Point counting, the present and often unreliable method of modal analysis of rocks, is unsatisfactory for many rock types. An instrumental technique that could be routinely performed by a technician is highly desirable. X-ray powder diffraction has been critically evaluated for this purpose and was found to be a suitable instrumental method when combined with density separation. A practical technique is thus proposed in which the rock powder is split into three density fractions by heavy-liquid separations and each fraction is quantitatively analyzed for its major constituents by X-ray diffraction. Altogether 28 minerals are considered, which are constituents of most igneous and many metamorphic rocks, and calibration curves are given for ten of these. With one exception, the precision was found to be reasonably good for the minerals determined. The accuracy of the technique could not be tested satisfactorily. The analytical time varies from 2 to 4 man-hours per sample depending on the rock type. Although the study is somewhat preliminary and the proposed technique is incomplete, it already shows some obvious advantages over point counting for many rock types.

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