Abstract

The color of an anisotropic mineral under crossed nicols in reflected light is a function of the rotational properties of the mineral for various wave lengths of light.When a mineral is placed in a position midway between the extinction positions as determined with crossed polarizer and analyser and the analyser is then rotated, a series of colors is produced. The relative dispersion of the apparent angles of rotation (DAr) for various wave lengths of light may be determined from the color sequence. Those wave lengths having the smallest angle of rotation are compensated first and the mineral grain shows the complementary color. Thus the relative dispersion of the apparent angles of rotation is the reverse of the succession of colors observed in the grain.A basic explanation for the color of anisotropic minerals with crossed polarizer and analyser is given and the possible application of these colors to mineral identification considered.

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