Some minerals show colors that are not part of the standard interference color sequence. Such colors are termed anomalous. Anomalous colors can result when:
Anomalous interference colors tend to be dark. Blue, violet, green and brown are all possible. These are actually useful diagnostic features. Chlorite tends to show anomalous greens and browns. Epidote commonly shows a dark "denim" blue color near extinction. When combined with its normal bright yellow and red interference colors, the effect is quite colorful.
Minerals with relatively weak color like chlorite, actinolite, glaucophane or hypersthene will show interference colors not very different from colorless minerals. The weak intrinsic color is not strong enough to affect the interference color very much. Strongly colored minerals like biotite or hornblende will be dominated by the intrinsic color. Colors very different from the intrinsic color are absorbed so strongly that they contribute little to the interference color. A deep green mineral will look green in crossed polarizers because everything but green is absorbed regardless of the retardation.
Some minerals are isotropic at certain wavelengths. Obviously that wavelength cannot contribute to an interference color. For example, a mineral that is isotropic in the green but not in the red or violet cannot show a green interference color. Its interference colors will only be red-violet. Since variations in optical properties with wavelength tend to be small, a mineral that is isotropic in one part of the spectrum will have low birefringence everywhere else, so it will not show brilliant interference colors. Anomalous interference colors tend to be dark. Instead of first order gray, a mineral that is isotropic in the green would appear dark purple.
Sometimes the privileged directions in a mineral vary with wavelength. This obviously cannot happen with uniaxial minerals, where the privileged directions are locked to a symmetry axis, so minerals whose optical orientations vary with wavelength tend to be biaxial. If the privileged directions vary with wavelength, all wavelengths will not go extinct at the same time and the mineral will appear colored instead of black. Since the mineral is near extinction even for those wavelengths, the color will be dark.
Created 15 Sep 1997, Last Update 14 Dec 2009
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