Shock-induced phase transitions in siderite up to 90 GPa and implications for deep carbon cycle

The phase stability of carbonates under mantle conditions is important for understanding the global carbon cycle. In this study, the Hugoniot data of a natural siderite (FeCO₃) were measured up to 90 GPa using the plane-plate impact method. Two successive phase transitions were observed at 38–40 GPa and 65–69 GPa, respectively. In comparison with the static compression results, the first phase transition was identified as a spin transition, and the second is attributed to the self-redox reaction. The volume change during the self-redox transition is consistent with the reaction products of tetrairon orthocarbonate Fe₄C₃O₁₂ and diamond. Using the measured Hugoniot data, we estimated the density of Fe₄C₃O₁₂ along the lower mantle conditions and found it to be higher than the seismic values. Our results suggest siderite plays an important role in the deep carbon cycle.