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Kinetics and Thermodynamics of Mg-Al Disorder in MgAl2O4-Spinel: A Review.

ABSTRACT: The MgAl2O4-spinel has wide applications in various industries and in geosciences. It shows a significant inter-site Mg-Al cation exchange (denoted by the inversion parameter x), which modifies structural features, such as the unit-cell parameters and the sizes of the component polyhedra, and influences the physical and chemical properties. Previous studies mainly focused on the kinetics and thermodynamics of the Mg-Al exchange reaction, with the aim to ascertain the correlation between the inversion parameter and temperature; these studies, however, reached conflicting results. Here, we first reviewed the kinetics studies on the Mg-Al cation exchange reaction, and then reviewed all thermodynamic experiments, with special attention paid to the Mg-Al cation exchange equilibrium and the quench process, which might have modified the cation distributions once attained at high temperatures. We also assessed the accuracies in the temperature measurements and in the quantifications of the x by different analytical methods. With some necessary temperature correction and data removal, we have landed with a generally reliable x-T dataset covering the T-x space of 873 < T < 1887 K and 0.18(1) < x < 0.357(60) (71 data pairs in total). Fitting these x-T data to three most commonly used thermodynamic models, we have obtained more accurate model parameters. Further, we also evaluated the constituent items of the Gibbs free energy for the Mg-Al cation exchange reaction with experimental results from different research fields and reached the conclusion that highly possibly the T ? S D should not be neglected. Based on this review, we suggest that: (1) Further kinetics study on the Mg-Al exchange reaction should be performed at both low T (<~973 K) and high T (>~1173 K); (2) further Mg-Al exchange equilibrium studies should be carried out at relatively low T and ambient P, as well as in vast ranges of simultaneous high P and high T; and (3) direct experimental measures about the entropies or the enthalpies of the MgAl2O4-spinels disordered to different extents should be conducted with full characterization of the starting materials and detailed description of the experimental procedures.

SUBMITTER: Ma Y

PROVIDER: S-EPMC6540108 | biostudies-literature | 2019 May

REPOSITORIES: biostudies-literature

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Kinetics and Thermodynamics of Mg-Al Disorder in MgAl2O4-Spinel: A Review.

Ma Yunlu Y Liu Xi X

Molecules (Basel, Switzerland) 20190501 9

The MgAl2O4-spinel has wide applications in various industries and in geosciences. It shows a significant inter-site Mg-Al cation exchange (denoted by the inversion parameter x), which modifies structural features, such as the unit-cell parameters and the sizes of the component polyhedra, and influences the physical and chemical properties. Previous studies mainly focused on the kinetics and thermodynamics of the Mg-Al exchange reaction, with the aim to ascertain the ...[more]

PMID: 31052437

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Project description:MgAl2O4-spinel has wide industrial and geological applications due to its special structural and physical-chemical features. It is presumably the most important endmember of complex natural spinel solid solutions, and therefore provides a structural model for a large group of minerals with the spinel structure. There exists a well known but still inadequately understood phenomenon in the structure of MgAl2O4-spinel, the Mg-Al cations readily exchanging their positions in response to variations of temperature, pressure, and composition. A large number of experiments were performed to investigate the Mg-Al cation order-disorder process usually quantified by the inversion parameter x (representing either the molar fraction of Al on the tetrahedral T-sites or the molar fraction of Mg on the octahedral M-sites in the spinel structure), and some thermodynamic models were thereby constructed to describe the x-T relation. However, experimental data at some key T were absent, so that the different performance of these thermodynamic models could not be carefully evaluated. This limited the interpolation and extrapolation of the thermodynamic models. By performing some prolonged annealing experiments with some almost pure natural MgAl2O4-spinel plates and quantifying the x values with single-crystal X-ray diffraction technique, we obtained some critical equilibrium x values at T down to 773 K. These new x-T data, along with those relatively reliable x values at relatively high T from early studies, clearly indicate that the CS94 Model (a model constructed by Carpenter and Salje in 1994) better describes the Mg-Al cation order-disorder reaction in MgAl2O4-spinel for a wide range of T. On the basis of the CS94 Model, a geothermometer was established, and its form is T-closure = 21362 × x3 - 12143 × x2 + 6401 × x - 10 (T-closure standing for the closure temperature of the Mg-Al cation exchange reaction). This geothermometer can be used to constrain the thermal history of the geological bodies containing MgAl2O4-spinel.

Dataset Information

Kinetics and Thermodynamics of Mg-Al Disorder in MgAl2O4-Spinel: A Review.

Publications

Kinetics and Thermodynamics of Mg-Al Disorder in MgAl<sub>2</sub>O<sub>4</sub>-Spinel: A Review.

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