| Nominally anhydrous silicate phases are capable of storing water in their structures as hydroxyl. This incorporated water greatly affects physical properties of the host minerals, such as seismic velocities, stability ranges, strain rates, and electrical conductivity. In addition, this reservoir of stored water in the Earth's mantle may have played a role in the evolution of the hydrosphere, as well as maintaining convection and tectonic processes. Wadsleyite, the stable polymorph of Mg2SiO4 from approximately 410 to 525 km depth in the Transition Zone, was studied by single crystal X-ray diffraction at ambient conditions and at pressure in the diamond anvil cell, and by polarized FTIR absorption. Iron-free wadsleyite samples with various water contents were synthesized with similar starting materials to isolate the effect of incorporated water on the physical properties. Analysis at ambient pressure found a systematic increase in unit cell volume, consistent with previous studies. Structure refinements at ambient pressure found evidence for ordering of the hydrogen position, based on shortening of the polyhedral edges that accommodate hydrogen. Positions of the O-H bonds were predicted from pleochroism of the absorption spectra of polarized IR radiation, and were in general agreement with predictions based on structure refinements. The O1 site is the most favorable hydroxyl site, with O3 also becoming favorable with increasing water content. Pressure-volume data were fit to a third-order Birch-Murnaghan equation of state for each sample. The resulting bulk moduli are 161(3), 158(2), and 155(3) GPa for wadsleyite with 0.4, 1.2, and 1.6 wt% H2O, respectively. The bulk moduli decrease with increasing water content, but do not follow a linear trend as they do in ringwoodite (gamma-Mg 2SiO4). The study of elastic properties of hydrous wadsleyite is complicated by a likely pressure-induced transition from orthorhombic to monoclinic symmetry, producing a rapid change in curvature of the d K/dP at low pressures. Bulk sound speed calculations from refined equations of state suggest that a hydrous transition zone, a strong dependence of dK/dT on water content, or both, are required to produce bulk sound speeds consistent with seismic observations.; A study on the compression of iron-bearing Phase A, is included to illustrate an alternate experimental technique and to demonstrate the effect of iron substitution on elastic properties of a magnesium silicate phase. |
