Phyllosilicate · Biaxial (−)
Glauconite in thin section
Green, pleochroic, moderate relief (birefringence about 0.020–0.032, masked by color). Usually rounded sand-size pellets. An authigenic mineral that forms mainly in marine shelf settings with slow sediment accumulation.
Plane-polarized light
| Color | Green |
|---|---|
| Pleochroism (color change on rotation) | Weak, Strong |
| Relief | Moderate |
| Cleavage | One |
| Shape | Pellets, Flakes |
Cross-polarized light
| Maximum birefringence | about 0.026 |
|---|---|
| Highest interference color | 2nd-order blues, greens |
| Extinction | Parallel |
| Twinning | None |
Interference colors are often masked by the mineral's own color.
Telling it apart
Chlorite has lower birefringence and often anomalous colors.
Where you'll find it
Greensand, marine sandstone and limestone
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