Polymorphism and Phase Stability of Hydrated Magnesium Carbonate Nesquehonite MgCO3·3H2O: Negative Axial Compressibility and Thermal Expansion in a Cementitious Material

Compressibility and Anisotropy of Trona: Unveiling the Structure of a Dense Na3H(CO3)2·2H2O Polymorph

Understanding the structural stability of hydrated carbonates and bicarbonates under different thermodynamic conditions is crucial, as they play an important role in the carbon cycle, in environmental chemistry, and geochemistry. Sodium sesquicarbonate dihydrate, Na3H(CO3)2·2H2O trona, is a naturally occurring evaporite mineral also found in magmatic environments. In this work, we carried out high-pressure (HP) in situ synchrotron powder and single-crystal X-ray diffraction (XRD) measurements on a naturally occurring trona specimen. A high-pressure monoclinic to triclinic phase transition occurred at 12.8 GPa, and the structure of the dense HP trona phase was solved. The coordination number of sodium atoms increased from 6 in the low-pressure polymorph to 7-8 in the HP one. Additionally, our results did not show any indication of the phase transition previously reported at 7 GPa based on Raman spectroscopy. The compressibility and anisotropy of low- and high-pressure phases were determined. Birch-Murnaghan equation of state parameters were fitted to the pressure-volume data sets, yielding a bulk modulus of 35(2) GPa (K' = 4.4(9)) for trona using neon as pressure-transmitting medium. Density functional theory calculations supported the experimental results, confirming the structural stability of the phases obtained by XRD.

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.