- 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 (MgFe<sub>2sub>O<sub>4sub>-Mg<sub>2sub>TiO<sub>4sub>) 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 Fe<sup>3+sup> and octahedrally coordinated Ti<sup>4+sup> (J<sup>Tsup><sub>Fesub><sup>Osup><sub>Tisub>). The predicted cation distribution evolves gradually from the Néel-Chevalier model to the Akimoto model as a function of increasing J<sup>Tsup><sub>Fesub><sup>Osup><sub>Tisub>, with J<sup>Tsup><sub>Fesub><sup>Osup><sub>Tisub> = 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 Fe<sup>2+sup> in end-member spinels, a comparison of model predictions for MgFe<sub>2sub>O<sub>4sub>-Mg<sub>2sub>TiO<sub>4sub> with observed cation ordering behavior in titanomagnetite (Fe<sub>3sub>O<sub>4sub>-Fe<sub>2sub>TiO<sub>4sub>) demonstrates that the analog breaks down for Fe<sub>3sub>O<sub>4sub>-rich compositions, where a value of J<sup>Tsup><sub>Fesub><sup>Osup><sub>Tisub> closer to zero is needed to explain the observed cation distribution. It is proposed that screening of Ti<sup>4+sup> by mobile charge carriers on the octahedral sublattice is responsible for the dramatic reduction in J<sup>Tsup><sub>Fesub><sup>Osup><sub>Tisub>. If confirmed, this conclusion will have significant implications for attempts to create a realistic thermodynamic model of titanomagnetite.