Some colored minerals change in color between two different colors when the stage is rotated under plane polarized light (PPL). This property is called pleochroism and it is caused by the unequal absorption of light by the mineral, depending on its orientation. A pleochroic mineral shows maximum absorption (darker color) when the polarized light vibrates parallel to the maximum index of refraction, and minimum absorption (paler color) when it is parallel to the minimum index of refraction. Since refractive indices are perpendicular on 2D sections, the mineral shows each of these ‘extreme’ colors twice during a complete 360° rotation. Intermediate orientations correspond to intermediate hues between the maximum and minimum colors of pleochroism. For example, the tourmaline crystal below varies in color from dark greenish brown when it is vertical to pale yellow when it is horizontal, showing a combination of these two colors at intermediate positions:
Variation of pleochroic colors in birefringent minerals
Pleochroism, exactly because it is due to changes in refractive index with crystal orientation, is a property of birefringent minerals. Cubic minerals, characterized by a single refractive index, cannot be pleochroic. Uniaxial minerals, like the tourmaline crystal shown above, are characterized by two refractive indices, a maximum and a minimum one (ε and ω), each corresponding to a different color of pleochroism: the maximum variation in color can be observed on a section that contains ε and ω (which is also that with the highest interference colors). All other sections show a less marked variation of the colors of pleochroism, with circular sections displaying no pleochroism at all. Biaxial minerals have three refractive indices, a maximum, a minimum, and an intermediate one (α, β, and γ), each corresponding to a different hue of pleochroism. For example, glaucophane shows α – colorless, β – lavender-blue, and γ – blue (this notation denotes the colors that are visible when a refractive index – in this case α, β, and γ, indeed – is parallel to the polarizer).
The light transmitted to the thin section from the polarizer vibrates on the E-W plane of the thin section (the horizontal of the observer). The observed pleochroic colors are related to the refractive index which is parallel to the horizontal. For example, the tourmaline shown above displays paler colors when the long axis of the mineral (corresponding to the optic axis ε) is horizontal, and darker ones when the short axis (hence the refractive index ω) is horizontal instead. Indeed, tourmaline is an optically negative mineral with ω > ε and hence maximum absorption (darker colors) along ω.