Evidence of interspersed co-existing CaCO (sub 3) -III and CaCO (sub 3) -IIIb structures in polycrystalline CaCO (sub 3) at high pressure

New experimental data are reported on high-pressure polymorphism of CaCO (sub 3) . The CaCO (sub 3) -III phase was stabilized using a large-volume press device and high-resolution X-ray powder diffraction (XRPD) patterns were collected from a few mm (super 3) of powder sample. The interpretation of XRPD indicates that CaCO (sub 3) -III and CaCO (sub 3) -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 CaCO (sub 3) -III and CaCO (sub 3) -IIIb are the structures most likely to occur above 2.5 GPa. Relevant co-axial crystallographic matrix transformations from lower-pressure polymorphs to both CaCO (sub 3) -III and CaCO (sub 3) -IIIb are discussed to illustrate a further possible occurrence of co-existing and interspersed stable polymorphs in carbonate systems.