Hydrous minerals play a major role in the transportation of water into Earth’s mantle and contribute to regulating the sustained presence of water on the surface of Earth. Therefore, to understand the deep-water cycle, it is important to study the properties of hydrous minerals under the conditions present in Earth’s mantle. Lawsonite CaAl₂Si₂O₇(OH)₂·H₂O is an important water carrier in subducting oceanic crusts. Recent experimental work^[1] showed orthorhombic to monoclinic phase transition of lawsonite at 9.3 GPa and suggested it to be potential water carrying mineral in the deep subducted slab. However, the stability limit of lawsonite in monoclinic structure has not been explored, which is necessary to prove the existence of lawsonite at subduction zones. In this work, we performed high pressure structural and lattice dynamical study of lawsonite in monoclinic structure to determine its stability limit at Earth’s mantle condition. Our study shows that the lawsonite is dynamically stable between 20-30 GPa. To further understand the behavior of the cell parameter at several pressures, the pressure dependence of compressibility and axial length ratio were studied. The decrease in the axial ratio a/a₀ is very high as compared to that of b/b₀ and c/c₀, indicating that the a-axis is most compressible. We show that the high-pressure monoclinic phase of lawsonite has unique spectroscopic fingerprints which will allow straightforward detection in experiments. Raman spectroscopic study indicates that H-bonding environment of the H₂O molecule does not change significantly under compression. Lawsonite is thus stable under the pressure and temperature conditions that exist in the Earth and is likely to be a major water carrier in colder, deep subducted slabs.

References.

[1] O’Bannon, E. III, C. M. Beavers, M. Kunz, and Q. Williams, The high-pressure phase of lawsonite: A single crystal study of a key mantle hydrous phase, J. Geophys. Res. Solid, 122, 6294– 6305, (2017).