In hand specimens, many minerals appear strongly colored, but minerals viewed with a microscope using PP light generally display a weak color or appear colorless. Many minerals in thin section or in grain mount are not thick enough to significantly absorb or enhance specific wavelengths of light. If minerals do appear colored, the color may change when we rotate the microscope stage because rotating the stage changes the orientation of the mineral’s crystal structure with respect to the polarized light. Some minerals absorb different wavelengths of light depending on light vibration direction. We call this property pleochroism. Biotite is an example of a mineral that normally displays marked pleochroism.
Pleochroism is an especially useful diagnostic property when identifying some minerals, but it can be overlooked. In thin sections, orthopyroxenes are commonly colorless, but some show a faint pleochroism from pink to green. Pleochroism of pyroxenes is an important property because it distinguishes the two major pyroxene subgroups: orthopyroxene and clinopyroxene. For minerals with noticeable pleochroism, reference tables describe the property by listing colors seen when looking at the mineral in different directions. For pleochroic uniaxial minerals, color varies between two hues. For biaxial minerals, color varies between three hues.
In contrast with pyroxenes, many amphiboles display strong colors and a very noticeable pleochroism in thin section. The biaxial mineral glaucophane (an amphibole) has pleochroism described by its pleochrotc formula:
X = colorless or pale blue
Y = lavender-blue or bluish green
Z = blue, greenish blue, or violet
X, Y, and Z refer to light vibrating parallel to each of three mutually perpendicular vibration directions in the crystal. In thin sections, glaucophane’s colors vary within the limits described for X, Y, and Z, depending on the crystal orientation, as we rotate the microscope stage. The biotite in Plates id pleochroic in green and brown, but the standard pleochroic formula for biotite might be:
X = colourless, light tan, pale greenish brown, or pale green
Y = Z = brown, olive brown, dark green, or dark red-brown
When we insert the upper polarizer, we see minerals in crossed polarized (XP) light, and we may see colors that are brighter and more pronounced than when we view the same grain in PP light. These are interference colors. They do not result from absorption of different wavelengths by the mineral (which is how minerals get their normal color). Instead, they result from the interference of light rays passing through the upper polarizer. They rarely resemble the true color of the mineral.
Interference colors depend on grain orientation, so different grains of the same mineral in one thin section normally display a range of interference colors. Because different minerals can display different ranges of interference colors, interference colors are useful for mineral identification Interference colors also vary wih the thickness of the grains, so it is important that thin sections be of uniform thickness. Additionally, the edge of same grains, grains near the edge of 9 thin section, of grains adjacent to holes in thin section (places where the sample is thin), may display abnormal interference colors.
Source
From Chapter 4 of Mineralogy, Third Edition, Dexter Perkins