HR: 0800h
AN: T41B-1199    [Abstracts]
TI: Structure and Decompression Melting of a Novel 2-D High Pressure Ice Phase Formed in Nano-Confinement Between Two Talc Surfaces
AU: * Kirkpatrick, R
EM: kirkpat@uiuc.edu
AF: Department of Geology, University of Illinois at Urbana-Champaign, 1301 W Green St, Urbana, IL 61801
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
AB: Molecular dynamics (MD) simulation of water confined in nano-spaces between layers of talc (composition Mg$_3$Si$_4$O$_{10}$(OH)$_2$ + 2H$_2$O) at 300K and pressures of approximately 0.45 GPa show the presence of a novel 2-D ice structure for the interlayer water, and the structural evolution with decreasing pressure provides insight into the mechanisms of its decompression melting. Talc is hydrophobic at ambient pressure and temperature, but at high pressure and temperature, weak hydrogen bonding between the talc and water plays an important role in stabilizing the structure. The results suggest that experimentally accessible elevated pressures may cause a wide range of previously unknown water structures in nano-confinement. At pressures significantly greater than 0.45 GPa, the water molecules are isolated and can not form an H-bonds with each other. At about 300K and 0.45 GPA, however, the interlayer spacing is large enough that a continuous H-bond network forms among the water molecules, basal oxygens, and OH-groups. In the talc 2-D ice, each water molecule is coordinated by six O$_{\rm b}$ atoms of one basal siloxane sheet and three water molecules. The water molecules are arranged in a buckled hexagonal array in the $a$-$b$ directions with two sub-layers along [001]. Each H$_2$O molecule has four H-bonds, accepting one from a talc OH-group and one from another water molecule, and donating one to an O$_{\rm b}$ and one to another water molecule. In plan view, the molecules are arranged in six-member rings reflecting the substrate talc structure. Decompression melting occurs by vacancy and interstitial formation due to migration of water molecules from the six-member rings to the centers of other six-member rings and eventual formation of liquid-like regions and large vacant cavities in the interlayer space. The results provide specific, experimentally testable predictions about the interlayer phase transition and suggest that pressure alone may cause significant effects on the structure of nano-confined water.
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