Feldspar · Biaxial (±)
Plagioclase in thin section
Polysynthetic (albite) twinning: parallel stripes that alternate light and dark as you rotate. Birefringence about 0.007–0.013, so 1st-order gray to white. Often clouded by alteration.
Plane-polarized light
| Color | Colorless |
|---|---|
| Pleochroism (color change on rotation) | None |
| Relief | Low |
| Cleavage | Two at ~90° |
| Shape | Laths / tabular, Anhedral / equant |
Cross-polarized light
| Maximum birefringence | about 0.010 |
|---|---|
| Highest interference color | 1st-order gray–white |
| Extinction | Inclined |
| Twinning | Polysynthetic stripes, Simple (two halves) |
Telling it apart
Untwinned plagioclase closely resembles quartz, but quartz is uniaxial and lacks cleavage. Microcline twins form a grid, not one set of stripes.
Where you'll find it
Basalt, gabbro, andesite, diorite, granite, many metamorphic rocks
Photo examples
Alex Strekeisen · Univ. of Geneva (albite) · Search Wikimedia Commons
Photos open on the source sites.
Identify an unknown grain Read interference colors Practice mineral ID
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