|
|
|
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 = |
1.522-1.527 1.526-1.532 1.530-1.538 0.005-0.013 |
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) |