Hydrous wadsleyite crystal structure and symmetric hydrogen bonding in the mantle
The transition zone acts as a geochemical filter to mantle material passing through the 410 and 660 km seismic discontinuities. The high water storage capacity of wadsleyite, Mg2SiO4, an abundant transition zone mineral, therefore plays an important role in the global water cycle. Hydration of wadsleyite by (OH)- associated with cation defects is well known to influence its physical and mechanical properties, however an atomic-scale understanding of the structure and hydrogen bonding at high pressures is needed to interpret the influence of water on the behavior of wadsleyite in the mantle transition zone. We have determined the pressure evolution of the wadsleyite crystal structure, lattice symmetry, and hydrogen-bond distances up to 35 GPa using single-crystal X-ray diffraction on well-characterized, Fe-bearing samples containing ~0.25 and ~2.0 wt.% H2O. Both compositions undergo a linear increase in the monoclinic distortion from orthorhombic symmetry above 10 GPa, with the less hydrous sample showing a greater increase in distortion on further compression. Although the proton positions cannot be modeled from the experimental data, the evolution of the primary hydrogen bond in the structure was monitored by the behavior of the O1–H…O4 hydrogen bond lengths along the M3 octahedron during compression. Above about 25 GPa, the hydrogen bond undergoes a transition to being completely incompressible, which is not observed in other O-O interatomic distances. We interpret this change in compression behavior as resulting from symmetrization of the hydrogen bond in wadsleyite. The observed strengthening of the hydrogen bond in wadsleyite has implications for the partitioning of hydrogen isotopes in the mantle transition zone.