In situ study of calcite-III dimorphism using dynamic diamond anvil cell

The phase transitions among the high-pressure polymorphic forms of CaCO₃ (cc-I, cc-II, cc-III, and cc-IIIb) are investigated by dynamic diamond anvil cell (dDAC) and in situ Raman spectroscopy. Experiments are carried out at room temperature and high pressures up to 12.8 GPa with the pressurizing rate varying from 0.006 GPa/s to 0.056 GPa/s. In situ observation shows that with the increase of pressure, calcite transforms from cc-I to cc-II at ～ 1.5 GPa and from cc-II to cc-III at ～ 2.5 GPa, and transitions are independent of the pressurizing rate. Further, as the pressure continues to increase, the cc-IIIb begins to appear and coexists with cc-III within a pressure range that is inversely proportional to the pressurizing rate. At the pressurizing rates of 0.006, 0.012, 0.021, and 0.056 GPa/s, the coexistence pressure ranges of cc-III and cc-IIIb are 2.8 GPa-9.8 GPa, 3.1 GPa-6.9 GPa, 2.7 GPa-6.0 GPa, and 2.8 GPa-4.5 GPa, respectively. The dependence of the coexistence on the pressurizing rate may result from the influence of pressurizing rate on the activation process of transition by reducing the energy barrier. The higher the pressurizing rate, the lower the energy barrier is, and the easier it is to pull the system out of the coexistence state. The results of this in situ study provide new insights into the understanding of the phase transition of calcite.