Pleochroism

Pleochroism is the ability of a colored anisotropic mineral to display different colors depending on the vibration direction of the light passing through it. The effect is seen in plane light - with the upper polarizer removed - as a visible change in hue or intensity when the microscope stage is rotated. It is one of the most diagnostic optical properties available in the petrographic microscope, because the specific colors and the degree of contrast between them are characteristic of particular minerals and, in some cases, of particular compositions. ^[1]

Why Pleochroism Occurs

The root cause of pleochroism is differential absorption. When polarized light travels through an anisotropic mineral, it is split into two rays - the fast ray and the slow ray - that vibrate perpendicular to each other. These two rays interact with the crystal structure differently and are absorbed to different degrees for different wavelengths of visible light. Because the human eye perceives different wavelength combinations as different colors, the two rays emerge with different colors. When the stage is rotated so that the fast ray vibration direction is aligned parallel to the lower polarizer, all transmitted light carries the fast ray color. A 90° rotation aligns the slow ray with the polarizer instead, and the mineral shifts to the slow ray color. Between these end positions, both rays contribute and the perceived color is typically intermediate. ^[1]

The color change only works because plane-polarized light forces only one vibration direction through the mineral at a time. Without the lower polarizer, mixed light would pass through all vibration directions simultaneously and no color change would be visible as the stage is rotated. This is why pleochroism is specifically a plane-light observation.

Isotropic Minerals: No Pleochroism

Isotropic minerals are never pleochroic. Because they do not undergo double refraction, there is no fast/slow ray split and no differential absorption between vibration directions. In plane light, isotropic minerals display a single uniform color regardless of how far the stage is rotated. ^[1]

This is an important diagnostic point: if a colored mineral shows no color change on stage rotation in plane light, it is either isotropic or being viewed along a direction where the two rays happen to have the same color. The latter situation is uncommon and can be resolved by examining additional grains.

Uniaxial Pleochroism

Colored uniaxial minerals are usually pleochroic. Because the indicatrix has only two principal vibration directions - the ordinary ray (ω) and the extraordinary ray (ε) - the pleochroism of a uniaxial mineral is fully described by stating the color of each of these two rays. ^[1]

The pleochroic formula lists the color of each ray beside its symbol. For example, common tourmaline (schorl) has the formula ω = dark green, ε = pale green. An alternative and more concise notation simply ranks the rays by absorption strength: ω > ε means the ordinary ray is more strongly absorbed and therefore darker. Pleochroism may also be described qualitatively as strong or weak, depending on how vivid the color difference is between the two orientations. ^[1]

Measuring Both Colors in the Same Sample

A practical difficulty is that both colors must be identified from grains in different orientations, because no single grain simultaneously displays maximum absorption in both the ω and ε directions. The procedure uses two grains. First, locate a grain oriented to display the lowest interference color between crossed polarizers - this grain has its optic axis nearly vertical, and the circular indicatrix section means that nearly all transmitted light is ordinary. In plane light, this grain yields the color of ω. Second, locate a grain oriented to display the highest interference color between crossed polarizers - this grain has its optic axis horizontal, and the principal indicatrix section brings both ω and ε into play. When this grain is rotated in plane light, it alternately shows the colors of ε and ω. Because ω is already known from the first grain, the color of ε is identified by default as the remaining color. ^[1]

Biaxial Pleochroism

Biaxial minerals have three principal vibration directions - parallel to X, Y, and Z of the indicatrix - and all three may absorb light differently. A complete pleochroic description of a biaxial mineral therefore requires three colors, one for each principal direction. The colors of light vibrating parallel to X, Y, and Z are listed separately. For example, hornblende may be described as X = yellow, Y = pale green, Z = dark green. As with uniaxial minerals, an abbreviated notation ranks the rays by absorption: Z > Y > X. ^[1]

Reading the Three Colors from a Thin Section

Just as with uniaxial minerals, different grains must be examined to extract each color. A grain oriented to show minimum retardation between crossed polarizers has its optic normal (Y axis) approximately vertical. In plane light, this grain displays the color associated with Y. A grain oriented to show maximum retardation has its X and Z indicatrix axes in the plane of the stage. When this grain is rotated in plane light, it alternates between the colors of X and Z. To tell X from Z, the accessory plate is used to identify which vibration direction is slow and which is fast. With the slow ray vibration direction parallel to the lower polarizer, the mineral displays the color of Z (the slow ray vibrates parallel to Z, the highest-RI direction). Rotating 90° then yields the color of X. ^[1]

The three-color requirement for biaxial minerals means that pleochroism is a more powerful discriminator for biaxial minerals than for uniaxial ones. Two minerals with similar appearance in crossed-polarized light may have very different pleochroic formulas in plane light, making the observation essential for confident identification of colored biaxial minerals such as amphiboles and pyroxenes.

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