Thermodynamics and kinetics of cation ordering in natural and synthetic Mg(Al,Fe3+)(2)O-4 spinels from in situ high-temperature X-ray diffraction

One natural and two synthetic spinels with compositions Ma(Al1-zFez3+)(2)O-4 (with 2z = 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 degrees C. Thermal expansion was monitored by measuring the cell edge variation. Cation disorder was monitored by measuring the variation of the oxygen positional parameter it, which is closely correlated with the inversion parameter i. All samples showed extensive Al reordering at the M site between 550 and 650 degrees C, followed by an increase of disorder at T > 650 degrees C due to both Mg-Al and Mg-Fe3+ 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 degrees C, i.e. at conditions under which equilibrium was achieved at any temperature. In fact, at T > 650 degrees C, both (IV)Fe(3+)and Al-IV increase with increasing T, following the equilibrium path. The reproducibility of Al-IV occupancies was very high, whereas Fe-IV(3+) occupancies were not satisfactorily matched. The cation distribution relaxation observed between 550 and 650 degrees C was interpreted on the basis of kinetic considerations. In this temperature ran,,e, inversion decreases to a minimum because the amounts of Al that reorder are far more abundant than those of Fe3+ that disorder. The Mg-Fe3+ exchange was confirmed to proceed at a faster rate than the Mg-Al exchange. Moreover, the Mg-Fe3+ exchange wits found to be active at laboratory times at about 550 degrees C, whereas the Mg-Al exchange was hard to monitor below 600 degrees C.