By means of a simple cadmium sulfide photoconductive cell, the spectral reflectivities of about 200 opaque ore minerals, mostly oriented, have been determined in air at wavelengths of about 470 mmu (blue), 520 mmu (green), 575 mmu (yellow), 600 mmu (orange), 700 mmu (red), and in white light ( approximately 610 mmu ). These values have been obtained on mineral areas as small as 70 mu by direct photoelectric comparison with quartz and mercury standards, whose reflectivities have been measured at ten points in the spectrum by the National Physical Laboratory. The shape and characteristics of the spectral reflectivity profile are significant in their control of color by the nature and distribution of absorption bands. Bireflecting sections show anisotropic colors, the intensity and nature of which vary with section orientation and express the magnitude and separation of the reflectivity profiles for the principal directions. Broad relationships are apparent between spectral profiles and conoscopic features of reflection rotation and apparent angle of rotation. The data have been tabulated as an aid to mineral identification, and in addition, the full range of reflectivity in white light for each mineral has been plotted against the mean microhardness value. From this plot, broad mineral groupings are apparent in similar reflectivity-microhardness areas for the oxides, simple sulfides, sulfosalts, iron-cobalt-nickel sulfarsenides, gold-silver tellurides, natural alloys, and native metals. The nature of the bonding and spectral dispersion for each of these groups is significant.

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