Glass · Isotropic
Volcanic glass in thin section
Isotropic and amorphous (non-crystalline): the product of rapidly cooled, quenched magma.
Plane-polarized light
| Color | Colorless, Brown |
|---|---|
| Pleochroism (color change on rotation) | None |
| Relief | Low, Negative (below the mount) |
| Cleavage | None |
| Shape | Anhedral / equant |
Cross-polarized light
| Maximum birefringence | about 0.000 |
|---|---|
| Highest interference color | Black at all angles |
| Twinning | None |
Telling it apart
Garnet is isotropic too but has high relief.
Where you'll find it
Pumice, tachylite, Pele's tears
Photo examples
Photos open on the source sites.
Identify an unknown grain Read interference colors Practice mineral ID
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