pleokroicism

Pleochroism is an optical phenomenon observed in certain anisotropic minerals and crystals, characterized by the variation in color when viewed under different orientations of polarized light. This effect arises due to the mineral’s internal structure, which causes light to be absorbed differently depending on the direction of propagation through the crystal lattice. The term originates from the Greek words *pleon* (more) and *chroma* (color), reflecting the multiple hues that may be perceived.

In pleochroic minerals, light is selectively absorbed along specific axes of the crystal, leading to distinct color changes when the crystal is rotated. For example, tourmaline often exhibits strong pleochroism, appearing green in one direction and pink in another. This property is particularly noticeable in thin sections under a petrographic microscope, where polarized light enhances the contrast between different orientations.

Pleochroism is closely related to birefringence, another optical property of anisotropic materials, but while birefringence affects the speed of light, pleochroism specifically influences color perception. The degree of pleochroism varies among minerals, with some showing subtle shifts in hue while others display dramatic contrasts. This characteristic is valuable in mineralogy for identifying and classifying crystals, as well as in gemology for assessing the quality and origin of colored stones.

The phenomenon is not limited to minerals; synthetic materials and certain biological structures, such as some proteins, can also exhibit pleochroism under specific conditions. Understanding pleochroism provides insights into the molecular and atomic arrangements within crystalline materials, contributing to fields like crystallography, materials science, and optics.