HR: 0800h
AN: T41B-1200    [Abstracts]
TI: High-temperature single-crystal neutron diffraction study of natural chondrodite
AU: * Kunz, M
EM: mkunz@lbl.gov
AF: Advanced Light Source,Lawrence Berkeley National Lab, 1, Cyclotron Rd MS 4R 0230, Berkeley, CA 94720 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: Burgi, H B
EM: hans-beat.buergi@krist.unibe.ch
AF: Laboratorium fuer chemische und mineralogische Kristallographie Universitaet Bern, Freiestrasse 3, Bern, BE CH-3012 Switzerland
AB: Chondrodite [Mg$_{5}$Si$_{2}$O$_{8}$(F,OH)$_{2}$] is a member of the humite group, which is a family of hydrous magnesium silicates with varying Mg$_{2}$SiO$_{4}$ : Mg(F,OH)$_{2}$ ratio. All humite minerals are structurally related to forsterite (Mg$_{2}$SiO$_{4}$), which is regarded as the most important mineral of the Earth's upper mantle. Based on their P-T stability, humites are candidate materials for transporting fluid phases and, in particular, OH into the deep Earth. Chondrodite is of particular interest because it exhibits an unusually high thermal stability. Unlike other dense, hydrous Mg-silicates (DHMS), OH-chondrodite remains stable at ambient pressure up to 873 K. In hydrous minerals, the thermal stability must be related, at least in part, to the behavior of the O-H$^{...}$O bond at high temperatures since dehydration is often the first breakdown reaction of such minerals. Raman studies of both OH- and the F-bearing chondrodite show a slight negative temperature dependence of the O-H vibrational frequency. This is unusual when compared with other DHMS phases, since it implies a decrease in the force-constant for the covalent O-H bond, which may be linked to a shortening of the OúúúH hydrogen bond. The temperature dependence of the H-atom environment in F-bearing chondrodite (Mg$_{4.64}$Fe$_{0.28}$Mn$_{0.014}$Ti$_{0.023}$(Si$_{1.01}$O$_{4}$)$_{2}$F$_{1.16}$OH$_{0.84}$) was investigated from 10 K to 900 K using single crystal neutron diffraction data collected in this study (500, 700 and 900 K), and previously published low-temperature data collected for the same crystal (10 K, 100 K and 300 K). The objectives of these experiments were: 1) to determine how the H-atom environment affects thermal stability; and 2) to investigate the correlation between Raman and neutron data. Atomic positions at each temperature were corrected for the partial substitution of O by F, and the apparent shortening of the O-H bond at high temperatures due to thermal motion of H and O/F. The refined temperature dependent O/F positions showed a physically reasonable evolution of the anisotropic displacement parameter with temperature. The Burgi-Capelli method (Burgi and Capelli 2000) was used to calculate the full mean-square displacement matrix and correct the O-H bond for the effect of thermal motion without any assumptions as to the correlation of O and H displacements. With this method, the observed temperature dependent displacement parameters for O and H, respectively, are used to refine the vibrational eigenvectors and their frequencies for the O-H unit. The refined stretching and bending frequencies ($\sim$2900 cm$^{-1}$ / $\sim$760 cm$^{-1}$) compare favorably with observed values ($\sim$3000 to 3500 cm$^{-1}$ / $\sim$500 to 800 cm$^{-1}$), lending credibility to the method. Results suggest that the O-H$^{...}$F geometry is controlled by the thermal expansion of the silicate framework whereas the H position is governed by the bond-valence requirement that maintains the H atom at a position of ideal bond valence sum. Burgi, H.B. and Capelli, S.C. (2000): Dynamics of molecules in crystals from multi-temperature anisotropic displacement parameters. I. Theory. Acta Crystallographica, A56, 403 - 412.
DE: 5410 Composition
DE: 3620 Crystal chemistry
DE: 3949 Thermal expansivity
DE: 3954 X ray, neutron, and electron spectroscopy and diffraction
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting