Pressure-induced structural changes and densification of vitreous MgSiO3 |
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| Authors: | SJ Gaudio S Sen |
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| Institution: | a Department of Geology, University of California, One Shields Avenue, Davis, CA 95616, USA
b Department of Chemical Engineering and Material Science, University of California, Davis, CA 95616, USA |
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| Abstract: | Compression of MgSiO3 glass in a 6/8 multianvil apparatus to 10.0 ± 0.5 GPa results in demonstrable changes in density and silicon coordination. Under high-pressure, samples were heated over a range of temperatures from 300 to 773 K, quenched to room temperature and decompressed at rates of 10.4 and 0.08 GPa/min. Recovered glasses have bulk densities that are 2.6-11.0% higher than the non-compressed glass. 29Si MAS NMR spectra of compressed glasses show narrowing of the 4]Si peak resulting from a reduction in the spread of the Si-O-Si bond angle distribution. After heating and rapid decompression, 29Si MAS NMR spectra of recovered glasses exhibit peaks assignable to 4]Si, 5]Si, and 6]Si with relative fractions of 0.945, 0.045, and 0.008, respectively. These changes in Si coordination and in Si-O-Si bond angle distribution with pressure only represent part of the structural changes associated with permanent densification of heated and unheated samples. The abundance of 6]Si is found to be insensitive to decompression rate, while 5]Si reverts to 4]Si on slow decompression at room temperature. These observations demonstrate that high-coordinated silicon species in MgSiO3 glass are formed on compression below glass transition temperatures and that pressure-induced structural changes can be preserved with rapid decompression. The ease with which 5]Si reverts to 4]Si during decompression suggests that the conversion of 4]Si → 5]Si principally involves short-range atomic displacement. The reversible and irreversible features of densification of MgSiO3 glass, provide insights into the fundamental structural and rheological properties of refractory silicate melts similar to those found in the Earth’s mantle. |
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