Determination of reflectivity of the ore minerals

Abstract

Increasing sensitivity and reliability of the exposure meters used in amateur and professional photography provides a simple and convenient method of measuring the reflectivity of ore minerals. A mineral species has a limited range of light reflecting power, and a determination of the reflection percentage provides a useful, positive aid in identifying minerals in polished section, in the same way that a determination of index of refraction is diagnostic for the nonopaque minerals.The relative reflecting powers of some 200 high index nonopaque and opaque mineral species, in the Murdoch suite at Harvard University, were determined with a General Electric photographic exposure meter coupled to a metallographic microscope. The values arrived at check very convincingly with those determined by other investigators; the method used by the writer presents a simplified apparatus.Absolute values of reflection were determined from relative values by assigning a reasonable reflection percentage (54.5 per cent) to a specimen of pyrite used as a comparison standard in all determinations. These absolute values compare well with those available in the literature. There is a definite relation between index of refraction and reflectivity, expressed in the Fresnel equation, and for high index nonopaque minerals, this relation was confirmed by comparing observed values of reflection against known indices of refraction.Values of reflecting power for the common species of the opaque or nearly opaque native elements, sulphides, sulphosalts, and oxides, commonly classed as the ore minerals, are presented in the form of two tables designed to aid in the rapid determination of these minerals in polished section or in cleavage fragments.