The valence and speciation of sulfur in glasses by X-ray absorption spectroscopy

The geochemical behavior of sulfur in magmas depends strongly on the oxidat ion state of sulfur, but this is not easily determined by standard analytic al methods. We have measured XANES absorption spectra at the sulfur K-edge and have found that such measurements are useful to characterize the oxidat ion state and speciation of sulfur in silicate glasses of geological releva nce. Measured spectra of a set of reference minerals show the effects of di fferent oxidation states and coordination numbers of sulfur: there is a lar ge shift in energy (similar to 10-12 eV) of the sulfur K-edge between S2- a nd S6+. This large and easily detectable difference makes possible the meas urement of the valence of sulfur in unknown samples by measuring the shift in energy of the absorption edge. This approach is applicable to both cryst alline and glassy materials, and useful results have been obtained on sampl es with as little as 450 ppm S. We have used XANES measurements to characte rize oxidation state and speciation of sulfur in a set of natural and synth etic sulfur-bearing glasses. The samples cover a range of composition from basaltic to almost rhyolitic, and some were synthesized over a range of pre ssure, temperature and oxygen fugacity; glass S content varies between 450 and 3000 ppm. XANES analyses, carried out in fluorescence mode at LURE, all owed determination of the sulfur oxidation state in all of the samples and clearly show that some samples contain a mixture of S2- and S6+; no other s ulfur species were observed. Quantitative determination of the abundance of sulfide and sulfate shows good agreement with independent measurements bas ed on electron-microprobe determination of the wavelength shift of sulfur K alpha X-rays.