K-feldspar · Biaxial (−)
Microcline in thin section
Grid (cross-hatched, 'tartan') twinning in XPL, from combined albite and pericline twins. Up to 1st-order white.
Plane-polarized light
| Color | Colorless |
|---|---|
| Pleochroism (color change on rotation) | None |
| Relief | Low, Negative (below the mount) |
| Cleavage | Two at ~90° |
| Shape | Anhedral / equant |
Cross-polarized light
| Maximum birefringence | about 0.007 |
|---|---|
| Highest interference color | 1st-order gray–white |
| Extinction | Inclined |
| Twinning | Tartan grid |
Telling it apart
Plagioclase twins are one set of parallel stripes. Orthoclase has no grid.
Where you'll find it
Granite, pegmatite, gneiss, arkose
Photo examples
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