolite.

Photomicrograph of andalusite in thin section in crossed polarized light. Click on the image for a larger rollover version with both ppl and xpl. Note the nearly square shape of the basal section, and the slight traces of the dark inclusions like the above two images.

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