The Valence and Speciation of Sulfur in Glasses by X-Ray Absorption Spectroscopy

Abstract

The geochemical behavior of sulfur in magmas depends strongly on the oxidation state of sulfur, but this is not easily determined by standard analytical methods. We have measured XANES absorption spectra at the sulfur K-edge and have found that such measurements are useful to characterize the oxidation state and speciation of sulfur in silicate glasses of geological relevance. Measured spectra of a set of reference minerals show the effects of different oxidation states and coordination numbers of sulfur; there is a large shift in energy (~10-12 eV) of the sulfur K-edge between S 2and S 6+. This large and easily detectable difference makes possible the measurement of the valence of sulfur in unknown samples by measuring the shift in energy of the absorption edge. This approach is applicable to both crystalline and glassy materials, and useful results have been obtained on samples with as little as 450 ppm S. We have used XANES measurements to characterize oxidation state and speciation of sulfur in a set of natural and synthetic sulfur-bearing glasses. The samples cover a range of composition from basaltic to almost rhyolitic, and some were synthesized over a range of pressure, temperature and oxygen fugacity; glass S content varies between 450 and 3000 ppm. XANES analyses, carried out in fluorescence mode at LURE, allowed determination of the sulfur oxidation state in all of the samples and clearly show that some samples contain a mixture of S 2and S 6+ ; no other sulfur species were observed. Quantitative determination of the abundance of sulfide and sulfate shows good agreement with independent measurements based on electron-microprobe determination of the wavelength shift of sulfur K␣ X-rays.

Xanes Spectroscopy of Sulfur in Earth Materials

“…The S K-and L-edge spectra (Figs. 2, 13, 14; Li et al 1995a, Kasrai et al 1996b) exhibit a progressive increase in the energy position of the edge feature with increase in oxidation state, refl ecting the progressive increase in charge on the absorber atom, to maximum values of 2482 eV and about 171 eV, respectively, for divalent metal sulfates, although Paris et al (2001) reported a value of 2483.1 eV for the Kedge peak of sulfate in a natural apatite. In Figure 13, which is after Li et al (1995a), the K and L edges of the sulfates MgSO 4 •2H 2 O, gypsum (CaSO 4 •2H 2 O), anhydrite, celestine and barite are correlated on a common energy-scale using the S K␣ 1 X-ray emission energy at 2307.8 eV and the S 1s binding energy for Kedge spectra and the S 2p 3/2 binding energy for L-edge spectra.…”

“…The energy position of the S K edge decreases almost linearly with increase in the number of 3d electrons, by about 1 eV from Ca 2+ to Zn 2+ . SILICATE GLASSES Paris et al (2001) used S K-edge XANES spectroscopy to study the speciation of S in some natural and synthetic glasses of basaltic to rhyolitic compositions and S contents of 0.045 to 0.30 wt.%. Spectra for a back-arc basalt, three samples of dacite from Mt.…”

“…St. Helens, laboratory-equilibrated at magnetite-hematite (MH) and Mn oxide (MNO) solid buffers of oxygen fugacity, and NBS-610 is a standard Na-Ca silicate glass equilibrated in air. The sharp feature B is the edge peak of sulfate (after Paris et al 2001).…”

“…Moreover, the strength of absorption peaks is proportional to the amount of absorber species present. This relation is quantitative for S K-edge XANES spectra of metal-sulfi de solid-solutions (Farrell & Fleet 2000, 2001, Farrell et al 2002, minor amounts of S species in coals (Hussain et al 1982, Huffman et al 1991, and minor to trace amounts of S in basaltic glass (Paris et al 2001, Fleet et al 2005a. Because both initial and fi nal electron states are important in defi ning the transition energy associated with the edge feature, the XANES spectrum yields information on the lower part of the conduction band, and the chemical state and local stereochemical environment of the absorber atom.…”

“…4. (a) Background-subtracted S L-edge XANES spectrum for galena, and (b) correlation plot between energy above threshold (⌬E) and 1/R 2 for peaks h (nearest-neighbor shell; S-Pb interaction), g (lattice spacing; d 111 ), f (second shell; S-S), and e (third shell; S-Pb) (after Kasrai et al 1991 Hitchcock et al 1987, Kitamura et al 1988, Vairavamurthy et al 1993a, b, Li et al 1995a, Womes et al 1997, Vairavamurthy 1998, Farrell & Fleet 2000, 2001, Farrell et al 2002, Paris et al 2001, Fleet et al 2005a. Representative K-and L 2,3 -edge XANES spectra for some minerals and synthetic compounds of known composition and structural or molecular state (reference compounds) are given in Figures 2 and 3, respectively.…”