• journal_title：American Mineralogist
• Contributor：Richard J. Harrison ; Erika J. Palin ; Natasha Perks
• Publisher：Mineralogical Society of America
• Date：2013-04-01
• Format：text/html
• Language：en
• Identifier：10.2138/am.2013.4318
• journal_abbrev：American Mineralogist
• issn：0003-004X
• volume：98
• issue：4
• firstpage：698
• section：Articles

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^T_Fe^O_Ti). The predicted cation distribution evolves gradually from the Néel-Chevalier model to the Akimoto model as a function of increasing J^T_Fe^O_Ti, with J^T_Fe^O_Ti = 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^T_Fe^O_Ti 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^T_Fe^O_Ti. If confirmed, this conclusion will have significant implications for attempts to create a realistic thermodynamic model of titanomagnetite.