Reevaluating CaCO3 Behavior at High Pressure: Formation of Ca2CO4 under Reduced Conditions

Due to its importance for the global carbon cycle, the behavior of CaCO3 under high pressures and temperatures has been extensively studied. Previous research has suggested the possibility of a transition to a disordered phase and/or amorphization of CaCO3. In our experiments, conducted using a multi-anvil apparatus with a boron nitride (BN) capsule, we also observed the disappearance of almost all diffraction peaks at pressures of ~27 GPa within a specific temperature range. However, at 1773 K, numerous diffraction peaks reappeared. The observed diffraction pattern could be perfectly indexed by more than 20 diffraction peaks, matching the structure of Ca2CO4-Pnma corresponding the recently discovered compound, calcium orthocarbonate. Upon decreasing the pressure, the Ca2CO4-Pnma phase persisted through three heating cycles in the pressure range of 20–27 GPa. At a pressure of ~20 GPa and a temperature of 1573 K, it transformed into a new phase or a mixture of phases, different from the expected CaCO3 + CaO. Our results challenge the previously proposed amorphization and transition to disordered state of CaCO3, suggesting instead that the observed phase transformation can be the result of interaction with the BN capsule.