Aluminosilicate · Biaxial (−)
Andalusite in thin section
Moderate relief, colorless or rarely pink with weak pleochroism. 1st-order gray, white or yellow (birefringence about 0.009–0.013). Parallel extinction, length fast. Two cleavages nearly at right angles, rarely visible. The variety chiastolite contains a dark cross of inclusions.
Plane-polarized light
| Color | Colorless, Pink |
|---|---|
| Pleochroism (color change on rotation) | None, Weak |
| Relief | Moderate |
| Cleavage | Two at ~90° |
| Shape | Prismatic, Euhedral |
Cross-polarized light
| Maximum birefringence | about 0.011 |
|---|---|
| Highest interference color | 1st-order gray–white, 1st-order yellow–red |
| Extinction | Parallel, Symmetric to cleavage |
| Twinning | None |
Telling it apart
Andalusite resembles enstatite but lacks the pyroxene cleavage. Sillimanite has higher birefringence.
Where you'll find it
Contact-metamorphic hornfels, low-pressure schist
Photo examples
Alex Strekeisen · Univ. of Geneva · 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