AbstractNew experimental data are reported on high-pressure polymorphism of CaCO₃. The CaCO₃-III phase was stabilized using a large-volume press device and high-resolution X-ray powder diffraction (XRPD) patterns were collected from a few mm³ of powder sample. The interpretation of XRPD indicates that CaCO₃-III and CaCO₃-IIIb structures are present simultaneously and are in similar proportions. The lack of any unindexed peaks demonstrates that these two polymorphs are the only phases in this experiment, indicating that CaCO₃-III and CaCO₃-IIIb are the structures most likely to occur above 2.5 GPa. Relevant co-axial crystallographic matrix transformations from lower-pressure polymorphs to both CaCO₃-III and CaCO₃-IIIb are discussed to illustrate a further possible occurrence of co-existing and interspersed stable polymorphs in carbonate systems.