Kinetics of cation ordering in synthetic Mg(Al, Fe3+)2O4 spinels

The kinetics of cation ordering (quench method) for two synthetic Mg(Al2-yFe3+)O-4 spinels (y similar to 0.39 and 0.54, samples F39 and F54, respectively) were studied by means of X-ray single-crystal diffraction. The equilibrium distribution of Mg-Al-Fe3+ between tetrahedral and octahedral sites was investigated at 750, 650 and 550 degrees C (in ordering run) starting from an equilibrium ordering state corresponding to 1000 degrees C, through several time-steps in order to monitor the rate of change of cation distribution. The cation distribution between T and M sites was calculated from the bond lengths using the Minuit program. The Mueller kinetic model, applied to the Mg-Al cation exchange, allowed the calculation of the kinetic ordering constants K, linearly related to temperature by the Arrhenius equations. The Fe3+ content influences the kinetics of the ordering process. The equilibrium for the three isotherms at 750,650 and 550 degrees C was reached at different elapsed times for the F39 and F54 spinels. In particular, the time for the F39 sample was, in all the ordering runs, higher than that of the F54 sample. Consequently, the activation energy (274 and 265 kJ/mol for low- and high-Fe3+ samples, respectively) for the intra-crystalline Mg-Al ordering slightly decreases with increasing Fe3+ content. The intracrystalline closure temperature (T-c) seems to be only marginally affected by the different Fe3+ content. As magnesiochromite spinels are among the most abundant phases in diamonds such results could have implications for diamond thermometry because magnesiochromite inclusions can contain as much as 45 % total iron as Fe3+.