Thermodynamics and constitution of silicate melts. The system PbO + PbF2+ SiO2

Abstract

Activities of PbF₂ in PbO + PbF₂ and PbO + PbF₂+ SiO₂ melts with up to 40 mol% PbF₂ were determined at 1073–1173 K by means of an electrochemical cell with CaF₂ as solid electrolyte. The results, together with previous measurements of the activity of PbO, were interpreted in terms of a theory in which the anionic constitution of the melts is represented by O^2–, F^– and an equilibrium array of silicate and fluorosilicate ions of formula Si_nO_3n+1–mF^{(2n+2–m)–}_m. The equilibrium constant k′ for the formation of fluorosilicate ions by the reaction F^–+ Si_nO_3n+1–mF^{(2n+2–m)–}_m= Si_nO_3n–mF^{(2n+1–m)–}_m+1+ O^2– was estimated to be 0.4 ± 0.025. This allowed the anionic distribution to be evaluated for basic melts. In addition to O^2– and F^–, the main anionic constituents in melts with X_PbO/X_SiO2 > 2.5 and X_PbF2⩽ 0.25 are SiO^4–₄, SiO₃F^3–, SiO₂F^2–₂, SiO₃F^–, Si₂O^6–₇ and Si₂O₆F^5–, with decreasing amounts of more highly fluorinated species. The volatile constituent SiF₄ is also present in small amount and becomes increasingly significant as X_SiO2/X_PbO is increased. Substitution of PbO by PbF₂ causes a net decrease in the mean chain length of the anions.