Polymerization and disproportionation of iron and sulfur in silicate melts: insights from an optical basicity-based approach

In silicate melts, the speciation state of altervalent elements depends on the extension and distribution of polymeric units in the system. Since polymerization of silicate melts is driven by acid–base properties of constituting oxides depending on their Lux–Flood reactivity, the bulk optical basicity of molten silicates and glasses allows us to compute the distribution of oxygen among three distinct polarization states (O2−, O−, O0) in conjugation with the polymeric model of Toop and Samis. Data available in literature are employed to investigate the mutual interaction between sulfur and iron in silicate melts to constrain the equilibrium between sulfide and sulfate species. Results put in evidence (i) the role of bulk melt chemistry, affecting through polymerization, and hence the transfer of electrons among the three polarization states of oxygens, the oxidation state of altervalent elements such as Fe and S and (ii) the limits of the Temkin approach implicit in chemical equations that consider sulfide and sulfate in melts as free anions. Improvements of the model’s accuracy for sulfur speciation may be attained by accounting for the various dissociation equilibria of molten sulfide M2/νS, oxide M2/ν and sulfate M2/νSO4 species in their standard state of pure component in the melt phase at temperature and pressure of interest instead of that of complete dissociation required by the Temkin model.