Abstract

The time and temperature evolution of intracrystalline Mg-Al exchange in synthetic MgAl2O4 spinel was studied by single-crystal X-ray diffraction of quenched samples, with the aim of contributing to rock cooling-rate estimation. Flux-grown, homogeneous crystals (0.1 mm in size) were annealed at 1000 °C and then were isothermally ordered at 900, 800, and 700 °C for 10 seconds to 40 days. The cation ordering process was investigated by measuring the spinel O atom positional parameter u, which had been previously demonstrated to be closely related to the inversion x at equilibrium by the linear equation (R2 = 0.995):

 
\[\mathit{x}\ =\ 21.396\ {-}\ 80.714\ \mathit{u}.\]

From x = 0.27 after annealing runs, the inversion decreased with time for all of the three ordering runs, and equilibrium was reached after about 4 min at T = 900 °C (x = 0.25), about 55 min at T = 800 °C (x = 0.23), and about 700 min at T = 700 °C (x = 0.21). The Mueller kinetic model was satisfactorily applied to the experimental data. Solution of Mueller’s integral gave the kinetic ordering constants K900 = 1.12 ± 0.57, K800 = 0.112 ± 0.047, and K700 = 0.0171 ± 0.0045 min−1, corresponding to t1/2 of 0.6, 6.2, and 40.5 min, respectively. The linear dependence of K with temperature (R2 = 0.99) was observed, and may be expressed by the Arrhenius equation:

 
\[ln\ \mathit{K}\ =\ 20.189\ {-}\ 23722/\mathit{T}(K)\]

An activation energy of 197 ± 22 kJ/mol was obtained for the intracrystalline Mg-Al ordering reaction, which compares well with the value of 217 kJ/mol reported in the recent literature for Mg-Fe3+ exchange in spinels.

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