Mica · Biaxial (−)
Biotite in thin section
Brown, brownish green or reddish brown flakes with strong pleochroism. Darker when the cleavage trace is parallel to the lower polarizer. Bird's-eye extinction. Birefringence 0.028–0.07, often masked by the mineral's color. Can show dark pleochroic halos around inclusions such as zircon, apatite or titanite.
Plane-polarized light
| Color | Brown, Green |
|---|---|
| Pleochroism (color change on rotation) | Strong |
| Relief | Moderate |
| Cleavage | One |
| Shape | Flakes |
Cross-polarized light
| Maximum birefringence | about 0.045 |
|---|---|
| Highest interference color | 2nd-order blues, greens, 3rd-order |
| Extinction | Parallel, Bird's-eye |
| Twinning | None |
Interference colors are often masked by the mineral's own color.
Telling it apart
Green or brown tourmaline looks similar but has no cleavage and is darker when its long axis is perpendicular to the lower polarizer. Amphiboles have two cleavages. Chlorite has lower birefringence and often anomalous colors.
Where you'll find it
Granite, granodiorite, schist, 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