HR: 1340h
AN: MR13B-1255    [Abstracts]
TI: Compositional effects on aluminum and silicon coordination changes in high pressure aluminosilicate glasses: NMR results
AU: * Kelsey, K E
EM: kkelsey@stanford.edu
AF: Stanford University, Dept of Geo and Env Sciences Bldg. 320, Stanford, CA 94305-2115, United States
AU: Stebbins, J F
EM: stebbins@stanford.edu
AF: Stanford University, Dept of Geo and Env Sciences Bldg. 320, Stanford, CA 94305-2115, United States
AU: Asimow, P D
EM: asimow@gps.caltech.edu
AF: California Inst. of Tech., Division of Geological and Planetary Sciences, Pasadena, CA 91125, United States
AU: Mosenfelder, J L
EM: jed@gps.caltech.edu
AF: California Inst. of Tech., Division of Geological and Planetary Sciences, Pasadena, CA 91125, United States
AB: Understanding the structure of high pressure aluminosilicate glasses can better constrain the macroscopic properties and igneous processes in the upper mantle. As melts densify, bond angles and distances change and the average coordination number (CN) for the cations increases. Both Al and Si can increase CN, and are affected by the type and amount of network modifying cation (i.e. Ca, Mg, Na, K, etc…) present. This study uses nuclear magnetic resonance (NMR) spectroscopy to investigate several aspects of the compositional controls on the structure of aluminosilicate glasses quenched from liquids at high pressure. ²⁹Si MAS NMR spectra of 6GPa Na₂Si₄O₉ glasses show the presence of 2 % ^[5]Si and 2 % ^[6]Si. A similar glass with 1.2 wt % Al₂O₃ and the same nominal NBO/T has very similar Si coordinations, even though the Al has an average CN of 5.4. Thus, although Al increases CN more readily than does Si, the addition of Al does not inhibit the formation of high coordination Si. Extending the previously investigated range of modifier cation field strength, ²⁷Al MAS NMR spectra of LaAlSi₃O₉ glasses show a substantial increase in average Al CN, from about 4.21 to 5.07 from ambient P to 8GPa, while Li₃AlSi₃O₉ glasses have consistently lower average Al CN, increasing from 4.02 to 4.48 from ambient P to 6 GPa. This agrees with previously noted trends with modifier cation field strength. However, at 6 and 8 GPa the average Al CN is less than expected in the La-containing glasses, suggesting that the La³⁺ environment changes as well. ¹⁷O 3QMAS NMR spectra of the Li-containing glasses are consistent with previously proposed mechanisms for the generation of ^[5],[6]Al at the expense of non-bridging oxygen, but the La-containing glasses have nearly unchanged oxygen environments with increasing pressure. Finally, we present ²⁷Al MAS NMR spectra of Na+Ca-bearing aluminosilicate glasses with up to 5 wt % FeO quenched from melts at 5 GPa and conclude that the substitution of small amounts of Fe²⁺ for Ca²⁺ does not significantly affect the average Al CN.
DE: 1011 Thermodynamics (0766, 3611, 8411)
DE: 3630 Experimental mineralogy and petrology
DE: 3640 Igneous petrology
DE: 3924 High-pressure behavior
DE: 3929 NMR, Mossbauer spectroscopy, and other magnetic techniques
SC: Mineral and Rock Physics [MR]
MN: 2007 Fall Meeting