Quantitative analysis of major elements (Na, Mg, Al, Si, K, Ca, Ti, and Fe) in rocks by classic wet-chemical procedures, in addition to being a costly and laborious process, has been shown to be subject to significant errors. Mineralogists and petrologists, increasingly concerned with determining small elemental differences in rock bodies, have turned to more rapid and precise methods in the field of spectral analysis. The development of X-ray spectrographic techniques in the light-element range has therefore been of particular interest. This paper summarizes the results of a test of the precision and sources of error in the X-ray method as applied to major elements in granitic rocks.
The test, using commerically available equipment and the fusion technique of specimen preparation, was designed for a five-way, completely nested variance analysis. Sources of variance are (1) different lengths of fusion time, (2) replications of a single fusion time, (3) replications of briquetting the fused specimens, (4) replications of individual runs in the spectrograph, and (5) replications of individual spectrograph readings of the Kα line intensity. A total of 64 readings per element were obtained and the significance of each factor tested for each element. Greatest sources of error are shown to be in factors 2 and 4. The total variance for each element is expressed as per cent relative deviation of counts per second. Calibration curves of natural and synthetic granitic rocks, fused with dilutions ranging from 13 to 36% rock in borax, provide conversions from relative deviation in counts per second to relative deviation in weight per cent. These values indicate that the precision of the X-ray method is directly comparable with wet chemistry for Si, probably better for Al, and distinctly superior for K, Ca, Ti, and Fe. Of the major rock-forming elements only Na and Mg are presently beyond the scope of the fusion method.