Pleochroism
Definition
The property of some minerals (hornblende, tourmaline, biotite) to show different colours when the plane of polarised light rotates relative to the crystal. Observed with one polar in (plane-polarised light) by rotating the stage.
- Property type
- Color change with polarized-light rotation
- Example minerals
- Hornblende, tourmaline, biotite
- Light condition
- Plane-polarized light, one polar in
- Observation method
- Rotate the microscope stage
Common questions
How does pleochroism differ from birefringence?+
Pleochroism is a color change seen with only one polarizer engaged, caused by differential light absorption along different crystal axes. Birefringence needs both polarizers crossed and shows as interference colors from the splitting of light into two rays. A mineral can show one property without the other.
Why is pleochroism useful for mineral identification?+
The specific colors a grain cycles through as the stage rotates, and the axis they align with, are diagnostic for particular mineral species, letting an examiner narrow down or confirm an identification made from other optical properties.
Related terms
- Birefringence
- The difference between the highest and lowest refractive indices of an anisotropic mineral, seen as a characteristic interference colour between crossed polars....
- Extinction Angle
- The angle, measured in degrees on the rotating stage of a petrographic microscope, between a crystallographic reference direction (cleavage trace or crystal...
- Grain Mount
- A microscope slide prepared by dispersing sand-grade grains in a mounting medium of known refractive index. Standard forensic grain mounts use Meltmount...
- QEMSCAN / MLA
- Automated mineralogy systems (QEMSCAN is from FEI/Thermo Fisher; MLA is the Mineral Liberation Analyser) that combine a scanning electron microscope with automated...
- SEM-EDX
- Scanning electron microscopy with energy-dispersive X-ray spectroscopy. The electron beam excites characteristic X-rays from each element present in the target grain, giving...