Smith College - Geology 222b - Petrology

Petrographic Data File

Formula KAlSi3O8  
Crystal System Monoclinic 2/m
Crystal Habit Fine-grained, tabular  
Cleavage perfect cleavage on {001}, good cleavage on {010} Right angle cleavage is characteristic but almost impossible to see in thin section due to very low relief.

Colorless in thin section

No pleochroism

Can be cloudy or pale brown due to inclusions.
Optic Sign Biaxial (-)  
2V 60-65 May have 2V as large as 85, but usually is less than 70
Optic Orientation Z=b 

X^a = 5o

Refractive Indices
alpha = 
beta = 
gamma = 

Max Birefringence .005-.007 1st order grey & white
Elongation Commonly elongated parallel to c or a  
Extinction   Extinct at 0-12 degrees from cleavage lines, but hard to see under thin section
Dispersion r > v  
Distinguishing Features Intermediate 2V, twinning: carlsbad twinning is the most common but pericline and albite twinning have also been observed, very LOW relief.  Orthoclase is often confused with quartz - try looking for cloudy patches on individual crystals in plain polarized light which indicate weathering of the crystal. And check to see if the crystal is uniaxial or biaxial.
Occurrence Found in granite, granodiorite, syenite, and other felsic rocks.  Also found in arkoses and certain  contact and regional metamorphic rocks.  Usually found with quartz and muscovite.
Editors Kristen Clark ('02), Lila Rose (03), Kriten Boyer (AC08), and Sarah Motti ('10). Laura Henry ('16)

This whitish orthoclase crystal exhibits low birefringince color under crossed polars
Orthoclase crystal showing extinction patterns.
Notice the cleavage of 90 degrees in the center of the crystal.
Orthoclase crystal at extinction--notice inclusions and undulatory extinction under cross polarized light.
Orthoclase crystals colorless under normal polarized light, very low birefringence colors (grey or white) under cross-polarized light.
Orthoclase crystal surrounding plagioclase crystal.
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U.C.L.A. Petrographic Workshop

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