Nesosilicate · Biaxial (+)
Staurolite in thin section
Pleochroic from colorless to golden yellow. Low 1st-order gray-yellow colors (birefringence about 0.009–0.015). Often porphyroblasts with inclusions and internal foliation. Twins at 60° and cross-shaped (90°) twins occur.
Plane-polarized light
| Color | Yellow |
|---|---|
| Pleochroism (color change on rotation) | Weak, Strong |
| Relief | High |
| Cleavage | None |
| Shape | Euhedral, Prismatic |
Cross-polarized light
| Maximum birefringence | about 0.012 |
|---|---|
| Highest interference color | 1st-order gray–white, 1st-order yellow–red |
| Extinction | Parallel |
| Twinning | None, Cross-shaped |
Telling it apart
Epidote is greener with brighter colors. Titanite has extreme birefringence. Tourmaline is darker when perpendicular to the polarizer.
Where you'll find it
Medium-grade pelitic schist
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