CaCO3 phase diagram studied with Raman spectroscopy at pressures up to 50 GPa and high temperatures and DFT modeling

We revise the high pressure, high temperature phase diagram of CaCO3 using Raman spectroscopy in conjunction with laser heated diamond anvil cell experiments. We confirm numerous aspects of earlier studies, including a recent X-ray diffraction study about the stability field of CaCO3-VII. Our Raman results show that CaCO3-VII is stable in the lower mantle at a depth of 690-1010 km. Our DFT calculations show that the phase transition from aragonite to CaCO3-VII at ≈25 GPa and from CaCO3-VII to post-aragonite at ≈40 GPa are accompanied by density changes of 2% and 3.5%, respectively. Shear sound velocities change by 9% and −12% across the transitions, respectively. Hence, a sufficient amount, at least locally, of CaCO3 in the Earth's mantle can be detectable by an increase of the shear velocity at 690 km and a decrease of the shear velocity at 1010 km depths.