In-situ High-Temperature XRD and FTIR for Calcite, Dolomite and Magnesite: Anharmonic Contribution to the Thermodynamic Properties  

作　　者：Xiang Wang Xiaoxiang Xu Yu Ye Chao Wang Dan Liu Xiaochao Shi Sha Wang Xi Zhu 

机构地区：[1]State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan 430074, China [2]Shanghai Key Lab of Chemical Assessment and Sustainability, School of Chemical Science and Engineering, Tongji University, Shanghai 200092, China

出　　处：《Journal of Earth Science》2019年第5期964-976,共13页地球科学学刊（英文版）

基　　金：supported by the National Key Research and Development Program of China(No.2016YFC0600204);the National Natural Science Foundation of China(Nos.41590621,41672041)

摘　　要：In-situ powder X-ray diffraction(XRD) and Fourier transform infrared(FTIR) spectra were measured on the natural crystals of calcite(Ca0.996 Mg0.004 CO3), dolomite(Ca0.497 Mg0.454 Fe0.046 Mn0.003 CO3) and magnesite(Mg0.988 Ca0.010 Fe0.002 CO3), 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(γi P) and isothermal(γi T) Grüneisen 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.In-situ powder X-ray diffraction（XRD） and Fourier transform infrared（FTIR） spectra were measured on the natural crystals of calcite（Ca0.996 Mg0.004 CO3）, dolomite(Ca0.497 Mg0.454 Fe0.046 Mn0.003 CO3) and magnesite(Mg0.988 Ca0.010 Fe0.002 CO3), 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（γi P） and isothermal（γi T） Grüneisen 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.

关 键 词：CALCITE DOLOMITE MAGNESITE high-temperature FTIR ANHARMONICITY thermodynamics 

分 类 号：P[天文地球]