Chemical potentials of oxygen for fayalite-quartz-lron and fayalite-quartz-magnetite equilibria

Abstract

The oxygen potentials corresponding to fayalite-quartz-iron (FQI) and fayalite-quartz-magnetite (FQM) equilibria have been determined using solid-state galvanic cells: Pt,Fe + Fe₂SiO₄ + SiO₂/(Y₂O₃)ZrO₂/Fe + "FeO,"Pt and Pt, Fe₃O₄ + Fe₂SiO₄ + SiO₂/(Y₂O₃)ZrO₂/Ni + NiO, Pt in the temperature ranges 900 to 1400 K and 1080 to 1340 K, respectively. The cells are written such that the right-hand electrodes are positive. Silica used in this study had the quartz structure. The emf of both cells was found to be reversible and to vary linearly with temperature. From the emf, Gibbs energy changes were deduced for the reactions: 0.106Fe (s) + 2Fe_0.947O (r.s.) + SiO₂ (qz) → Fe₂SiO₄ (ol) ΔG°= -39,140+ 15.59T(± 150) J mol^-1 and 3Fe₂SiO₄ (ol) + O₂ (g) → 2Fe₃O₄ (sp) + 3SiO₂ (qz) δG° = -471,750 + 160.06 T(±1100) J mol^-1 The "third-law" analysis of fayalite-quartz-wustite and fayalite-quartz-magnetite equilibria gives value for δG° ₂₉₈ as -35.22 (±0.1) and -528.10 (±0.1) kJ mol^-1, respectively, independent of temperature. The Gibbs energy of formation of the spinel form of Fe₂SiO₄ is derived by com-bining the present results on FQI equilibrium with the high-pressure data on olivine to spinel transformation of Fe₂SiO₄.