Phyllosilicate · Biaxial (±)
Chlorite in thin section
Green, with one perfect cleavage. Birefringence low (0–0.02), often with anomalous interference colors such as indigo-violet or olive brown. Pleochroism increases with iron content.
Plane-polarized light
| Color | Green |
|---|---|
| Pleochroism (color change on rotation) | Weak, Strong |
| Relief | Low, Moderate, High |
| Cleavage | One |
| Shape | Flakes, Needles / fibrous |
Cross-polarized light
| Maximum birefringence | about 0.006 |
|---|---|
| Highest interference color | 1st-order gray–white, Anomalous (inky blue / brown) |
| Extinction | Parallel, Inclined |
| Twinning | None |
Telling it apart
Distinguished from muscovite and biotite by green color, pleochroism and weak birefringence. Serpentine is less pleochroic and has lower refractive indices.
Where you'll find it
Greenschist, altered basalt, slate, phyllite
Photo examples
Alex Strekeisen · Univ. of Geneva · Search Wikimedia Commons
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