EXPERIMENTAL DETERMINATION OF ACTIVITIES IN FETIO3-MNTIO3 ILMENITE SOLID SOLUTION BY REDOX REVERSALS

Abstract

Solid solutions of (Fe,Mn)TiO3 were synthesized, mostly at 0.10 XMn intervals, at 1 bar, 900°C and log f O2 = –17.50. Analysis by EMP indicate an ideal stoichiometry for the Fe-Mn ilmenites with (Fe+Mn) = Ti = 1.000 when normalized to 3 oxygens. Their unit cell volume increases linearly with XMn. The composition of Fe-Mn ilmenite coexisting with metallic Fe and rutile was reversed at 1 bar, 700–900°C and fixed f O2 in a gas-mixing furnace. Oxygen fugacity was controlled by mixing CO2 and H2 gas and was continuously monitored with an yttrium-stabilized zirconia electrolyte. Solution properties of Fe-Mn ilmenite were derived from the experimental data by mathematical programming (Engi and Feenstra, in preparation) including notably the results of Fe-Mn exchange experiments between ilmenite and garnet (Feenstra and Engi, submitted) and anchoring the standard state properties to the updated thermodynamic dataset of Berman and Aranovich (1996). The thermodynamic analysis resulted in positive deviations from ideality for (Fe,Mn)TiO3 ilmenite, which is well described by an asymmetric Margules model with WHFeFeMn = –9.703 and WHFeMnMn = –23.234 kJ/mol, WSFeFeMn = –19.65 and WSFeMnMn = –22.06 J/(K·mol). The excess free energy for Fe-Mn ilmenite derived from the redox reversals is larger than in the symmetric ilmenite model (WGFeMn = +2.2 kJ/mol) determined by O'Neill et al. from emf measurements on the assemblage iron-rutile-(Fe,Mn)ilmenite.