HR: 1330h
AN: T42A-0268    [PDF]
TI: Mobility of Water Molecules on Brucite and Talc Surfaces by \textit{Ab Initio} Potential Energy Surface and Molecular Dynamics Simulations
AU: * Sakuma, H
EM: hsaku@geo.titech.ac.jp
AF: Department of Earth and Planetary Sciences, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro, Tokyo, 1528551 Japan
AU: * Sakuma, H
EM: hsaku@geo.titech.ac.jp
AF: Institute of Geology and Paleontology, Graduate School of Science, Tohoku University, Aoba, Aramaki, Aoba, Sendai, 9808578 Japan
AU: Tsuchiya, T
EM: takut@cems.umn.edu
AF: Department of Chemical Engineering and Materials Science (CEMS), University of Minnesota, 421 Washington Av. SE, Minneapolis, MN 55455 United States
AU: Kawamura, K
EM: kats@geo.titech.ac.jp
AF: Department of Earth and Planetary Sciences, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro, Tokyo, 1528551 Japan
AU: Otsuki, K
EM: otsuki@dges.tohoku.ac.jp
AF: Institute of Geology and Paleontology, Graduate School of Science, Tohoku University, Aoba, Aramaki, Aoba, Sendai, 9808578 Japan
AB: The interaction between minerals and water molecules is a fundamentally important property that influences fluid distribution and migration in the earth's crust, and transportation of H$_2$O into the earth's mantle. In this work, we focused attention on the mobility of water molecules on the cleavage surfaces of brucite and talc, and performed the molecular dynamics simulations. Brucite is in most common as an alteration product of periclase in contact metamorphosed dolomites. It is also found as a low-temperature hydrothermal vein mineral in serpentinites and chlorite schists. Talc occurs most frequently as a product of metamorphism of hydrothermal alteration of ultramafic rocks and is an important phase in the oceanic crust. The first step in performing the simulations of mineral-water interfaces is to develop a reliable and consistent model for the interaction of water molecules with solid surfaces. Fundamental understanding of such interaction often requires physico-chemical knowledge at a molecular level, which is not easily obtained from experimental works, electronic state calculations provide remarkable information of the interactions between surface species and water molecules. The first-principles method based on the density functional theory is the most useful method in the present day. By using the first-principles method, we calculated the energy curves with respect to water configuration on brucite and talc surfaces and the \textit{ab initio} model potentials were obtained by fitting the parameters to the calculated energy curves. Using this \textit{ab initio} model potentials, molecular dynamics simulations of water thin film embedded between brucite and talc surfaces have been performed at various temperature-pressure conditions. The thicknesses of water thin film were employed from 0.5 to 2.0 nm. Since it is expected that physical properties of water would change with the distance from mineral surfaces, we analysed the mobility for each sliced layers having 0.25 nm thickness along with a normal axis for surface plane. The self-diffusion coefficient $D_S$ and reorientation time $\tau^2_y$ of water at each sliced layer were calculated. The $\tau^2_y$ corresponds to $\tau_{NMR}$ measured by NMR experiments. The calculated $D_S$ of nearest brucite and talc surfaces are 1.45 and 2.0 times and the $\tau^2_y$ are 0.77 and 0.5 times as large as the bulk values at ambient condition, respectively. These anomalies are limited in monolayer nearest to brucite and talc surfaces and the water in the other layers are almost equal to bulk state. This rapid mobility of water on brucite and talc surfaces results from the weak interaction between water and surfaces. Thus rapid mobility of water might affect on the fluid migration in the earth.
DE: 3947 Surfaces and interfaces
SC: Tectonophysics [T]
MN: 2003 Fall Meeting