Abstract 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 Fe-bearing magnesite using synchrotron Mössbauer, infrared (IR), and Raman spectroscopies in diamond anvil cells up to ∼71 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 the wavenumbers of both Raman and IR modes, and a variation in the L/ν4 mode intensity ratio. 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 ∼45–50 GPa in Fe-bearing magnesite. Raman and IR measurements display discontinuous shifts in both lattice (L) and internal vibrational (ν1 and ν4) modes across the spin transition. Previous studies on the MgCO3 endmember have reported additional lattice distortion near 50 GPa, close to the pressure at which the spin transition occurs in Fe-rich compositions. The relative changes in the Grüneisen parameters (Δγ) of the lattice mode across the spin transition for Fe-containing compositions and the further distortion for MgCO3 endmember exhibit a linear dependence on Fe content, whereas those of the internal modes show a second-order, non-linear trend, particularly for the ν4 mode. Both lattice and internal modes may reflect the combined effect of lattice distortion and the spin transition, but the two cannot be distinguished from the behavior of the lattice mode. While lattice distortions have been discussed for the Mg-endmember, they have not been reported for Fe-bearing or Fe-rich compositions, possibly because the lattice distortions may be masked by the dominant effects of the spin transition, particularly in Fe-rich compositions. Since lattice modes are expected to primarily govern elastic wave propagation and the contribution of internal-mode variations is limited, the Fe-content dependence of sound velocities after the spin transition is expected to be approximately linear. This suggests that Fe-bearing carbonates may lower seismic wave velocities in the deep mantle where they are sufficiently abundant.
Mashino et al. (Wed,) studied this question.