Dense Hydrated Magnesium Carbonate MgCO₃·3H₂O Phases

The study of the structural stability of carbonates under different pressure and temperature conditions is important for modeling the carbon budget in the Earth's interior and the stability of carbonation products of carbon capture and storage (CCS) solutions. In this work, we confirm the existence of the two dense polymorphs of the hydrated magnesium carbonate MgCO₃·3H₂O nesquehonite mineral previously reported, and we characterize their structures using synchrotron single-crystal X-ray diffraction at 3.1 and 11.6 GPa. Phase transitions entail the distortion and atomic rearrangement of the Mg-centered polyhedra and the tilting of the [CO₃] carbonate units. In particular, the Mg coordination number increases from 6 in nesquehonite to 7 in the second high-pressure phase, while maintaining a topology based on complex MgCO₃(H₂O)₂ chains. We also studied their vibrational behavior upon compression using Raman spectroscopy and complemented the experimental results with density-functional theory (DFT) calculations. The role played by hydrogen bonds in the compressibility and the polymorphism of this hydrated carbonate is also discussed.