|
Orthoclase |
|
Property |
Value |
Comments |
| 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. |
| Color/Pleochroism |
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 = |
1.520
1.525
1.527 |
|
| 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) |
