Aluminosilicate · Biaxial (−)
Kyanite in thin section
High relief, colorless to pale blue, bladed prisms. Low to moderate colors (birefringence about 0.012–0.016). Two cleavages that meet at about 79°. Inclined extinction (triclinic).
Plane-polarized light
| Color | Colorless, Blue / lavender |
|---|---|
| Pleochroism (color change on rotation) | None, Weak |
| Relief | High |
| Cleavage | Two at another angle |
| Shape | Prismatic, Laths / tabular |
Cross-polarized light
| Maximum birefringence | about 0.014 |
|---|---|
| Highest interference color | 1st-order gray–white, 1st-order yellow–red |
| Extinction | Inclined |
| Twinning | None, Simple (two halves) |
Telling it apart
Andalusite has parallel extinction and moderate relief. Sillimanite basal sections are diamond-shaped with one cleavage.
Where you'll find it
Medium- to high-pressure pelitic schist and gneiss
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