Contamination introduced by grinding methods is determined quantitatively with high precision and accuracy by the x-ray spectrographic analysis of all major oxides in rocks. This is done by performing two complete analyses of rock powders of identical origin but ground by different grinding media. Original composition of uncrushed rock is computed on the basis of these two analyses. This method is feasible because of the speed of the x-ray analysis. It is accurate because the same matrix with only slight compositional changes due to contamination is used. Data are given on six volcanic rocks analyzed for Si, Al, Ti, Fe, Mn, Mg, Ca, K, and Na.
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