Smith College - Geology 222b - Petrology

Petrographic Data File

Formula (K, Na)AlSi3O8 K-feldspar (KAlSi3O8) with albite (NaAlSi3O8) intergrowth
Crystal System Not applicable  
Crystal Habit Twinned lamellae Lammellae  roughly parallel to {100}
Cleavage perfect cleavage {001) and good cleavage {010} Does not stand out in thin section
Color/Pleochroism Gray and black in cross polarized light, colorless in plain polarized light, dark gray in hand sample  
Optic Sign Not applicable Interference figure not possible
2V Not applicable Interference figure not possible
Optic Orientation Not applicable Interference figure not possible
Refractive Indices
alpha = 
beta = 
gamma = 
delta = 


Indices between those of microcline and albite

The orthoclase(K-feldspar) will have the lower refractive index, and the albite will have a higher refractive index.

Max Birefringence ~ 0.007 Do higher than first-order white
Elongation Not applicable Interference figure not possible
Extinction Parallel  
Dispersion Not applicable Interference figure not possible
Distinguishing Features Exsolution lamellae visible in thin section and sometimes in handsection. Mircoperthite is only seen microscopically, cryptoperthite is submicroscopic and needs x-ray diffraction to see extremely fine lamellae. Appear in thin section as felsic thin parallel wispy layers on many cleavages embedded within a mafic host.
Occurrence The less abundant phase forms into irregular, crystallographically-oriented planar bands within the more abundant host during cooling. The exsolution lamellae of Na- and K-rich feldspar are created in the bulk composition of the K-feldspar. A Na-rich host with K-rich lamellae is an antiperthite.
Editors Maria Rowley (00), Sharon Mason (05), Danielle Schmandt (07)

Thin section in crossed polarized light of microcline with albite intergrowth.  The microcline has tartan twinning, while the albite is the globular gray areas.  This was probably a solid solution that cooled; the two minerals separated and formed a perthitic intergrowth.
Thin section in crossed polarized light of perthite.  The lamellae are K- and Na-fledspars. 
Perthite in sample K-21 showing coarse intergrowth.
Perthite in plane polarized light, click to go to rollover link, so you can view it in plane polarized light and crossed polarized light
Thin section shows lamellae bands with twinning.
Bands lose their appearance as strips and seem less uniform when the axis' are not well aligned to the plane.

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