High-temperature cation distributions in Fe₃O₄-MgAl₂O₄-MgFe₂O₄-FeAl₂O₄ spinels from thermopower and conductivity measurement

The distribution of cations between octahedral and tetrahedral sites in Fe₃O₄‐FeAl₂O₄, Fe₃O₄‐MgFe₂O₄, and Fe₃O₄‐MgAl₂O₄ solid solutions has been determined in situ at 600 °C to 1400 °C and 1‐atm pressure. The method uses a combination of thermopower and electrical conductivity measurements to characterize the partitioning of Fe²⁺ and Fe³⁺ between the two sites. This enables determination of all cation occupancies on the Fe₃O₄‐FeAl₂O₄ and Fe₃O₄‐MgFe₂O₄ joins, but requires use of a model to fix one other parameter in Fe₃O₄‐MgAl₂O₄ solid solutions.

The data have been used to evaluate the applicability of currently used cation‐distribution models to spinels. The join Fe₃O₄‐MgFe₂O₄ fits reasonably well to either the simple constant $KCdFe2+−Fe3+$ and $KCdMg−Fe3+$ model (Navrotsky and Kleppa, 1967) or to the more complex O’Neill‐Navrotsky (1983, 1984) model in which –RT In K_Cd is predicted to be a linear function of the occupancy of tetrahedral sites by trivalent cations. Cation‐distribution data for Fe₃O₄‐FeAl₂O₄ solid solutions are in quantitative agreement with the O’Neill‐Navrotsky model in that $−RTlnKCdFe2+−Fe3+$ and $−RTlnKCdFe2+−Al$ are linear functions of the degree of inversion. The observed variations of $−RTlnKCdFe2+−Al$ do not, however, agree with those in end‐member hercynite so that this join only agrees qualitatively with the model. Fe₃O₄‐MgAl₂O₄ solutions are intermediate in behavior between the other two joins, their cation distributions $(−RTlnKCdFe2+−Fe3+,−RTlnKCdFe2+−Al, and −RTlnKCdMg−Fe2+)$ being linear functions of tetrahedral trivalent ions and in broad agreement with the data for pure magnetite, hercynite, and magnesioferrite, respectively.