16-35 The Big Mix-Up: Testing the Limits of X-Ray Fluorescence Spectroscopy in Element Concentration Analysis of Mineral Mixtures

Session: From Thin Section to Outcrop: Exploration of Undergraduate Research (Posters)

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    This project investigates the ability of handheld XRF spectroscopy to accurately quantify elemental concentrations from mixtures. Samples of nearly powdered quartz, calcite, and a brown earth pigment from Otranto, Italy composed of boehmite, goethite, hematite, and anatase were mixed based on weight percent. For each set of mixtures, 9 samples were created ranging in ratios from 1:10 to 10:1. Including the pure end-members, 30 total samples were prepared. Each sample was analyzed with a Bruker Tracer III handheld XRF spectrometer 10 times for 60 seconds using a He atmosphere at a current and voltage of 24.2 mA and 15 keV, respectively. A conversion file within Bruker’s S1CalProccess software was used to convert peak height to concentration for Mg, Al, Si, P, S, K, Ca, Ba, Ti, V, Cr, and Mn. This file considers inter-elemental interference and baseline corrections and was developed using 41 mudstone standards with known elemental concentrations. 

    The results show that the concentration of the most abundant element in each end-member (Si for quartz, Ca for calcite, and Al for brown earth) systematically increases along nearly linear trends as the amount of the end-member increases. For the simplest set of mixtures, quartz and calcite, the concentration of Ca nearly follows the expected concentration of a theoretical mixture of pure quartz and pure calcite; however, Si is lower than expected suggesting the calibration file underestimates Si concentrations. Uncertainty, based on the standard deviation of 10 replicate analyses, is greatest for Al and lowest for Ca, highlighting the inherent limitations of XRF spectroscopy to quantify lighter elements. Aluminum, Si, and Ti concentrations are higher in mixtures of brown earth with quartz compared tobrown earth mixed with calcite. For Al and Si this is consistent with residual inter-elemental spectral interference that may not have been completely removed by the conversion file. Concentrations of the minor elements V and Mn generally increase as the proportion of brown earth increases, but the trends are not linear.

    Semi-quantitative elemental concentration data determined by handheld XRF analysis in this study yielded reliable first-order compositional trends, particularly for major elements. However, deviations from theoretical mixing, higher uncertainty for lighter elements, and non-linear behavior of minor elements indicate calibration and matrix limitations.

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