Experimental investigation of the bonding of sulfur in highly reduced silicate glasses and melts

Elucidating the role of sulfur on the structure of silicate glasses and melts at elevated pressures and temperatures is important for understanding transport properties, such as electrical conductivity and viscosity, of magma oceans and mantle-derived melts. These properties are fundamental for modeling the evolution of terrestrial planets and moons. Despite several investigations of sulfur speciation in glasses, questions remain regarding the effect of S on complex glasses at highly reducing conditions relevant to Mercury. Glasses were synthetized with compositions representative of the Northern Volcanic Plains of Mercury and containing quantities of S up to 5 wt %. Multiple spectroscopic methods and microprobe analyses were employed to probe the glasses, including in situ impedance spectroscopy at 2- and 4-GPa pressures and temperatures up to 1740 K using a multi-anvil press, Si-29 NMR spectroscopy, and Raman spectroscopy. Electrical activation energies (Ea) in the glassy state range from 0.56 to 1.10 eV, in agreement with sodium as the main charge carrier. The electrical measurements indicate that sulfide improves Na+ transport and may overcome a known impeding effect of the divalent cation Ca2+. The glass transition temperature lies between 700 and 750 K, and for temperatures up to 970 K Ea decreases (0.35-0.68 eV) and the conductivities of the samples converge (similar to 5-8 x 10(-3) S/m). At T-quench, the melt fraction is 50-70% and melt conductivity varies from 0.7 to 2.2 S/m, with the sample containing 5 wt% S the most conductive among the set. Si-29 NMR spectra reveal that a high fraction of S bonds with Si in these complex glasses, a characteristic that has not been recognized previously. Raman spectra and maps reveal regions rich in Ca-S or Mg-S bonds. The evidence of sulfide interactions with both Si and Ca/Mg suggest that alkaline earth sulfides can be considered weak network modifiers in these glasses, under highly reduced conditions.