Evolution of the crystal structure of hydrous wadsleyite at high pressure up to 33 GPa

Abstract:

Wadsleyite (beta-Mg₂SiO₄) is thought to be the dominant silicate mineral in the upper part of the transition zone from about 410 to 520 km depth. While known to incorporate significant amounts of hydroxyl defects, the influence of hydration on the crystal structure of wadsleyite at high pressure has not been investigated. Using synchrotron X-ray diffraction at sector 13 (GSECARS) of the Advanced Photon Source, we carried out single-crystal structure refinements on two Fe-bearing (Fo90) wadsleyite samples; one containing about 0.25 wt% H₂O and one containing about 2.0 wt% H₂O. Nominally orthorhombic, hydrous wadsleyite is known to display a slight monoclinic distortion (beta angle >90 degrees) resulting from Mg/Fe and vacancy ordering associated with hydroxyl (Smyth et al. 1997). Here we observe a slight monoclinic beta angle by about one standard deviation in both samples at room pressure. However, the less hydrous sample exhibits a transition at about 9-10 GPa after which the beta angle increases linearly to about 90.57(4) degrees at 35 GPa. The more hydrous sample also shows increasing beta with pressure above 10 GPa but is much less pronounced. A possible mechanism to explain the change in symmetry with pressure is being developed. In the more hydrous sample, examination of the potentially hydrogen bonded O...O interatomic distances including O1-O4, O4-O4, and O1-O2 show linearly decreasing distances up to about 25 GPa, whereupon further compression to 35 GPa, the distances do not change further. The high-pressure structure data indicate a potential symmetrization of the hydrogen bonds in wadsleyite above 25 GPa.