Amphibole · Biaxial (−)
Glaucophane in thin section
Distinctly pleochroic from colorless to blue, with amphibole cleavages at 56°/124°. Birefringence about 0.021, partly masked by color.
Plane-polarized light
| Color | Colorless, Blue / lavender |
|---|---|
| Pleochroism (color change on rotation) | Strong |
| Relief | Moderate, High |
| Cleavage | Two at 56° / 124° |
| Shape | Prismatic, Needles / fibrous |
Cross-polarized light
| Maximum birefringence | about 0.021 |
|---|---|
| Highest interference color | 1st-order yellow–red, 2nd-order blues, greens |
| Extinction | Inclined, Parallel |
| Twinning | None |
Interference colors are often masked by the mineral's own color.
Telling it apart
Riebeckite is another distinctly pleochroic blue amphibole. Tourmaline has no cleavage.
Where you'll find it
Blueschist
Photo examples
Alex Strekeisen · 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