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Thermodynamics and kinetics of cation ordering in natural and synthetic Mg(Al,Fe³⁺)₂O₄ spinels from in situ high-temperature X-ray diffraction

¹ Dipartimento di Mineralogia e Petrologia, University of Padova, Corso Garibaldi 37, I-35137, Italy
² Dipartimento di Scienze della Terra, University of Roma "La Sapienza", P.le A. Moro 5, I-00185, Italy

Correspondence: ^* E-mail: alberto.dalnegro{at}unipd.it

One natural and two synthetic spinels with compositions Mg(Al_1–zFe_z³⁺)₂O₄ (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 °C. Thermal expansion was monitored by measuring the cell edge variation. Cation disorder was monitored by measuring the variation of the oxygen positional parameter u, which is closely correlated with the inversion parameter i. All samples showed extensive Al reordering at the M site between 550 and 650 °C, followed by an increase of disorder at T > 650 °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 °C, i.e., at conditions under which equilibrium was achieved at any temperature. In fact, at T > 650 °C, both ^IVFe³⁺ and ^IVAl increase with increasing T, following the equilibrium path. The reproducibility of ^IVAl occupancies was very high, whereas ^IVFe³⁺ occupancies were not satisfactorily matched. The cation distribution relaxation observed between 550 and 650 °C was interpreted on the basis of kinetic considerations. In this temperature range, 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 was found to be active at laboratory times at about 550 °C, whereas the Mg-Al exchange was hard to monitor below 600 °C.

Key Words: X-ray diffraction • in situ-HT • order-disorder • spinels

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