Thermodynamics and kinetics of cation ordering in natural and synthetic Mg(Al,Fe3+)2O4 spinels from in situ high-temperature X-ray diffraction

One natural and two synthetic spinels with compositions Mg(Al 1− z Fe z 3+ ) 2 O 4 (with 2 z = 0.078, 0.138, and 0.200, respectively) were studied by in situ, high-temperature, single-crystal X-ray diffraction. All samples were heated from room temperature to 1050 °C. Thermal expansion was monitored by measuring the cell edge variation. Cation disorder was monitored by measuring the variation of the oxygen positional parameter u , which is closely correlated with the inversion parameter i . All samples showed extensive Al reordering at the M site between 550 and 650 °C, followed by an increase of disorder at T > 650 °C due to both Mg-Al and Mg-Fe 3+ intersite exchanges. The measured cation distributions were compared with those calculated using the general thermodynamic model for spinel binary solid-solutions of O’Neill and Navrotsky (1984). Measured and calculated inversion parameters compare satisfactorily at T > 650 °C, i.e., at conditions under which equilibrium was achieved at any temperature. In fact, at T > 650 °C, both IV Fe 3+ and IV Al increase with increasing T , following the equilibrium path. The reproducibility of IV Al occupancies was very high, whereas IV Fe 3+ occupancies were not satisfactorily matched. The cation distribution relaxation observed between 550 and 650 °C was interpreted on the basis of kinetic considerations. In this temperature range, inversion decreases to a minimum because the amounts of Al that reorder are far more abundant than those of Fe 3+ that disorder. The Mg-Fe 3+ exchange was confirmed to proceed at a faster rate than the Mg-Al exchange. Moreover, the Mg-Fe 3+ exchange was found to be active at laboratory times at about 550 °C, whereas the Mg-Al exchange was hard to monitor below 600 °C.