Aluminosilicate · Biaxial (+)
Sillimanite in thin section
Elongate prisms or fine fibers ('fibrolite'), high relief. One perfect cleavage; diamond-shaped basal sections have the cleavage parallel to the long axis. Birefringence is listed as about 0.020–0.022, though some sources describe its colors as low (pale yellow). Orthorhombic, so elongate sections extinguish parallel.
Plane-polarized light
| Color | Colorless |
|---|---|
| Pleochroism (color change on rotation) | None |
| Relief | High |
| Cleavage | One |
| Shape | Needles / fibrous, Prismatic |
Cross-polarized light
| Maximum birefringence | about 0.021 |
|---|---|
| Highest interference color | 1st-order yellow–red, 2nd-order blues, greens |
| Extinction | Parallel |
| Twinning | None |
Telling it apart
Andalusite has lower birefringence and more distinct cleavage. Kyanite has two cleavages at about 79°.
Where you'll find it
High-grade pelitic schist and gneiss, hornfels
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