Study On High Temperature And High Pressure Stability Of Carbonate And Deep Earth’s Carbon Cycle

Carbonates belong to the most abundant Earth materials and play an important role in the deep Earth’s carbon cycle.The total amount of carbon in the atmosphere,oceans and other near-surface reservoirs is negligible compared to the amount stored in the Earth’s mantle.Understanding where and how carbon is stored within the Earth’s interior is of great interest.There are experimental evidences that most of the carbon on Earth exists in the form of carbonate minerals.Therefore,the stability and structural transformation of carbonates in the deep Earth mantle are of considerable interest due to their importance in the carbon storage and Earth’s global carbon cycle.Since people cannot reach the deep Earth mantle at pressures and temperatures to hundreds of gigapascals and thousands of degrees,we can choose to simulate the temperature and pressure at any depth through diamond anvil cell（DAC）experiment.The high pressure phase transformation and structural stability of BaCO₃ was studied by employing Raman spectroscopy,synchrotron X-ray diffraction combined with diamond anvil cells.Meanwhile,the differential scanning calorimeter（DSC）was employed to characterize the process of high temperature phase transition of BaCO₃.First run was to investigate the high pressure phase transition of witherite by Raman spectroscopy with four types of pressure transmitting medium（PTM）,methanol:ethanol（4:1）,silicon oil,helium and neon gas.Experimental result shows that BaCO₃transforms from orthorhombic structure（Pmcn）to hexagonal structure（P-31c）was observed at 9.2¹0.6 GPa,the phase transition points obtained by different PTM are different.After releasing the pressure v₁ symmetric stretching vibration of the CO₃^2-retained in its hexagonal（P-31c）structure,lattice modes and v₄ in-plane symmetric bending vibration of CO₃^2-retrogress to its original orthorhombic（Pmcn）structure.The values of Mode Grüneisen parameters（γ_i）for hexagonal phase and of aragonite phase of different experiment indicating that hexagonal phase is more incompressible than aragonite phase of BaCO₃,meanwhile,the carbonate vibration modes are less compressible than lattice modes.Second run was conducted with an aim of deeply understanding the phase transition behavior of BaCO₃ under high pressure and providing more detailed crystal structure analysis by synchrotron X-ray diffraction,at approximately 10.5 GPa a first-order transformation that orthorhombic structure to hexagonal structure was observed.At 10.5 GPa diffraction peaks representing the hexagonal phase,with space group P-31c.The volume pressure data are used to calculate Equation of State（EoS）parameters V₀=309.18?³,K₀=58.5 GPa,K₀′=1.4 for Orthorhombic phase,V₀=131.75?³,K₀=39.1 GPa,K₀′=4（fixed）for Hexagonal phase.In the last experimental run the high temperature phase transition behavior of BaCO₃ is analyzed and discussed by using DSC.According to the DSC analysis results,there were two endothermic peaks of BaCO₃ at 808°C and 955°C,which corresponded to the transitions of orthorhombic phase to hexagonal phase and hexagonal phase to cubic phase,respectively.By studying the phase transition behavior of BaCO₃ under high pressure and high temperature and compared to the previously found,not only further proves the compressibility and structural stability of aragonite structured carbonates under extreme condition,but also contributes to the understanding of the carbon reservoirs in the Earth mantle and the Earth’s global carbon cycle.