Smith College - Geology 222b - Petrology

Petrographic Data File

Kyanite
Property
Value
Comments
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°

 

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
Refractive Indices
alpha = 
beta = 
gamma = 
delta = 


1.710-1.718
1.719-1.725
1.724-1.734
0.012-0.016

 
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.
Nesse, William D. (1986) Introduction to optical mineralogy. New York: Oxford University Press.

Editors Wendy Kelly ('05), Rhiannon Nolan ('19)

 

Photomicrograph of kyanite in plain and polarized light showing a twinned crystal displaying clear mirror-image cleavage and opposite interference colors. Click on the image to see a larger image with roll over tthat shows the crystal in cross-polarized light.

Photomicrograph of kyanite in plain and polarized light showing distinct cleavage breaks, colorless in thin section in plain light, this crystal shows a low interference color in polarized light. Click on the image to see a larger image with roll over tthat shows the crystal in cross-polarized light.

Photomicrograph of kyanite next to a garnet crystal and surrounded by muscovite in plane polarized light. Kyanite typically displays pale green pleochroism and no distinct cleavage. Click on the image to see a larger image with xpl rollover.


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