Pleochroism is the change in color of a colored birefringent mineral that is observed at the petrographic microscope under plane polarized light when the stage is rotated. The name itself derives, indeed from the Greek pléōn khrôma, more colors. This phenomenon arises from the uneven absorption of light by a mineral depending on its orientation – and hence the orientation of its refractive indices – with respect to the incoming light beam and can be observed with polarized light. Polarized light is characterized by photons that oscillate on a single plane. In general, under the petrographic microscope, this direction corresponds to the horizontal of the observer, the orientation of the polarizer. The section of a pleochroic mineral on a thin section will have a maximum and a minimum refraction index. By rotating the stage, we vary the refraction index that is parallel to the polarized light beam between these two values. When polarized light is oriented parallel to the refraction index with the maximum absorption, a darker color will emerge. Vice versa, when the polarized light vibrates parallel to the orientation with lower absorption, the mineral shows a paler color. The orientation of the maximum and minimum refraction indices is always perpendicular in 2D sections. Therefore, a pleochroic mineral will display the maximum and minimum colors observable on the section twice during a complete 360° rotation. Since pleochroism depends on the variation of refraction indices, it cannot be observed in cubic minerals or in mineral grains cut parallel to the cyclic section (i.e. the 2D section where there is a single refraction index in all orientations).

Notation of pleochroism
Pleochroism is usually described by specifying the color that is observed parallel to the maximum and minimum index of refraction in uniaxial minerals and the maximum, intermediate, and minimum refraction indices in biaxial minerals. For example, tourmaline – a uniaxial trigonal mineral – may show ε – pale green/brown and ω – dark green/blue/brown, whereas glaucophane (monoclinic, biaxial) varies in color from α – colorless to β – lavender-blue and γ – blue.

Examples of pleochroic minerals

Video. Tourmaline shows maximum absorption (darker colors) when its long axis is perpendicular to the polarizer (vertical) and minimum absorption (paler colors) when it is parallel to the polarizer (horizontal). Tourmaline is optically negative and its smaller refraction index is oriented parallel to its long axis (ω > ε). The smaller, equidimensional grain in the video is a basal tourmaline section: in this section, the refraction indices are the same. Therefore, its color remains the same as we rotate the stage. PPL. Width: 3 mm. Leucogranite, Calamita Schists. Monte Calamita, Isola d’Elba, Italy.

Video. Chloritoid shows a distinct pleochroism with variable color from pale yellow/green to green/blue/indigo. The color becomes paler when the shorter c-axis is oriented parallel to the polarizer. PPL. Width: 1.2 mm. Chloritoid schist, Massa Unit. Monte Brugiana, Alpi Apuane, Italy.

Video. Piemontite is characterized by vivid yellow to pink and violet colors that change rapidly when rotating the stage due to the pleochroism of the mineral. PPL. Width: 2.5 mm. Piemontite schist, Sanbagawa metamorphic belt. Asemigawa, Kochi, Shikoku, Japan.