Amphibole · Biaxial (−)
Hornblende in thin section
Distinctly colored and pleochroic in shades of green, yellow-green, blue-green and brown. Amphibole cleavages intersect at 56° and 124°, best seen in basal sections. Extinction 12–34° in elongate sections. Highest colors usually upper 1st to lower 2nd order, often masked by the mineral's color.
Plane-polarized light
| Color | Green, Brown |
|---|---|
| Pleochroism (color change on rotation) | Strong |
| Relief | Moderate, High |
| Cleavage | Two at 56° / 124° |
| Shape | Prismatic, Euhedral |
Cross-polarized light
| Maximum birefringence | about 0.020 |
|---|---|
| Highest interference color | 1st-order yellow–red, 2nd-order blues, greens |
| Extinction | Inclined |
| Twinning | Simple (two halves), None |
Interference colors are often masked by the mineral's own color.
Telling it apart
Tremolite is light colored with lower indices; actinolite has lower indices and a lower extinction angle. Biotite has one cleavage. Augite has cleavages at about 87°/93°.
Where you'll find it
Diorite, andesite, granodiorite, amphibolite
Photo examples
Athabasca University · 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