Carbonate · Uniaxial (−)
Calcite in thin section
Relief changes strongly as you rotate in PPL ('twinkling'). Extreme birefringence (0.172) gives creamy high-order colors. Rhombohedral cleavage (two directions at about 75°) and lamellar twinning. Extinction symmetrical or inclined to cleavage.
Plane-polarized light
| Color | Colorless |
|---|---|
| Pleochroism (color change on rotation) | None |
| Relief | Changes on rotation |
| Cleavage | Rhombohedral |
| Shape | Anhedral / equant |
Cross-polarized light
| Maximum birefringence | about 0.172 |
|---|---|
| Highest interference color | Creamy high-order |
| Extinction | Symmetric to cleavage, Inclined |
| Twinning | Polysynthetic stripes |
Telling it apart
Dolomite is more commonly euhedral and may be cloudy. Alizarin Red S stain turns calcite red and leaves dolomite unstained.
Where you'll find it
Limestone, marble, carbonatite, veins
Photo examples
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