Pressure-induced structural evolution and mechanical response of Mg5(CO3)4(OH)2·4H2O hydromagnesite, a sustainable cement-related mineral

The structural stability and compression behavior of hydrated magnesium carbonates are critical for understanding their role in permanent CO 2 sequestration and for optimizing low-carbon cementitious materials. This work investigates the compressibility, anisotropy, and structural evolution of Mg 5 (CO 3 ) 4 (OH) 2 ·4H 2 O hydromagnesite mineral under high-pressure conditions using a combination of single-crystal and powder X-ray diffraction, high-pressure Raman spectroscopy, and density functional theory calculations. The results reveal a previously unreported pressure-induced order–disorder–order phase-transition sequence occurring at 0.6(1) and 7.7(1) GPa, driven by positional disorder of a carbonate group along the direction of maximum compressibility. No changes in structural topology or space-group symmetry are observed throughout compression. Quantitative Euler-angle analysis demonstrates that the structural evolution is governed by progressive carbonate tilting and rigid-body rotations of [MgO 6 ] octahedra. A third-order Birch–Murnaghan equation of state yields V 0 = 687.71(14) Å 3 , K 0 = 40.7(6) GPa, and K′ 0 = 5.18(14). Raman spectroscopy confirms the vibrational stability of hydromagnesite over the investigated pressure range and reveals an initial stiffening of O H stretching vibrations, followed by progressive strengthening of hydrogen bonding above ~7.2 GPa. These findings provide a comprehensive crystallographic, elastic, and spectroscopic description of hydromagnesite under compression and offer new insights into the behavior of hydrated magnesium carbonates relevant to carbon storage, geochemical processes, and MgO-based cement technologies.