In-situ High-Temperature XRD and FTIR for Calcite, Dolomite and Magnesite: Anharmonic Contribution to the Thermodynamic Properties
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In-situ powder X-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectra were measured on the natural crystals of calcite (Ca0.996Mg0.004CO3), dolomite (Ca0.497Mg0.454Fe0.046Mn0.003CO3) and magnesite (Mg0.988Ca0.010Fe0.002CO3), with a temperature up to 796 K. The thermal expansion coefficients were evaluated for these carbonate minerals, resulting in the values of 2.7×10-5, 3.3×10-5 and 3.5×10-5 K-1 for calcite, dolomite and magnesite, respectively. The magnitude of these coefficients is in the same order as those for the isothermal and elastic moduli of these carbonates (e.g., calcite<dolomite<magnesite). The IR-active internal modes of the CO3 group systematically shift to lower frequencies at elevated temperature, and the isobaric (γiP) and isothermal (γiT) Gruneisen parameters for the internal modes are generally smaller than 0.5. The corresponding anharmonic parameters (ai) are typically within the range of -1.5.+1×10-5 K-1, which are significantly smaller in magnitude than those for the external modes. We also calculate the thermodynamic properties (internal energy, heat capacities and entropy) at high temperatures for these carbonates, and the anharmonic contribution to thermodynamics shows an order of calcite>dolomite>magnesite. The Debye model (harmonic approximation) would be valid for magnesite to simulating the thermodynamic properties and isotope fractionation β-factor at high P-T condition.
Key wordscalcite dolomite magnesite high-temperature FTIR anharmonicity thermodynamics
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Many thanks to Profs. Kurt Leinenweber and Joseph Smyth for helpful and constructive discussion and revision on this manuscript. This work was supported by the National Key Research and Development Program of China (No. 2016YFC0600204), and the National Natural Science Foundation of China (Nos. 41590621, 41672041). EPMA and in-situ high-T FTIR experiments were carried out at the State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences (Wuhan), while in-situ high-T powder XRD measurements were conducted at School of Chemical Science and Engineering, Tongji University. The final publication is available at Springer via https://doi.org/10.1007/s12583-019-1236-7.
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