Pressure-induced phase transition in MnCO3 and its implications on the deep carbon cycle

The high-pressure behavior of manganese-rich carbonate, rhodochrosite, has been characterized up to 62GPa by synchrotron-based midinfrared spectroscopy and X-ray diffraction. Modifications in both the infrared spectra and the X-ray diffraction patterns were observed above similar to 35GPa, indicating the presence of a high-pressure phase transition at these pressures. We found that rhodochrosite adopts a structure close to CaCO3-VI with a triclinic unit cell (a=2.87 angstrom, b=4.83 angstrom, c=5.49 angstrom, =99.86 degrees, =94.95 degrees, and =90.95 degrees at 62GPa). Using first-principles calculations based on density functional theory, we confirmed these observations and assigned modes in the new infrared signature of the high-pressure phase. These results suggest that high-pressure metastable phase of calcite may play an important role in carbon storage and transport in the deep Earth.