Feldspathoid · Uniaxial (−)
Nepheline in thin section
Colorless, low relief, very low gray colors (birefringence about 0.003–0.004). Uniaxial negative.
Plane-polarized light
| Color | Colorless |
|---|---|
| Pleochroism (color change on rotation) | None |
| Relief | Low |
| Cleavage | None |
| Shape | Euhedral, Anhedral / equant |
Cross-polarized light
| Maximum birefringence | about 0.004 |
|---|---|
| Highest interference color | 1st-order gray–white |
| Extinction | Parallel |
| Twinning | None |
Telling it apart
Resembles the feldspars but lacks good cleavage and twinning and has lower birefringence. Quartz is uniaxial positive.
Where you'll find it
Nepheline syenite, phonolite, alkaline basalt
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