Framework silicate · Uniaxial (+)
Quartz in thin section
Recognized by low relief, low birefringence (about 0.009, 1st-order gray to white), and no cleavage or twinning. Uniaxial positive. Plastically strained grains show sweeping (undulose) extinction.
Plane-polarized light
| Color | Colorless |
|---|---|
| Pleochroism (color change on rotation) | None |
| Relief | Low |
| Cleavage | None |
| Shape | Anhedral / equant |
Cross-polarized light
| Maximum birefringence | about 0.009 |
|---|---|
| Highest interference color | 1st-order gray–white |
| Extinction | Undulose (sweeping), Parallel |
| Twinning | None |
Telling it apart
Plagioclase is biaxial, has cleavage and is usually twinned. Orthoclase looks similar but has negative relief. Cordierite alters readily to yellowish pinite and is often twinned.
Where you'll find it
Granite, rhyolite, sandstone, quartzite, schist, gneiss
Photo examples
Alex Strekeisen · 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