Oxidation state of iron in silicate glasses and melts: a thermochemical model

• Published in: Chemical geology 2001-04, Vol.174 (1), p.157-179
• Main Authors: Ottonello, Giulio, Moretti, Roberto, Marini, Luigi, Vetuschi Zuccolini, Marino
• Format: Article
• Language: English
• Subjects: Glasses; Iron redox; Melts; Mössbauer; Polymer model
• ISSN: 0009-2541; 1872-6836
• DOI: 10.1016/S0009-2541(00)00314-4

The acid and base dissociation constants of FeO and Fe 2O 3 components in silicate melts are defined in terms of observed relationships between atomistic properties of dissolved oxides (nephelauxetic parameters, electronegativity, fractional ionic character of the bond) and polymerization constants in simple systems. These constants are obtained from the Toop–Samis model depicting the Gibbs free energy of mixing of binary MO–SiO 2 melts, which is coupled with the amphoteric treatment of altervalent dissolved oxides. Model parameterization is carried out on the basis of the extended set of data concerning thermodynamic activity of FeO in melts buffered by equilibrium with pure iron metal and a gaseous phase and on the various measurements of bulk redox state of iron in chemically complex melts at various T, fO 2 conditions. Dissociation constants are related to thermodynamic parameters of the main dissolved species (O 2−, Fe 2+, Fe 3+, FeO 2 −) without any significant error progression. As an ancillary result, thermochemical calculations allow to quantify to some extent the systematic errors in the Fe II/Fe III bulk redox ratio arising from the utilization of Mössbauer spectroscopy on quenched melts and glasses.