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Kinetics of cation ordering in synthetic MgAl₂O₄ spinel

¹ Dipartimento di Scienze della Terra, Università di Roma "La Sapienza," Piazzale Aldo Moro 5, I-00185 Roma, Italy
² Dipartimento di Scienze della Terra, Università di Trieste, Via Edoardo Weiss 8, I-34127, Trieste, Italy

Correspondence: ^* E-mail: gianni.andreozzi{at}uniroma1.it

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):

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 t_1/2 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:

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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