Cyclosilicate · Biaxial (±)
Cordierite in thin section
Looks like quartz or feldspar: colorless, low relief, 1st-order gray to white (never above 1st-order red). Often complexly twinned (polysynthetic or cyclic star patterns in basal sections). Alters readily to yellow-greenish 'pinite'.
Plane-polarized light
| Color | Colorless, Blue / lavender |
|---|---|
| Pleochroism (color change on rotation) | None, Weak |
| Relief | Low, Negative (below the mount) |
| Cleavage | None |
| Shape | Anhedral / equant, Euhedral |
Cross-polarized light
| Maximum birefringence | about 0.010 |
|---|---|
| Highest interference color | 1st-order gray–white, 1st-order yellow–red |
| Extinction | Inclined, Parallel |
| Twinning | Sector / wedge, Polysynthetic stripes, None |
Telling it apart
Quartz is uniaxial and lacks twinning. Plagioclase twins are straight parallel stripes.
Where you'll find it
Hornfels, granulite, high-temperature low-pressure gneiss
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