High-temperature Vibrational Spectroscopy Of Calcite Group And Aragonite Group Carbonates

Carbon is one of the most important elements for life.It can form various inorganic and organic compounds together with different elements due to its unique chemical properties,and widely exists in different spheres in the forms of solid,liquid,or gas.Carbonates are the most abundant carbon-bearing phases in the Earth and play an important role in the geological evolution.The vibrational spectroscopy has been widely used to study the phase transition,as well as the thermodynamic properties of carbonates at high temperature and pressure.In this study,Raman spectroscopy and mid-infrared spectroscopy are used to explore the calcite group and aragonite group carbonates at high temperature.The results show that the average thermal expansion coefficient??_V?of carbonates follow the order calcite<dolomite<magnesite at 297?800 K,and the thermal expansion of the c-axis is significantly higher than that of the a-axis.The isobaric and isothermal Grüneisen parameters(?_iP,and?_iT)of the internal vibration of the three carbonates are from 0 to 0.5,and the absolute value of the anharmonic parameters are less than 1.3×10^-5 K^-1.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 high-temperature vibrational spectroscopy shows that the aragonite transforms to calcite at 773K and witherite transform to a trigonal phase at 1150 K.The isobaric and isothermal Grüneisen parameters(?_iP and?_iT,0.9?3.5)of the external vibration modes of the four aragonite group carbonates?CaCO₃,SrCO₃,PbCO₃,BaCO₃?were significantly higher than the internal vibration modes?<0.4?.The effect of the anharmonicity on PbCO₃should be greater than the other three aragonite carbonates,and the acoustic Debye temperature is significantly lower.The internal vibration modes have a linear relationship with the relative mass of the corresponding cations of aragonite group carbonates.The abnormally low frequency of PbCO₃ should be related to the larger atomic mass of Pb.The unit cell parameter of Sr_xCa_1-xCO₃ solid solutions gradually increased with the increase of Sr content.The variation rates of the unit cell parameters follow the sequence b>c>a as the composition changing.With the Sr content increasing,both the Raman-active and IR-active vibrational modes systematically show a redshift for the solid solution of Sr_xCa_1-xCO₃.As the mass of cations increasing,the ¹³C/¹²C-?factor of aragonite-type carbonates gradually decreases.At room temperature,¹³C tends to be concentrated in carbonates.At temperatures above 400 K,¹³C tends to be concentrated inCO₂,and the ¹³C enrichment priority isCO₂>aragonite-type carbonates>graphite.The¹³C substitution has no effects on the melting point of the calcite and the phase transition temperature of the aragonite.Within the margin of error,the calcite and aragonite have the same isostatic Grüneisen parameters and anharmonic parameters with the corresponding phase contains ¹³C.The effects of anharmonicity on isotopic fractionation are not significant in the study.This study conducts a detail research on the effects of cations of carbonates on the unit cell parameters,vibrational modes,thermodynamic properties,and carbon isotope fractionation.The carbon isotope fractionation is also calculated by using the vibration frequencies between carbonates and other carbon-bearing phases and the behavioral trends of carbon isotope have been clarified roughly.Our works can provide a reliable reference for the carbon isotope fractionation and carbon cycle of the earth.