THE BULK ANALYSIS OF SILICATE ROCKS BY PORTABLE X-RAY-FLUORESCENCE - EFFECT OF SAMPLE MINERALOGY IN RELATION TO THE SIZE OF THE EXCITED VOLUME

This paper examines the limitations arising from sample mineralogy whe n a portable Xray fluorescence instrument is applied to the direct in situ analysis of silicate rocks. Estimates were made of the size at th e excited volume from which the X-ray fluorescence signal originates b y calculating the critical penetration depth for selected X-ray lines. Measurements were made of the variations in detected intensities over the area of the P-XRF analyser window and showed that, using radioiso tope sources incorporated in the instrument used in this study (Spectr ace TN9000), excitation intensities were six to ten times greater at t he centre compared to the edge of the analyser window. These data indi cated that the region of the sample at the centre of the window will m ake an enhanced contribution to detected spectra. Replicate measuremen ts on slabs of rock selected to represent fine-to coarse-grain size te xtures indicated the magnitude of the sampling precision that can be a chieved in the direct analysis of silicate rocks. Typical values were better than 5% relative standard deviation of the mean from an average of five determinations on fine-to medium-grained rocks and better tha n 10% relative standard deviation in a single determination on these s amples.