GEOCHEMICAL ANALYSIS OF SILICATE ROCKS AND SOILS BY XRF USING PRESSED POWDERS AND A 2-STAGE CALIBRATION PROCEDURE

A two-stage calibration procedure has been developed for major-element analyses of silicate rocks and soils using XRF and pressed powders. The first step involves fitting the composition-intensity data matrix of the standards to the Lucas-Tooth-Pyne (LTP) model using stepwise regression. Compositions of the standards are then regressed stepwise against the composition predicted by the LTP regression using a linear model of the form Ci = f(C'1,C'2,...,C'i,...,C'n) where the primes denote the compositions predicted by the LTP model. The second-stage regression significantly improved the correlation coefficient (R2) of the calibrations for SiO2, Al2O3 and Na2O over those produced by the LTP model alone. The procedure has been used to analyze 185 sample unknowns covering a wide range of composition (granite, diabase, sericite, biotites and soils developed thereon including laterites). Of these 95% had sum of constituents between 98.75% and 101.25%. Eight of these samples have been analyzed by independent methods. In general the agreement between pairs of analyses are very good. Thus, the calibration procedure appears to produce analyses over a wide range of compositions that are sufficiently accurate for routine work. Although the procedure requires a large number of standards (∼ 50), once the initial investment has been made the calibration can be upgraded and its range of application expanded as new standards become available. © 1990.