Abstract

Hydrous wadsleyite (β-Mg2SiO4) with 2.8 wt% water content has been synthesized at 15 GPa and 1250 °C in a multi-anvil press. The unit-cell parameters are: a = 5.6686(8), b = 11.569(1), c = 8.2449(9) Å, β = 90.14(1)°, and V = 540.7(1) Å3, 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) [V0 = 542.7(8) Å3, KT0 = 137(5) GPa, K′ = 4.6(3)] still fits the data well. In comparison, the second-order fit gives V0 = 542.7(8) Å3, KT = 147(2) GPa. The relation between isothermal bulk modulus of hydrous wadsleyite KT0 and water content CH2O is: KT0 = 171(1)–12(1) CH2O (up to 2.8 wt% water). The axial-compressibility βc is larger than both βa and βb, consistent with previous studies and analogous to the largest coefficient of thermal expansion along the c-axis.

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