Crystal structure of hydrous wadsleyite with 2.8% H (sub 2) O and compressibility to 60 GPa

Hydrous wadsleyite (beta -Mg (sub 2) SiO (sub 4) ) 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) Aa, beta =90.14(1) degrees , and V=540.7(1) Aa (super 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 (sub 0) =542.7(8) Aa (super 3) , K (sub T0) =137(5) GPa, K'=4.6(3)] still fits the data well. In comparison, the second-order fit gives V (sub 0) =542.7(8) Aa (super 3) , K (sub T) =147(2) GPa. The relation between isothermal bulk modulus of hydrous wadsleyite K (sub T0) and water content C (sub H (sub 2) O) is: K (sub T0) =171(1)-12(1) C (sub H (sub 2) O) (up to 2.8 wt% water). The axial-compressibility beta (sub c) is larger than both beta (sub a) and beta (sub b) , consistent with previous studies and analogous to the largest coefficient of thermal expansion along the c-axis.