Raman signatures of the distortion and stability of MgCO3 to 75 GPa

Abstract

Knowledge of the stability of carbonate minerals at high pressure is essential to better understand 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 reported Raman modes of natural magnesite MgCO 3 up to 75 GPa at room temperature using helium as a pressure-transmitting medium (PTM). Our Raman spectra of MgCO 3 showed the splitting of T and ν 4 modes initiates at approximate 30 and 50 GPa, respectively, which could be associated with its lattice distortions The MgCO 3 structure was referred to as MgCO 3-Ⅰb at 30-50 GPa and as MgCO 3-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 those Raman bands in MgCO 3 under relatively hydrostatic conditions is consistent with the onset pressure of structural transition to MgCO 3-Ⅱ reported by theoretical predictions and high pressure-temperature experiments. This study suggests that hydrostatic conditions may significantly affect the structural evolution of MgCO 3 with increasing pressure, which shall be considered for modeling the carbon cycle in the Earth's lower mantle.

Phase Stability and Vibrational Properties of Iron-Bearing Carbonates at High Pressure

“…The use of He can maintain the hydrostatic conditions at 50 GPa [32], and thus avoid the influence of deviatoric stress. For more detailed experimental information, one can refer to our previous study [33]. Additionally, the Raman spectra of iron-bearing dolomite were collected by using an eXcelon digital CCD spectroscopy system (PIXIS 400, Princeton Instruments co., USA) coupled with an 1800 G/mm ruled grating with 532 nm blaze wavelength.…”

Effects of hydrostaticity and Mn-substitution on dolomite stability at high pressure

“…The spectral resolution was ~2 cm -1 with a holographic diffraction grating of 1800 lines/mm. Please refer to our previous study for experimental information in detail (Zhao et al, 2021).…”

“…The presence of carbonate minerals can dramatically affect the physical and chemical properties of the mantle, such as phase stability, melting, viscosity, electrical conductivity, thermal conductivity, and elasticity (Fu et al, 2017;Gaillard et al, 2008;Yao et al, 2018;Zhao et al, 2019;Gui et al 2021). Thus, studying the structural evolution of carbonate minerals at high pressure is crucial to constrain the deep carbon cycle as well as mantle dynamics (Farsang et al, 2021a;Fu et al, 2017;Isshiki et al, 2004;Liu et al, 2015;Mao et al, 2020;Sun et al, 2020;Zhao et al, 2021). Among all the carbonate minerals subducting into the Earth's deep interior, the dolomite group (e.g., dolomite CaMg(CO 3 ) 2 ) has been extensively investigated as one of the most prominent deep carbon carriers.…”

“…In particular, those high-pressure phases of (Mg,Fe)-dolomite are considered to be important carbon carriers in the deep mantle (Mao et al, 2011;Merlini et al, 2012). Recently, it has been found that improved hydrostaticity can greatly influence the structural evolution of MgCO 3 at high pressure up to 80 GPa, where the pressure-transmitting medium (PTM) was helium (Zhao et al, 2021). However, the effects of hydrostaticity on those phase transitions in dolomite have not been investigated yet because of the lack of high-pressure studies on dolomite minerals using a helium PTM (Efthimiopoulos et al, 2018).…”

First-principles calculations of high-pressure physical properties anisotropy for magnesite

“…However, the structure of its high-pressure phase and its phase transition boundary are controversial. The experiment shows that the phase transition pressure range from magnesite(space group R 3 c) to magnesite-II(space group C2/m) is 75-115 GPa [2][3][4][5][6] , while the theoretical result is 75-101 GPa [7][8][9][10][11][12][13] .…”