Abstract
The importance for the global carbon cycle, the P – T phase diagram of CaCO 3 has been under extensive investigation since the invention of the high-pressure techniques. However, this study is far from being completed. In the present work, we show the existence of two new high-pressure polymorphs of CaCO 3 . The crystal structure prediction performed here reveals a new polymorph corresponding to distorted aragonite structure and named aragonite-II. In situ diamond anvil cell experiments confirm the presence of aragonite-II at 35 GPa and allow identification of another high-pressure polymorph at 50 GPa, named CaCO 3 -VII. CaCO 3 -VII is a structural analogue of CaCO 3 - P 2 1 / c -l, predicted theoretically earlier. The P – T phase diagram obtained based on a quasi-harmonic approximation shows the stability field of CaCO 3 -VII and aragonite-II at 30–50 GPa and 0–1200 K. Synthesized earlier in experiments on cold compression of calcite, CaCO 3 -VI was found to be metastable in the whole pressure–temperature range.