“Free” oxide ions in silicate melts: Thermodynamic considerations and probable effects of temperature

Reactions of simple metal oxides with silica often describe the melt solution in terms of equilibria among three energetically distinct oxygen ion species: bridging oxygens connecting two network cations, non-bridging oxygens shared by one network cation and multiple modifier cations of lower charge and larger radius, and “free” oxide ion bonded only to modifiers. Interest in the role and concentration of “free” oxide ions has recently been renewed by spectroscopic studies that have directly determined its abundance in a few glassy and amorphous silicates with low silica contents and high cation field strengths, as well as being inferred to be present by less direct measurements of bridging/non-bridging oxygen ratios in more silica-rich compositions. Here we review and evaluate recent evidence on “free” oxide concentrations in silicate glasses and melts, and discuss the important clues about effects of composition that come from comparisons of heats of formation of silicates and measurements of oxide component activities: relative proportions of oxygen species are expected to depend strongly on the charge and size of the modifier cation and its degree of covalent/ionic bonding. We also present simple thermodynamic approximations that allow temperature effects on “free” oxide concentrations to be estimated. In most compositions, this relative high energy species is expected to become more abundant at high temperatures, and may reach quite significant concentrations (> 10% of total oxygens?) in hot, ultramafic, Mg- and Fe-rich melts in nature. In contrast, in most common, glass-forming compositions the concentrations of “free” oxide ion are probably much lower, particularly on cooling to glass transition temperatures, but this species has been suggested to potentially have an important role in melt dynamics even at low abundances. In any case, further experimental determinations, and theoretical evaluations, of this somewhat elusive and controversial species are well warranted.