Dissolution mechanisms of tetravalent sulphur in silicate melts: Evidences from sulphur K edge XANES studies on glasses

Incorporation of sulphur in silicate glasses produced by reactions between melts and sulphur-bearing sources at high temperatures was studied by X-ray absorption near edge structure (XANES) spectroscopy at the sulphur K edge. Three types of syntheses were performed using sodium trisilicate, soda lime silicate, and float glass as starting materials: (i) controlled bubbling of glass melts with SO2 or SO2/O-2 gas mixtures at ambient pressure, (ii) melting of glass powder containing various salts (Na2SO4, Na2SO3, Na2S) in excess to the expected solubility in the melt and (iii) fusion of glass powder in gold capsules at 1000 degrees C and 100 MPa with an added amount of salts below the expected sulphur solubility. In some of the latter experiments water was introduced to vary the oxygen fugacity in the system. Sulphur concentrations in the glasses were determined by combustion and subsequent IR spectroscopy; water concentration in the glasses was measured by IR microspectroscopy. Bulk composition and homogeneity of glasses were checked by electron microprobe analyses. XANES spectra obtained at room temperature give evidence for incorporation of sulfate (S6+) and/or sulfide (monosulfide [S2-] and/or polysulfide [S-x(2-)]) in all glasses, depending on starting materials and run conditions. Clear evidence for sulfite (S4+) was not found in any of the spectra, implying that this sulphur species in glass products at room temperature is at or below the detection limit. We suggest that tetravalent sulphur (as SO2 or Na2SO3) disproportionates into sulfate and sulfide when being dissolved in simple silicate melts or during cooling of the glass melt.