HR: 0800h
AN: T41B-1198    [Abstracts]
TI: Molecular Modeling of the 10-\AA\ Phase at Subduction Zone Conditions
AU: * Wang, J
EM: jianwei7@uiuc.edu
AF: Department of Geology, University of Illinois at Urbana-Champaign, 1301 W Green St, Urbana, IL 61801
AU: Kalinichev, A G
EM: kalinich@uiuc.edu
AF: Department of Geology, University of Illinois at Urbana-Champaign, 1301 W Green St, Urbana, IL 61801
AU: Kirkpatrick, R J
EM: kirkpat@uiuc.edu
AF: Department of Geology, University of Illinois at Urbana-Champaign, 1301 W Green St, Urbana, IL 61801
AB: Molecular dynamics (MD) modeling of the 10-\AA\ phase, Mg$_3$Si$_4$O$_{10}$(OH)$_2$$\dot$$x$H$_2$O, with $x$ = 2/3, 1.0 and 2.0 shows complex structural changes with pressure, temperature and water content and provides new insight into the structures and stabilization of these phases under subduction zone conditions. The structure(s) of this phase and its role as a reservoir of water in the mantle have been controversial, and these calculations provide specific predictions that can be tested by in situ diffraction studies. At ambient conditions, the computed structures of talc ($x$=0) and the 10-\AA\ phases with $x$=2/3 and 1.0 are stable over the 350 ps period of the MD simulations. Under these conditions, the $x$=2/3 and 1.0 10-\AA\ phases show phlogopite-like layer stacking in good agreement with previously published structures based on powder X-ray diffraction data for samples quenched from high pressure and temperature experiments. The calculations show that the 10-\AA\ phase with $x$=2.0 is unstable at ambient conditions. The computed structures at $P$=5.5 GPa and $T$=750K, well within the known stability field of the 10-\AA\ phase, change significantly with water content, reflecting changing H-bonding configurations. For $x$=2/3, the layer stacking is talc-like, and for $x$=1.0 it is phlogopite-like. The calculations show that transformation between these two stackings occurs readily, and that the talc-like stacking for $x$=2/3 composition is unlikely to be quenchable to ambient conditions. For $x$=2.0, the layer stacking at $P$=5.5 GPa and $T$=750K is different than any previously proposed structure for a 10-\AA\ phase. In this structure, the neighboring basal oxygens of adjacent TOT layers are displaced by $b/3$ (about 3 \AA) resulting in the Si atoms of one siloxane sheet being located above the center of the six-member ring across the interlayer. The water molecules are located 1.2 \AA\ above the center of all six-member rings and accept H-bonds from the OH groups located below the rings. The $b/3$-displaced structure does not readily transform to either the talc-like or phlogopite-like structure, because neither of these stackings can accommodate two water molecules per formula unit. There is likely to be a compositional discontinuity and phase transition between the $b/3$-displaced phase and the phase with phlogopite-like stacking.
DE: 8124 Earth's interior--composition and state (old 8105)
DE: 3620 Crystal chemistry
DE: 3924 High-pressure behavior
DE: 3939 Physical thermodynamics
DE: 3947 Surfaces and interfaces
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting