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内容記述 |
The behavior of iron-bearing magnesium carbonate (Mg,Fe)CO3 at high pressures has significant implications for deep carbon cycling in Earth’s mantle, as this composition more realistically reflects the carbonates expected in the Earth’s deep interior. In this study, we have investigated high-pressure vibrational properties of breunnerite, a Mg-rich (Mg,Fe)CO3 phase, using synchrotron Mössbauer, infrared (IR), and Raman spectroscopies in diamond anvil cells up to ~70 GPa at room temperature. Above 29 GPa, Raman and IR spectra reveal signatures of pressure-induced lattice distortion, including splitting of lattice-coupled (T) and in-plane bending (ν4) modes of internal CO32- vibrations, changes in the pressure-dependence of both Raman and IR modes, and a variation in the L/ν4 mode intensity ratio across 29 GPa. While lattice distortion has been proposed in pure MgCO3, even minor Fe substitution alters the response of internal modes. This result suggests that low Fe content can modify the local bonding environment, which may influence the high-pressure and -temperature stability of the MgCO3-FeCO3 solid solution. Mössbauer spectra show that a high-spin to low-spin transition occurs between ~47-49 GPa in breunnerite. Raman and IR measurements display discontinuous shifts in both lattice (L) and internal vibrational (ν1 and ν4) modes between 45-50 GPa and non-linear Fe-content dependence of the vibrational properties above 50 GPa, which may reflect a combined influence of the ionic radius reduction in Fe2+ accompanying the spin transition and further pressure-induced lattice distortion. These Fe-dependent variations suggest that sound velocity behavior is likewise non-linear in the MgCO3-FeCO3 solid solution, implying a more complex elastic response of lower-mantle carbonates. |
