Cation-distribution energetics and heats of mixing in MgFe₂O₄-MgAl₂O₄, ZnFe₂O₄-ZnAl₂O₄, and NiAl₂O₄-ZnAl₂O₄spinels; study by high-temperature calorimetry Available to Purchase

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.