HR: 0800h
AN: MR41A-0905    [Abstracts]
TI: Structure and Hydroxyl Bonding Environment in Hydrous Wadsleyite
AU: * Holl, C M
EM: hollc@colorado.edu
AF: University of Colorado, Campus Box 399, Boulder, CO 80503 United States
AU: Smyth, J R
EM: joseph.smyth@colorado.edu
AF: University of Colorado, Campus Box 399, Boulder, CO 80503 United States
AU: Frost, D J
EM: Dan.Frost@Uni-Bayreuth.DE
AF: Bayerisches Geoinstitut, Universitaet Bayreuth Universitaetsstrasse 30, Bayreuth, D-95440 Germany
AB: Three samples of hydrous Fo₁₀₀ wadsleyite (β-Mg₂SiO₄) with various water contents up to 1.6 wt% were examined by single-crystal X-ray diffraction, paying particular attention to suspected protonation sites. The unit cell expands systematically with hydration, increasing approximately 0.2% per 1 wt% H₂O. This volume change is equivalent to a pressure change on anhydrous wadsleyite of approximately 3 kbar. Calculated density change from measured cell volumes and site occupancies is -2.1% per 1 wt% H₂O. Of the three cation octahedra in the structure, the M3 site showed the greatest volume expansion, change in angle variance, and decrease in site occupancy with increasing hydration. Five O-O distances in the structure decreased with increasing hydration: the shared O4-O4 edge between the M1 and M2 octahedra, the unique O4-O4 edge on M2, and three M3 octahedral edges. Of these, the O1-O4 edge of M3 showed the greatest decrease in length with hydration. Shortening may be due to the presence of a hydrogen bond along the direction of the O-O vector, relaxing the repulsion between anions. These results suggest that the preferred hydration mechanism is protonation of the non-silicate oxygen site O1, with protonation of the O4 site less important. No evidence was observed for protonation of the bridging oxygen site O2.
DE: 3954 X-ray, neutron, and electron spectroscopy and diffraction
SC: Mineral and Rock Physics [MR]
MN: Fall Meeting 2005