Accessory · Uniaxial (−)
Apatite in thin section
Small needles or hexagonal basal sections with moderate relief and very low 1st-order gray colors (birefringence about 0.001–0.007). Parallel extinction, length fast.
Plane-polarized light
| Color | Colorless |
|---|---|
| Pleochroism (color change on rotation) | None |
| Relief | Moderate |
| Cleavage | None |
| Shape | Needles / fibrous, Euhedral |
Cross-polarized light
| Maximum birefringence | about 0.004 |
|---|---|
| Highest interference color | 1st-order gray–white |
| Extinction | Parallel |
| Twinning | None |
Telling it apart
Distinguished from garnet by its weak birefringence. Colored apatite is darker when its long axis is parallel to the polarizer, unlike tourmaline.
Where you'll find it
Accessory in most igneous rocks
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