Bonding and structural changes in siderite at high pressure

We studied the high pressure behavior of siderite (FeCO3), an iron-rich carbonate that may be an important reservoir for carbon storage within the deep Earth, experimentally in a diamond anvil cell as well as with theoretical calculations. From our Raman spectroscopy measurements, we found that the symmetric stretching mode (ν1) of the carbonate ion splits at 46 GPa with the new peak appearing at lower frequency. X-ray powder diffraction results and DFT calculations indicate that siderite undergoes an isostructural volume collapse associated with an electronic spin transition in the Fe2+ at approximately the same pressure.