Abstract: Anisotropic opaque minerals viewed in reflected light microscopy show two sets of colours: the colours seen in plane polarized light which change as the section is rotated on the microscope stage, and the colours seen between crossed polars which change as the analyser is uncrossed. These latter colours are known variously as polarization colours or anisotropic rotation tints, but are here referred to as anisotropy colours. They are commonly a diagnostic aid to correct mineral identification. All these colours occur as a consequence of the dispersion of the relative permittivity (dielectric) tensor—the variation in the values of the tensor with wavelength of incident light and in low symmetry crystals, the variation in the directions of the principal axes of the tensor with wavelength.

In this paper, it is shown that the colour seen in plane polarized light and the anisotropy colours can be predicted for any orientation of section, at any stage angle, and for any degree of uncrossing of the analyser by calculations based on the dielectric tensor values, and these predicted colours compare favourably with the observed values. Three minerals are studied in this paper as examples: stannite, covelline, and bournonite.