• journal_title：American Mineralogist
• Contributor：Giovanni B. Andreozzi ; Francesco Princivalle
• Publisher：Mineralogical Society of America
• Date：2002-
• Format：text/html
• Language：en
• journal_abbrev：American Mineralogist
• issn：0003-004X
• volume：87
• issue：7
• firstpage：838
• section：Articles

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):

formula" id="disp-formula-1">f" />

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:

formula" id="disp-formula-2">f" />

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.