Fe3+/Sigma Fe vs. FeL alpha peak energy for minerals and glasses: Recent advances with the electron microprobe

This paper describes a preliminary study that attempts to determine the oxi dation state of Fe (Fe3+/Sigma Fe) with the electron microprobe (EMP) by me asuring the self-absorption induced shift of the FeL alpha peak emitted fro m minerals and glasses. In transition metals of the first row, the L-spectr a exhibit common distortions, namely peak position shifts, peak shape alter ations, and changes in the L beta /L alpha ratios, caused by the large diff erence in the self-absorption coefficients (mu/rho) on either sides of the L-3 absorption edges that are in close proximity to the L alpha peak maxima . Measurements performed on alpha -Fe2O3 and FexO oxides have shown that se lf-absorption effects are stronger for the later oxide, leading to enhanced Fe2+L alpha peak shift toward longer wavelengths as the beam energy increa ses. First measurements performed on silicates have confirmed that enhanced self-absorption of FeL alpha occurs on Fe2+ sites. The measurements consis ted of plotting the FeL alpha peak position at a fixed beam energy (15 keV) against the total Fe concentration for two series of Fe2+- and Fe3+- heari ng silicates. In a first step, these data have: shown that both Fe2+L alpha and Fe3+L alpha peaks shift continuously toward longer wavelengths as the Fe concentration increases, with enhanced shifts for Fe2+L alpha. For silic ates containing only Fe2+ or Fe3+, no effects of the site geometry were det ected on the variations of the FeLa peak position. A second set of plots ha s shown the variations of the peak position relative to the previous Fe2+-F e3+ curves of step 1, as a function of the nominal Fe3+/Sigma Fe, for a ser ies of reference minerals (hydrated and non-hydrated) and basaltic glasses. Data from chain and sheet silicates (e.g., pyroxenes, amphiboles, micas) e xhibited strong deviations compared to other phases (e.g., garnets, Al-rich spinels, glasses), due to reduced self-absorption of FeL alpha. Intervalen ce-charge transfer (IVCT) mechanisms between Fe2+ and Fe3+ sites may be the origin of these deviations. These crystal-structure effects limit the accu racy of the method for mixed Fe2+-Fe3+ valence silicates. Precisions achiev ed for further Fe3+/Sigma Fe measurements strongly depend on the total Fe c oncentration. For basaltic glasses containing an average of 8 wt% Fe and 10 % Fe3+/Sigma Fe, the precision is about +/-2%, (absolute). For low Fe conce ntrations (below 3.5 wt%), the uncertainty in the peak position measured by the EMP spectrometers leads to error bars that are similar to with the sep aration of the curves fitted to the Fe2+ and Fe3+ plots, which is propagate d as prohibitive lack of precision for Fe3+/Sigma Fe (>70% relative). A maj or limitation of microbeam methods in general deals with beam damage. This aspect has been carefully studied for basaltic glasses, and optimal beam co nditions have been established tin general, electron doses higher than thos e corresponding to 130 nA and 30 mum beam diameter should be avoided to pre vent large beam induced oxidation phenomena). Additional work, in progress, concerns: (1) other beam-sensitive phases such as hydrated glasses: and (2 ) minerals in which FeL alpha is affected by large matrix effect correction s (e.g., Cr- and Ti-rich oxides where FeL alpha is strongly absorbed), for which the self-absorption-induced shift of FeL alpha is different from that of common silicates and glasses.