Amphibole · Biaxial (−)
Actinolite / tremolite in thin section
Colorless (tremolite) to pale or deep green (actinolite), with pleochroism that strengthens with iron content. Amphibole cleavages at 56°/124°. Inclined extinction about 10–21° in elongate sections. Colors up to upper 1st or low 2nd order.
Plane-polarized light
| Color | Colorless, Green |
|---|---|
| Pleochroism (color change on rotation) | None, Weak, Strong |
| Relief | Moderate, High |
| Cleavage | Two at 56° / 124° |
| Shape | Needles / fibrous, Prismatic |
Cross-polarized light
| Maximum birefringence | about 0.022 |
|---|---|
| Highest interference color | 1st-order yellow–red, 2nd-order blues, greens |
| Extinction | Inclined, Parallel |
| Twinning | None, Simple (two halves), Polysynthetic stripes |
Telling it apart
Hornblende has higher indices and a larger extinction angle. Colorless richterite can be hard to tell from tremolite.
Where you'll find it
Greenschist, metamorphosed mafic rocks, marble (tremolite)
Photo examples
Univ. of Geneva (actinolite) · 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