Pleochroism

Pleochroism is the optical phenomenon in which a substance shows different colors when observed from different crystallographic directions or under polarized light. In anisotropic crystals, light traveling along different axes is absorbed differently, so the transmitted color varies with orientation. When viewed in a polarizing microscope and the stage is rotated, the sample may reveal distinct pleochroic colors along different axes.

There are two main forms: dichroism, in which two colors are observed, and trichroism, in which three colors may be seen. Uniaxial crystals typically display two colors, while biaxial crystals can show three. The effect arises from directional differences in electronic transitions and absorption coefficients, caused by the crystal field and molecular orientation within the lattice. Pleochroism applies to transmitted light; reflected light can also reveal color differences in some materials.

Observation and measurement methods include using petrographic microscopes with polarizers; the color changes with rotation of the stage. Quantitative analysis can be performed with spectrophotometric or colorimetric techniques. Pleochroism is exploited in gemology and mineral identification. Common pleochroic minerals include tourmaline and cordierite; their colors differ along the optical axes, aiding identification. Pleochroism is distinct from iridescence, which arises from interference effects in microstructures, and from photochromism, where color changes with illumination rather than orientation.