Color and Pleochrosim
When we talk about the color of a mineral in thin section, we are referring to color seen when viewing the mineral in plane polarized (PP) light.
Minerals may be colorless, weakly colored or strongly colored. Often, the color of a mineral in hand specimen and in thin section have the same hue (e.g., red minerals will be red in both views, green minerals will be green in both views, etc.), although the color in thin section is typically not as intense. Sometimes color in thin section does not match color in hand specimen, especially for dark colored minerals. For example, hornblende is generally black when viewed in hand specimen. In thin section it is likely to appear in various shades of brown or green. The different coloration in hand specimen and thin section is the result of two things. First, the color seen in thin section is the color of transmitted light; the color in hand specimen is the color of reflected light. Second, we use plane polarized light when viewing minerals in thin section.
If a mineral grain is heterogeneous (is not the same composition everywhere), it may exhibit patchy or concentric color zonation (different colors in different places).
Because we use polarized light when viewing minerals in thin section, the color we see may vary as we rotate the microscope stage. Minerals that change color with stage rotation are said to have pleochroism. Pleochrosim is a consequence of the fact that, for anisotropic minerls, different colors of polarized light are absorbed depending on the direction of light vibration. Some minerals show extreme pleochroism, others show more subtle pleochroism, and some show none at all.
Below are some examples of minerals that exhibit quite pronounced pleochroism. All photos shown on this page are in PP light.
Sapphirine is a remarkable blue mineral that often shows extreme pleochroism. These views are of the same part of a thin section, but the stage was rotated, so they are 90o different in orientation. The sapphirine goes from deep blue on the left to a lighter blue on the right.
Orthopyroxene is also present. It has a light pinkish or orange color on the left and is greenish on the right.
The large grain in the center of these views is orthopyroxene. It is pinkish on the left, but after 90o stage rotation appears green on the right. Pink to green pleochroism is one of the properties that helps distinguish orthopyroxene from clinopyroxene. The pleochroism shown here is extreme; it is not so pronounced for most orthoyroxenes.
This view shows biotite, pleochroic from tan to brown. The two views show the effect of a 90o stage rotation. This is typical color and pleochroism for biotite, but some biotites are pleochroic in other colors, including green and red.
This hornblende shows pleochroism from green to blue-green. Hornblende is generally pleochroic, often in shades of green, blue, brown, and less commonly red or yellow. This example is in shades of green only, but brown hues are generally more common.