K-feldspar · Biaxial (−)
Sanidine in thin section
The K-feldspar typical of felsic volcanic rocks. Carlsbad twins common, 1st-order gray (birefringence about 0.007).
Plane-polarized light
| Color | Colorless |
|---|---|
| Pleochroism (color change on rotation) | None |
| Relief | Low, Negative (below the mount) |
| Cleavage | Two at ~90° |
| Shape | Laths / tabular, Laths / tabular |
Cross-polarized light
| Maximum birefringence | about 0.007 |
|---|---|
| Highest interference color | 1st-order gray–white |
| Extinction | Inclined, Parallel |
| Twinning | Simple (two halves), None |
Telling it apart
Low sanidine has a smaller 2V than orthoclase. Quartz looks similar but is uniaxial.
Where you'll find it
Rhyolite, trachyte, obsidian, tuff
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