Chemical potentials of oxygen for fayalite-quartz-lron and fayalite-quartz-magnetite equilibria
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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) + 2Fe0.9470 (r.s.) + SiO2 (qz)--~ FeESiO 4 (ol) AG ~
15.59T(+-150)
J m o l -~
and 3Fe2SiO 4 (ol) + 0 2 (g) ~
2 F e 3 0 4 (sp) + 3SIO2 (qz)
AG ~ = -471,750 + 160.06 T ( - l l 0 0 )
J m o l -~
The "third-law" analysis of fayalite-quartz-wustite and fayalite-quartz-magnetite equilibria gives value for AH~98 as -35.22 ( - 0 . 1 ) and - 5 2 8 . 1 0 (-0.1) kJ mol -~, respectively, independent of temperature. The Gibbs energy of formation of the spinel form of FezSiO4 is derived by combining the present results on FQI equilibrium with the high-pressure data on olivine to spinel transformation of Fe2SiO4.
I.
INTRODUCTION
As part of a larger research program on silicates and, their solid-solutions, tL2j electrochemical measurements have been undertaken to determine the thermodynamic stability of fayalite at high temperature. Accurate information on solid phases is useful for the evaluation of data on liquid silicates and for the computation of phase diagrams. At high pressures, fayalite (Fe2SiO4) transforms from the olivine structure to a more dense spinel f o r m . [3] Both the equilibria, FQI and FQM, are extensively used as oxygen buffers. The compound F e 2 S i O 4 is reported to exhibit negligible homogeneity range, t41 There have been a large number of studies on FQI and FQM equilibria using different techniques, but the agreement between the results of various investigators has not been entirely satisfactory. The present investigation aims at generating more reliable thermodynamic data on_FQI and FQM equilibria at high temperature. The Gibbs energy change for FQI equilibrium has been determined by Lebedev and Levitskii[SJ (1123 to 1423 K) and Schwerdtfeger and Muan 16] (1273, 1373, and 1423 K) by CO/CO2 gas equilibrium technique. Nafziger and Muan 17~ (1423 and 1449 K) and Williams ts] (1198 to 1451 K) employed the C O 2 / H 2 gas equilibration techK.T. JACOB, Professor and Chairman, G.M. KALE, Graduate Student, and G.N.K. IYENGAR, Professor, are with the Department of Metallurgy, Indian Institute of Science, Bangalore 560 012, India. Manuscript submitted October 7, 1988. METALLURGICALTRANSACTIONS B
nique, whereas Berliner and Shapovalova tgj (1033 to 1403 K) used a H2/H20 gas mixture. Taylor and Schmalzried tl~ (1173 to 1373 K), Schwab and Sohnlein tlq (1233 to 1428 K), Schwab and Kustner t~2] (1099 to 1414 K), and, recently, O'Neill t13] (1000 to 1400 K) employed an emf technique using stabilized zirconia as the solid electrolyte, whereas Rog and Kozinski t~4] (870 to 1200 K) used Fe ยง exchanged /3-alumina as solid electrolyte in their emf measurements to determine the free energy of formation of FezSiO4 from stoichiometric FeO and SIO2. The crys
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