MODAL ANALYSIS OF GRANITIG ROCKS BY X.RAY DIFFRACTION

A relatively rapid and practical method for determining mineral proportions in granitic rocks by X-ray diffraction is derived from 32 samples of tie Cornelia (Arizona) pluton. Forty-three additional samples from that pluton provide an independent test of tle method and 4l specimens fromtle Rocky Hill (California) and Cornucopia (Oregon) plutons test its applicability to otho granitoid rocks. Optically derived modes of known precision are used as standards and for testing performance of the method. Quartz, K-feldspar, amphibole and biotite are determined with average errors estimated to be no larger than those associated with optical modes. Average errors for plagioclase probably are somewhat larger in the X-ray estimates. Except for biotite, the X-ray peaks employed seem not to have been used previously in quantitative work of this type, and the data transformations that are integral to the method are ne\r'. Estimates derive from fixed-time counts on single peaks for each phase except plagioclase; for it, an intensity-ratio method using all five peaks is employed. A complete analysis for tle live phases requires 50minutes of machinetime, withtheattendon of aninstrument technician. Sample powders are prepared and pressure-mounted exactly as for X-ray-fluorescence analysis; thus, tle six-minute preparation time per sample (by batch methods) will not constitute an additional cost in many studic. Development work is required for'use in otler laboratoris, but thorough testing shows that the princrpal capabilities needed by petrologists in the routine study of ganitic rock compositions are approximately as well achieved by tlis method as they are by the microscope.