HR: 09:30h
AN: MR11A-07    [Abstracts]
TI: Atomistic and Nanoscale Origins of Macroscopic Properties of Silicate Melts at High-Pressure: Spectroscopy & Quantum Chemical Calculations
AU: * Lee, S
EM: sungklee@snu.ac.kr
AF: Seoul National University, School of Earth and Environmental Sciences, Seoul, 151-742 Korea, Republic of
AU: Fei, Y
EM: fei2gl.ciw.edu
AF: Carnegie Institution of Washington, Geophysical Laboratory 5251 Broad Branch Rd. NW, Washington, DC 20015 United States
AU: Cody, G
EM: cody@gl.ciw.edu
AF: Carnegie Institution of Washington, Geophysical Laboratory 5251 Broad Branch Rd. NW, Washington, DC 20015 United States
AU: Mysen, B
EM: mysen@gl.ciw.edu
AF: Carnegie Institution of Washington, Geophysical Laboratory 5251 Broad Branch Rd. NW, Washington, DC 20015 United States
AU: Mao, H
EM: mao@gl.ciw.edu
AF: Carnegie Institution of Washington, Geophysical Laboratory 5251 Broad Branch Rd. NW, Washington, DC 20015 United States
AU: Eng, P
EM: eng@cars.uchicago.edu
AF: University of Chicago, Consortium for Advanced Radiation Sources, Chicago, Il 60637 United States
AB: Atomic structure of amorphous oxide melts at high pressure controls their macroscopic properties and geophysical progresses in the Earth's interior. Advances in NMR spectroscopy, x-ray optics, and theoretical analyses enable us to determine the structure of silicate glasses and provides clues to the microscopic origins of melt properties and relevant geochemical processes, such as generation, migration, and dynamics of magmas at high pressures (e.g. Lee et al. Geophy. Res. Letts. 2003, 30, p1845; Lee et al. Phys. Rev. Letts. 2005, 94, p165507; Lee et al. Nature Materials 2005, accepted). Here we report recent progress on pressure-induced structural changes in various amorphous oxide glasses and melt at high pressures using multi-nuclear solid state NMR, and synchrotron X-rays, and quantum simulations. In prototypical amorphous borates, and silicates, as well as complex aluminosilicate glasses and melts, the fractions of highly coordinated framework units (e.g. five coordinated ^[5,6]Si, ^[5,6]Al, ^[4]B) increase with increasing pressure with multiple densification mechanisms. The distribution of these framework cations at high pressure is not completely random but favors formation of oxygen linking dissimilar Si pairs such as ^[5,6]Si-O-^[4]Al. Whereas the general trend in the effect of pressure on the structure is similar in those amorphous oxides, detailed pressure-induced structural changes are largely dependent on the degree of polymerization in the melts, types and fractions of network modifying cations at isobaric conditions. Topological disorder due to Si-O bond length distribution increases with pressure and is also larger for more polymerized amorphous oxides. Na-23 NMR spectra for sodium silicate and aluminosilicate glasses revealed that Na-O distance in the binary sodium silicates increases with pressure but that in the aluminosilicate glasses decreases with pressure. These results demonstrate that the pressure-induced structural changes in the silicate melts at high pressure and the corresponding changes in properties are complex function of composition. Pressure dependence of thermodynamics and transport properties such as diffusivity and viscosity of melts were directly calculated from experimentally measured atomic-scale disorder, and from variation of non-bridging oxygen fraction with pressure from spectroscopic data. These calculation results suggest that non-linear pressure dependence of viscosity of silicate melts stems from both the isobaric NBO fraction and its pressure dependence, yielding improved prospects for understanding the atomistic origins of magmatic processes in Earth's interior.
DE: 3630 Experimental mineralogy and petrology
DE: 3924 High-pressure behavior
DE: 3929 NMR, Mossbauer spectroscopy, and other magnetic techniques
DE: 3939 Physical thermodynamics
SC: Mineral and Rock Physics [MR]
MN: Fall Meeting 2005