Light Scattering Study On The Stability Of Carbonate Minerals At High Pressure Conditions

The study on the stability of carbonate minerals under high temperature and high pressure plays an important role in a deeper understanding of the change or evolution of the deep part of the earth.Carbonate minerals are the most abundant minerals on the surface of the earth,and the main source of natural energy.The deep part of the earth is an unstable state of high temperature and high pressure.Carbonate minerals directly participate in the circulation of CO₂ in the earth's interior and surface as carriers,so CO₂ solidifies to look for stable carbonate minerals.Therefore,it is important to understand and study the structural changes and stability of carbonate minerals and the way of carbon cycle in the deep earth.Because of the complex high temperature and high pressure inside the earth,so we use the diamond anvil(DAC)technology to achieve higher pressure in the experiment to study the stability of minerals under high pressure.Due to the variety and complexity of minerals in nature,we used pure PbCO₃samples and Na_1.757Ca_0.793K_0.285Zr_0.036(CO₃)_1.89 samples prepared by solid-state method to carry out high-pressure experiments by X-ray diffractometer,Raman spectroscopy and electron projection microscopy.In the experiment,ruby is used as pressure calibration material,and discussed ruby a under high pressure by the Raman spectra.Na_1.757Ca_0.793K_0.285Zr_0.036(CO₃)_1.89 sample belongs to the hexagonal structure and space group is P63mc,a=10.060(?),b=10.060(?),c=12.720(?),V=1114.84(?)³,?=90°,?=90°,?=120^°,Z=8.Using diamond anvil cell(DAC)technology we have studied the stability of Pb CO₃ at high pressure.Solid NaCl,mixture of methanol,ethanol,water(16:3:1)(M EW)and methanol,ethanol(4:1)(M E)were used as pressure transmitting medium,and the pressure in this study reached up to 25 GPa,25GPa and 67 GPa respectively.It is found that Pb CO₃ undergoes three phase transitions at around 10 GPa,15 GPa and30 GPa,respectively.In addition,softening of the out of bending vibration mode of CO₃^2-group were observed.The Mode Grüneisen parameters(?)were calculated for the different pressure transmitting media,and found that the Pb CO₃ undergoes phase transitions at slightly different pressures,in respect pressure transmitting medium.It also found that the pressure has a greater influence on the lattice vibration than the CO₃^2-group,which is due to the larger distance of the Pb²⁺-O bond.Secondly,we used the solid-phase method to prepare samples of the Na₂CO₃-CaCO₃system.The sample tested by XRD has a hexagonal crystal structure and belongs to the P63mc space group.Subsequently,Na₂CO₃-CaCO₃system was examined in a diamond anvil cell up to a pressure of 30 GPa.It was found that the sample started from 11.77 GPa with a new peak at 1145.86 cm^-1 at the?₁-symmetric stretching vibration in this experiment.The?₄ in-plane bend widened and the amplitude decreased.It is a precursor to a phase change.The sample undergoes a phase change when the pressure about 15 GPa.When the pressure up to 20 GPa,there is second phase transition of the sample and?₄ in-plane bend widens sharply and disappears with the increase of pressure.When the pressure is above 26 GPa,only the?₁-symmetric stretching mode can be distinguished,and sample is high disorder in this pressure.Under high pressure,if amorphous occurs,some peaks will not appear because of their wide half width and high disorder.In the process of decompression,it is found that some peaks of the symmetric vibration mode are superimposed,and all peaks of the sample appear after the pressure is released.The sample does not appear amorphization when the pressure up to 30 GPa.