A high-pressure phase transition of calcite-III

We document the presence of a high pressure phase transition in metastable calcite-III using infrared spectroscopy. The post-calcite-III transition initiates at a pressure of 15.5 (+/- 2) GPa, and is completed between 25 and 30 GPa. The transition is particularly apparent in the nu(4)-in-plane bending vibration of the carbonate group, in which two new peaks progressively supplant the doublet associated with calcite-III. Additionally, both the nu(3)-asymmetric and nu(3)-symmetric stretches of the carbonate group in the high-pressure phase occur at substantially lower frequencies than the extrapolated positions of the corresponding calcite-III peaks. The geometry of the carbonate unit within the high-pressure phase is likely closer to trigonal symmetry than in the calcite-III structure, and the C-O bond is probably longer than in the lower pressure calcite-III phase.