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The study of opaque minerals in reflected light is a basic technique for investigation of metallic ores. In geology and mineralogy it has an important role in identification of the minerals of ores, in analysis of textural relations, and in recognition of stages and processes of mineral deposition. In the mineral industries, it is employed in studies of mill products aimed at design, improvement, or control of mineral dressing procedures. In ore microscopy, mineral identification is based largely on study of physical, optical, and chemical properties. Physical properties investigated are crystal form and habit, cleavage and parting, scratch hardness, indentation hardness, polishing hardness, and tenacity. Much attention is currently being given to determinations of indentation hardness. Ore minerals comprise both isotropic and anisotropic substances. For isotropic minerals, the fundamental constants are the index of refraction and index of absorption, but neither can readily be determined with the microscope. Color and reflectivity relative to associated minerals are the properties commonly observed, but reflectivity is also quantitatively determinable by photometric or photoelectric methods. For anisotropic minerals, color, bireflectance, reflectivity, and elongation, together with anisotropism and polarization colors both in air and in oil, can be determined qualitatively in polarized light, and some minerals can be identified solely from such observations. Dispersion phenomena of polarization figures are likewise of value. Quantitative work has consisted largely of measurements of either mean or uniradial reflectivities. Apparatus is available, however, for quantitative analysis of elliptically polarized light, by measurement of the rotation angle and path difference, and by determination of the sign of the path difference. Theory indicates that optical symmetry may be determinable for some minerals. Existing apparatus needs improvement for accurate measurement of path differences. Chemical etching and qualitative tests for specific elements are both employed in mineral identification. Etching is also used for revealing textural features. Tests for specific elements are made either on mineral samples gouged or drilled from polished sections or by contact printing from polished sections. X-ray diffraction methods are important adjuncts to ore microscopy and have supplanted microchemical tests to a considerable extent. Spectrographic and spectrometric methods, though less widely used, have many actual and potential applications. In the study of ores, X-ray and spectrographic methods are especially effective if used in conjunction with microscope study, which gives essential information on the distribution and relationships of minerals in ores. In future work in ore microscopy, it seems safe to predict that there will be increasing emphasis on quantitative measurement of microhardness and optical properties as aids to identification. The joint use of ore microscopy, X-ray methods, and spectrographic methods also seems likely to be extended, for the combination of techniques can furnish solutions to a wide range of problems encountered in geology, mineralogy, and mineral dressing.
Cameron, Eugene N.
Professor of Geology, University of Wisconsin, Madison, Wis.