The high‐pressure phase of lawsonite: A single crystal study of a key mantle hydrous phase

Abstract

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.

Electrical Investigation of Natural Lawsonite and Application to Subduction Contexts

“…Electrical transitions at 4 and 9.7 GPa are consistent with the orthorhombic P to C and orthorhombic to monoclinic transitions, respectively. O'B et al: O'Bannon et al ( and references therein); B&A: Ballaran and Angel (); L et al: Liebscher et al ().…”

“…Thus, these constant ram pressure large volume press experiments lie far closer to constant pressure conditions than the constant volume end‐member (Leinenweber et al, ). This estimated uncertainty on pressure is possibly smaller than the one associated with phase transformation in lawsonite (transformation causing a change in the sample volume that was accounted for as part of conductivity calculations using the structural study by O'Bannon et al, , as explained below).…”

“…Because we cannot estimate these effects well without having an in situ measurement capability, we consider the length after the experiment to be the preferred sample length for conductivity calculations. In the case of the experiment conducted at fixed temperature up to 10 GPa (BB185), the volume change associated with the transitions at 4 and 9.5 GPa and provided in O'Bannon et al () was accounted for as part of the geometric factor calculations: the sample length was recalculated at each transition, occurring at 4 and 9.7 GPa.…”

Chemical Crystallography at the Advanced Light Source

“…Form I of tris(μ2-3,5-diisopropyl-1,2,4-triazolato-κ 2 N 1 :N 2 )trigold(I) exhibited four successive phase transitions to 3 GPa, and each phase was characterised on beamline 11.3.1 [51]. Lawsonite, CaAl2Si2O7(OH)2·H2O, a subduction-zone mineral of geochemical interest, was studied to 12 GPa on 12.2.2 and underwent a phase transition at 9 GPa that had only been observed previously by spectroscopy [52]. The high-pressure behaviour of the hybrid Pb-halide perovskites, CH3NH3PbI3 and CH3NH3PbBr3, was observed to include at least one phase change in each compound before 2 GPa [53].…”

X-Ray Diffraction under Extreme Conditions at the Advanced Light Source

“…On beamline 12.2.2, the current endstation 2 system, with a single ϕ rotation axis coupled to the Perkin Elmer detector, has enabled user groups to explore the high-pressure behavior [50] of a wide range of samples, including minerals [51], perovskites [52], metal organic frameworks (MOFs) [53], and more. The endstation 2 system can accommodate DACs of nearly any reasonable mass and shape, but a successful single crystal experiment depends on having the largest angular opening possible.…”