A computational model of cation ordering in the magnesioferrite-qandilite (MgFe2O4-Mg2TiO4) solid solution and its potential application to titanomagnetite (Fe3O4-Fe2TiO4)

Cation ordering in the magnesioferrite-qandilite (MgFe2O4-Mg2TiO4) solid solution has been investigated using an interatomic potential model combined with Monte Carlo simulations. The dominant chemical interaction controlling the thermodynamic mixing behavior of the solid solution is a positive nearest-neighbor pairwise interaction between tetrahedrally coordinated Fe3+ and octahedrally coordinated Ti4+ (J(FeTi)(TO)). The predicted cation distribution evolves gradually from the Neel-Chevalier model to the Akimoto model as a function of increasing J(FeTi)(TO), with J(FeTi)(TO) = 1000 +/- 100 K providing an adequate description of both the temperature and composition dependence of the cation distribution and the presence of a miscibility gap. Although Mg is a good analog of Fe2+ in end-member spinels, a comparison of model predictions for MgFe2O4-Mg2TiO4 with observed cation ordering behavior in titanomagnetite (Fe3O4-Fe2TiO4) demonstrates that the analog breaks down for Fe3O4-rich compositions, where a value of J(FeTi)(TO) closer to zero is needed to explain the observed cation distribution. It is proposed that screening of Ti4+ by mobile charge carriers on the octahedral sublattice is responsible for the dramatic reduction in J(FeTi)(TO). If confirmed, this conclusion will have significant implications for attempts to create a realistic thermodynamic model of titanomagnetite.