HR: 14:55h
AN: T43E-06    [Abstracts]
TI: The structure and composition of the 10-$\AA$ phase
AU: * Pawley, A R
EM: alison.pawley@man.ac.uk
AF: School of Earth, Atmospheric and Environmental Sciences, University of Manchester, Oxford Road, Manchester, M13 9PL United Kingdom
AU: Welch, M D
EM: mdw@nhm.ac.uk
AF: Department of Mineralogy, Natural History Museum, Cromwell Road, London, SW7 5BD United Kingdom
AB: 10-$\AA$ phase is a high-pressure hydrous phase stable up to at least 9 GPa. It forms from the hydration of talc above $\sim$ 5 GPa, and is therefore a candidate for transporting significant amounts of H$_{2}$O deep into the Earth's mantle in subduction zones. However, its equilibrium structure and composition are uncertain. A fixed MgO:SiO$_{2}$ ratio of 3:4 has long been assumed, but the H$_{2}$O content appears to depend on run duration, increasing with time up to a likely maximum of x = 2 in the proposed formula Mg$_{3}$Si$_{4}$O$_{10}$(OH)$_{2}$.xH$_{2}$O. We have obtained new structural and compositional data of a sample of well-crystallised deuterated 10-$\AA$ phase, which reveal an unexpected structural complexity, with implications for its stability in the Earth and capacity to store H$_{2}$O in subduction zones. The sample was synthesised in a multi-anvil device at 6.5 GPa, 600 $\deg$C, 400 hr from a stoichiometric mixture of Mg(OD)$_{2}$ and SiO$_{2}$ glass. $^{29}$Si MAS NMR spectroscopy reveals the presence of a significant Q$^{2}$ Si peak in addition to the normal Q$^{3}$ Si, indicating that approximately 1/16 of the expected talc-like Si sites are not occupied by Si. $^{2}$H NMR spectroscopy distinguishes at least 3 different types of D sites, one of which is highly mobile and is likely due to molecular D$_{2}$O, with the others due to OD groups. IR and Raman spectra show several more OH environments than found in talc, suggesting that charge balancing of the Si deficit involves additional OH groups (0.8 extra OH p.f.u.). We propose that the Si "vacancies" are hydrogarnet-type groups within the tetrahedral sheets. Electron microprobe analysis shows a Si deficit of 0.17 atoms p.f.u., implying that the Si "vacancies" occur as single entities rather than as clusters. No superlattice reflections are visible in hk0 sections of single-crystal XRD patterns; however, this does not rule out ordering of vacancies within individual sheets. On the other hand, 3-fold superlattice reflections occur along c*, superimposed on a modulated diffuse scattering profile.
DE: 3620 Crystal chemistry
DE: 3924 High-pressure behavior
DE: 3929 NMR, Mossbauer spectroscopy, and other magnetic techniques
DE: 3934 Optical, infrared, and Raman spectroscopy
DE: 3954 X ray, neutron, and electron spectroscopy and diffraction
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting