UC Santa Cruz

Lawsonite CaAl2Si2O7(OH)2·H2O is an important water carrier in subducting oceanic crusts and the primary hydrous phase in basalt at depths greater than ~80 km. We have conducted high-pressure synchrotron single-crystal X-ray diffraction experiments on natural lawsonite at room temperature up to ~10.0 GPa to study its high-pressure polymorphism. We find that lawsonite remains orthorhombic with Cmcm symmetry up to ~9.3 GPa and shows nearly isotropic compression. Above ~9.3 GPa, lawsonite becomes monoclinic with P21/m symmetry. Across the phase transition, the Ca polyhedron becomes markedly distorted, and the average positions of the H2O molecules and hydroxyls change. The changes observed in the H-atom positions under compression are different than the low-temperature changes in this material. We resolve for the first time the H-bonding configuration of the high-pressure monoclinic phase of lawsonite. A bond valence approach is deployed to determine that the phase transition from orthorhombic to monoclinic is primarily driven by the Si2O7 groups, and in particular their bridging oxygen atoms (O1). The changes in the structure strongly indicate that entropy increases across the symmetry-lowering transition and hence that the slope of the phase transition is negative. Monoclinic 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. Monoclinic lawsonite also likely has enhanced electrical conductivity along its c axis due to its dynamically disordered hydrogen atoms.