Knowledge of the stability of carbonate minerals at high pressure is essential to better understand the carbon cycle deep inside the Earth. The evolution of Raman modes of carbonates with increasing pressure can straightforwardly illustrate lattice softening and stiffening. Here, we report Raman modes of natural magnesite MgCO3 up to 75 GPa at room temperature using helium as a pressure-transmitting medium (PTM). Our Raman spectra of MgCO3 show the splitting of T and ν4 modes initiated at approximate 30 and 50 GPa, respectively, which may be associated with its lattice distortions. The MgCO3 structure was referred to as MgCO3-Ib at 30–50 GPa and as MgCO3-Ic at 50–75 GPa. Intriguingly, at 75.4 GPa some new vibrational signatures appeared around 250–350 and ~800 cm–1. The emergence of these Raman bands in MgCO3 under relatively hydrostatic conditions is consistent with the onset pressure of structural transition to MgCO3-II revealed by theoretical predictions and high-pressure and high-temperature experiments. This study suggests that hydrostatic conditions may significantly affect the structural evolution of MgCO3 with increasing pressure, which shall be considered for modeling the carbon cycle in the Earth's lower mantle.

中文翻译：

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MgCO3到75 GPa的变形和稳定性的拉曼信号

了解碳酸盐矿物在高压下的稳定性对于更好地了解地球深处的碳循环至关重要。碳酸盐的拉曼模式随着压力的增加而演变，可以直接说明晶格的软化和硬化。在这里，我们报道了使用氦气作为压力传递介质（PTM）在室温下高达75 GPa的天然菱镁矿MgCO3的拉曼模式。我们的MgCO3拉曼光谱显示分别在大约30 GPa和50 GPa时开始的T和ν4模式分裂，这可能与其晶格畸变有关。MgCO3结构在30–50 GPa时称为MgCO3-Ib，在50–75 GPa时称为MgCO3-Ic。有趣的是，在75.4 GPa处，大约250–350和〜800 cm–1处出现了一些新的振动信号。这些拉曼谱带在相对静水的条件下在MgCO3中的出现与理论预测以及高压和高温实验揭示的向MgCO3-II的结构转变的起始压力是一致的。这项研究表明，静水压条件可能会随着压力的增加显着影响MgCO3的结构演变，应考虑将其用于模拟地球下地幔中的碳循环。