K-feldspar · Biaxial (−)
Orthoclase in thin section
Negative relief and low 1st-order gray (birefringence about 0.005–0.006). Typically Carlsbad twins (two halves). Can host perthite lamellae.
Plane-polarized light
| Color | Colorless |
|---|---|
| Pleochroism (color change on rotation) | None |
| Relief | Low, Negative (below the mount) |
| Cleavage | Two at ~90° |
| Shape | Laths / tabular, Anhedral / equant |
Cross-polarized light
| Maximum birefringence | about 0.006 |
|---|---|
| Highest interference color | 1st-order gray–white |
| Extinction | Inclined, Parallel |
| Twinning | Simple (two halves), None |
Telling it apart
Microcline has grid twinning. Quartz looks similar but has positive relief. Sanidine is usually told apart by its 2V angle.
Where you'll find it
Granite, syenite, arkose
Photo examples
Alex Strekeisen (perthite) · Search Wikimedia Commons
Photos open on the source sites.
Identify an unknown grain Read interference colors Practice mineral ID
Related minerals
Sources
OpticMin, ATHENA, Univ. of Geneva (P. Perroud) · Alex Strekeisen thin-section collection · Distinguishing features of minerals (UT Arlington) · MIT OCW 12.109 amphibole lab · SEPM Strata: carbonate staining · Wikipedia mineral pages (infobox optical data) · Athabasca University rocks and minerals · Smith College petrography: perthite