Sulfur chemistry in a borosilicate melt Part 3. Iron-sulfur interactions and the amber chromophore

Abstract

The mutual interactions of iron and sulfur in a borosilicate melt were defined as a function of the iron content, sulfur concentration, melt temperature, and oxygen fugacity. Over the range of conditions for which ions of SO4 2- - Fe3+ - Fe2+ or S2- - Fe2+ - Fe0 existed in the melt, sulfur and iron behaved as two independent redox systems. Mutual interaction occurred only for the conditions when S2- ions were in equilibrium with Fe3+ ions in the melt (for example, an oxygen fugacity of 10^-9 to 10^-11 bar at 1150 °C). The Fe3+ ion oxidized the S2- ion to the supersulfide ion, S2 -, which was identified as the amber chromophore. When the iron content of the melt was 1 wt% or less, the supersulfide ion was produced only in low concentrations; and consequently, the redox reaction producing it did not measurably affect sulfur solubility in the melt. In melts containing 10 wt% total iron, the mutual interaction of Fe3+ and S2- ions was sufficient to enhance the sulfur solubility through the formation of polysulfide species in the melt.