Enthalpies of mixing of spinels in the systems MgFe2O4-MgAl2O4, ZnFe2O4-ZnAl2O4, and NiAl2O4-ZnAl2O4, determined by oxide-melt solution calorimetry at 973 K, can be described by regular solution parameters of 40.4, 20.1, and 40.1 kJ·mol-1, respectively. These values contradict the systematics, based on cation size and cation distribution, proposed by O'Neill and Navrotsky (1984, American Mineralogist, 69, 733-755). Transposed-temperature-drop calorimetry (room temperature to 973 K) of samples quenched from different temperatures was used to obtain enthalpies of annealing associated with re-equilibration of cation distribution. These enthalpy data and independently measured cation-distribution data in the literature are consistent with the O'Neill and Navrotsky disordering model in which ΔH = αx + βx2, where x is the degree of inversion. Values of β needed to fit the enthalpy data are in the range of −30 to −35 kJ·mol−1 for NiAl2O4, MgAl2O4, and MgFe2O4, rather than near the average value of −20 kJ·mol−1 proposed for 2-3 spinels by O'Neill and Navrotsky. In the range 298–973 K, the system MgAl2O4-MgFe2O4 shows a small negative excess heat capacity whereas NiAl2O4-ZnAl2O4 shows no excess heat capacity.

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