HR: 11:35h
AN: V41E-06    [PDF]
TI: Structures and Properties of Silicate Glasses and Melts at High Pressure: Multi-nuclear 2 Dimensional Solid State NMR and Statistical Mechanical Modeling
AU: * Lee, S
EM: s.lee@gl.ciw.edu
AF: Geophysical Laboratory, Carnegie Institution of Washington, Washington, DC 20015 United States
AU: Fei, Y
EM: fei@gl.ciw.edu
AF: Geophysical Laboratory, Carnegie Institution of Washington, Washington, DC 20015 United States
AU: Cody, G D
EM: cody@gl.ciw.edu
AF: Geophysical Laboratory, Carnegie Institution of Washington, Washington, DC 20015 United States
AU: Mysen, B O
EM: mysen@gl.ciw.edu
AF: Geophysical Laboratory, Carnegie Institution of Washington, Washington, DC 20015 United States
AB: Essential to the transport and thermodynamic properties of silicates at high pressure is the full understanding of atomic arrangement of system under pressure. Whereas there have been significant progresses in our understanding of the atomic structures of crystalline materials or molecules at high pressure, much less is known about the structures of amorphous silicates including glasses and melts. This is largely because conventional X-rays or optical spectroscopy can only provide limited information of the various aspects of atomic disorder in densified amorphous silicates. Recent development and advances of 2 dimensional solid state NMR have offered much improved resolution, allowing us to probe structural details of amorphous silicates (Lee, Fei, Cody, \& Mysen, Geophys. Res. Lett., 2003, 30, 1845; Lee and Stebbins, J. Phys. Chem. B., 2003, 107, 3141). Here we present recent 2D NMR (MQMAS) spectra of silicate glasses, quenched from melts at 6-15 GPa in a multi-anvil apparatus at Geophysical Laboratory, which reveals previously unknown details of melt structures at high pressure. The atomic structures of model glasses at high pressure are significantly different from those at ambient pressure and show evidence of extensive chemical ordering among highly coordinated network polyhedra, such as $^{[5,6]}$Al and $^{[5,6]}$Si, which affects corresponding macroscopic properties. New oxygen sites in serious of sodium silicate and aluminosilicate glasses with varying degree of polymerization at high pressure include $^{[5,6]}$Al-O-$^{[4]}$Si, $^{[5,6]}$Si-O-$^{[4]}$Si and Na-O-$^{[5,6]}$Si. The fractions of these sites increase with increasing pressure mainly at the expanse of Na-O-$^{[4]}$Si. The fraction of $^{[5,6]}$Al in aluminosilicate glasses increases with pressure, but decreases with increasing degree of polymerization of melts from (Na$_{2}$O)$_{0.75}$(Al$_{2}$O$_{3}$)$_{0.25}$3SiO$_{2}$ to NaAlSi$_{3}$O$_{8}$ at constant pressure. The effect of these structural changes and chemical ordering to the diffusivity and configurational thermodynamic properties of silicate glasses at high pressure were also explored using statistical mechanical modeling and quantum chemical calculations, in conjunction with information of the extent of disorder from NMR data. These results and methods shed light on a new opportunity of studying atomic structures of amorphous solids at high pressure and provide improved prospects on microscopic origins of melt properties at the 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: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting