HR: 0800h
AN: T41B-1191    [Abstracts]
TI: High pressure neutron powder diffraction study of OD-chondrodite
AU: * Feese, I G
EM: Solesorientes@yahoo.com
AF: Department of Physics, University of Louisville, Louisville, KY 40292 United States
AU: Lager, G A
EM: galager@louisville.edu
AF: Department of Geography and Geosciences, University of Louisville, Louisville, KY 40292 United States
AU: Marshall, W G
AF: ISIS Facility, Rutherford Laboratory, Chilton, Didcot, OX11 0QX United Kingdom
AU: Ulmer, P
AF: Institute for Mineralogy and Petrology, ETH, Zurich, CH-8092 Switzerland
AB: Chondrodite [Mg$_{5}$Si$_{2}$O$_{8}$(F,OH)$_{2}$], a member of the humite group, has been the subject of a number of recent studies because of its high P-T stability, association with mantle-derived rocks and structural similarity to olivine. In particular, OD-chondrodite is a potential repository for water in the upper mantle and serves as a model for the OH defect structure in olivine. High-pressure neutron powder diffraction studies of the intermediate F-bearing solid solution have shown that hydrogen bonding in this phase strengthens slightly with pressure in response to a rotation of the OH vector. In this study, neutron powder data have been collected to 5.87 GPa for the OD-chondrodite in order to investigate the affect of the OD $<$=$>$ F substitution on the hydrogen-bond geometry and the mechanism of compression. In contrast to F-bearing chondrodite, there are two D atoms disordered over four possible positions in the OD end-member. The two O-D bond lengths remain essentially constant with increasing pressure. Both H-bond lengths associated with the D1 site show significant increases with pressure, in contrast to F-bearing chondrodite. At the D2 site, one H-bond length increases whereas the second bond length decreases. The largest angular change is associated with O-D1$^{...}$O5, which increases from $\sim$155 to 165$\deg$. The increase in H-bond lengths appears to be consistent with the positive pressure shifts ($\sim$30 cm$^{-1}$) observed for the OH bands in infrared spectra of OD-chondrodite. Strain calculations show that the minimum axis of compression is perpendicular to the closest packed layers in both OD and F-bearing chondrodite. The maximum direction of compression, which is approximately parallel to the b crystallographic axis in F-bearing chondrodite, is rotated $\sim$24$\deg$ clockwise in the bc plane in OD-chondrodite.
DE: 3620 Crystal chemistry
DE: 3900 MINERAL PHYSICS
DE: 3954 X ray, neutron, and electron spectroscopy and diffraction
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting