Phase equilibria in the systems Fe₂O₃-MgO-TiO₂ and FeO-MgO-TiO₂ between 1173 and 1473 K, and Fe²⁺-Mg mixing properties of ilmenite, ferrous-pseudobrookite and ulvöspinel solid solutions

Abstract

Detailed phase relations have been determined within the systems Fe₂O₃-MgO-TiO₂ and FeO-MgO-TiO₂. Experiments were performed over the temperature interval 1173–1473 K by equilibrating pelletized, fine-grained oxide mixtures in either inert calcia-stabilized zirconia pots (Fe₂O₃-MgO-TiO₂ system) or evacuated silica tubes (FeO-MgO-TiO₂ system). Equilibrium phase assemblages were determined by combined optical microscope, X-ray diffraction and EMP examination. Phase relations in the Fe₂O₃-MgO-TiO₂ ternary are dominated by the instability of the M₂O₃ solid solution relative to the phase assemblage M₃O₄ + M₃O₅. A miscibility gap along the M₂O₃ binary also gives rise to two, 3-phase fields (α-M₂O₃ + M₃O₅ + M₃O₄ and α′-M₂O₃ + M₃O₅ + M₃O₄) separated by the M₃O₄ + M₃O₅ phase field. Phase relations in the FeO-MgO-TiO₂ ternary were divided into two sub-systems. For the FeTiO₃-MgTiO₃-TiO₂ sub-ternary, there is complete solid solution along the M₂O₃ and M₃O₅ binary joins at high temperature. At low temperatures (T < 1373 K) the M₃O₅ pseudobrookite solid solution decomposes to M₂O₃ + TiO₂. Increasing the concentration of MgO in M₃O₅ phase results in a decrease in the temperature at which M₃O₅ becomes unstable and compositional tie lines linking M₂O₃ and TiO₂ fan out, before the appearance of a three-phase region where M₂O₃, M₃O₅, and TiO₂ coexist. Within the expanded FeO-MgO-TiO₂ system, at temperatures above ∼1273 K there is a continuous solid solution along the M₃O₄ binary. At low temperatures (T < 1273 K) the Mg₂TiO₄ end-member breaks down to MgO and MgTiO₃. The M₃O₄ phase shows significant non-stoichiometry, down to at least 1173 K. Fe²⁺-Mg partitioning data were obtained for coexisting M₂O₃-M₃O₅ and M₂O₃-M₃O₄ pairs in the FeO-MgO-TiO₂ ternary. Assuming a regular solution mixing model for all phases, the M₂O₃ and M₃O₄ solid solutions were both found to exhibit moderate positive deviations from ideality (∼2600 J/mol), whereas the data for the M₃O₅ binary suggest close to ideal behaviour.