Iron oxidation states in silicate glass fragments and glass inclusions with a XANES micro-probe

The iron oxidation states and its environment in volcanic silicate glasses have been studied by combining micro-X-ray absorption near edge structure ( XANES) experiments at the iron K-edge in silicate glass fragments and glass inclusions trapped during the crystal growth. Experiments were performed a t the ESRF (ID22 beam-line) - a third generation synchrotron - using a focu sed beam (2 x 10 mum(2)) together with a high brilliance and a good energy resolution. The selected silicate glasses, basaltic in composition, contain between 5.5 and 8 wt% total Fe. In all XANES spectra, pre-peaks exhibit a multi-component structure, which is the convolution of information related to the valence state of iron (2(+) , 3(+)), the site geometry and transitio n types. The Fe3+/Sigma Fe ratio of silicate glass fragments deduced from t he XANES spectra is correlated to values determined by chemistry on bulk sa mples within the range 0.05-0.85, XANES spectra from glass inclusions hoste d in natural olivine crystals may be interpreted considering the glass frag ments as reference samples. (C) 2001 Elsevier Science B.V. All rights reser ved.