Crystal structure of hydrous wadsleyite with 2.8% H2O and compressibility to 60 GPa

Hydrous wadsleyite (beta-Mg2SiO4) with 2.8 wt% water content has been synthesized at 15 GPa and 1250 degrees C in a multi-anvil press. The unit-cell parameters are: a = 5.6686(8), b = 11.569(1), c = 8.2449(9) angstrom, beta = 90.14(1)degrees, and V = 540.7(1) angstrom(3), and the space group is I2/m. The structure was refined in space groups Imma and I2/m. The room-pressure structure differs from that of anhydrous wadsleyite principally in the increased cation distances around O1, the non-silicate oxygen. The compression of a single crystal of this wadsleyite was measured up to 61.3(7) GPa at room temperature in a diamond anvil cell with neon as pressure medium by X-ray diffraction at Sector 13 at the Advanced Photon Source, Argonne National Laboratory. The experimental pressure range was far beyond the wadsleyite-ringwoodite phase-transition pressure at 525 km depth (17.5 GPa), while a third-order Birch-Murnaghan equation of state (EoS) [V-0 = 542.7(8) angstrom(3), K-T0 = 137(5) GPa, K' = 4.6(3)] still fits the data well. In comparison, the second-order fit gives V-0 = 542.7(8) angstrom(3), K-T = 147(2) GPa. The relation between isothermal bulk modulus of hydrous wadsleyite K-T0 and water content C-H2O is: KT0 = 171(1)-12(1) CH.,0 (up to 2.8 wt% water). The axial-compressibility beta(c) is larger than both beta(a) and beta(b), consistent with previous studies and analogous to the largest coefficient of thermal expansion along the c-axis.