HR: 1330h
AN: V42A-0329    [PDF]
TI: Aluminum coordinations for aluminosilicate glasses at 1 atm. and 3GPa: a $^{27}$Al MAS NMR study
AU: * Allwardt, J R
EM: allwardt@pangea.stanford.edu
AF: Dept. of Geological and Environmental Sciences, Stanford University, Dept of GES, Stanford, CA 94305
AU: Stebbins, J F
EM: Stebbins@pangea.stanford.edu
AF: Dept. of Geological and Environmental Sciences, Stanford University, Dept of GES, Stanford, CA 94305
AU: Withers, A C
EM: withe012@tc.umn.edu
AF: Dept. of Geology and Geophysics, 310 Pillsbury Drive SE 108 Pillsbury Hall, Minneapolis, MN 94305
AU: Hirschmann, M M
EM: Marc.M.Hirschmann-1@umn.edu
AF: Dept. of Geology and Geophysics, 310 Pillsbury Drive SE 108 Pillsbury Hall, Minneapolis, MN 94305
AB: In this study, we used $^{27}$Al MAS NMR to investigate the distribution of different aluminum coordinations in ambient pressure and moderate pressure (3GPa) aluminosilicate glasses with compositions of M$^{x+}$$_{(9-3x)}$Al$_{2}$Si$_{6}$O$_{18}$, where M= Ca$^{2+}$, Na$^{+}$, and K$^{+}$. These spectra were collected at 18.8 Tesla to minimize the peak broadening due to second-order quadrupolar effects, which causes overlapping peaks for different coordinations of Al. Aluminum is thought to be mostly tetrahedrally coordinated in ambient-pressure aluminosilicate glasses where M$^{+}$$_{(2-x)}$O $>=$ Al$_{2}$O$_{3}$, but $^{27}$Al NMR studies have shown that small concentrations of high-coordinated Al are present in both charge balanced (Toplis et al. 2000; Stebbins et al. 2001) and "depolymerized" (M$^{+}$$_{(2-x)}$O $>$ Al$_{2}$O$_{3}$) aluminosilicate glasses (Stebbins and Farnan 1992). The $^{27}$Al spectra of samples of this study show that these ambient pressure glasses all contain $^{[5]}$Al. Interestingly, the NAS (Na-aluminosilicate) glass contains more $^{[5]}$Al than the KAS glass (0.6 vs. 0.3, respectively), which is opposite of that observed for SiO$_{[5]}$ groups in binary silicate glasses (Stebbins and McMillan 1993). The CAS glass spectrum shows a shoulder at 25 ppm, but the $^{[4]}$Al peak is too broad to assign a reliable percentage for the amount of $^{[5]}$Al. Previous studies of high-pressure glasses have shown that Al transforms to higher coordinations with increasing pressures (Yarger et al. 1995). In this study, we observe that the $^{27}$Al MAS spectrum of the 3 GPa NAS glass shows that 7% $^{[5]}$Al and 1% $^{[6]}$Al species exist. Additionally, the spectra of the KAS glass shows that 2% $^{[5]}$Al and possibly some $^{[6]}$Al are present at 3 GPa. These spectra clearly show for the first time that pressure induced Al-coordination changes occur in aluminosilicate glasses at pressures of 3 GPa. Recent $^{27}$Al NMR spectra of high-pressure "depolymerized" aluminosilicate glasses with different fictive temperatures have shown that the amount of high-coordinated Al increases with increasing temperature, which suggests that these results represent a minimum value for the melt (Allwardt et al 2003). Further work will focus on obtaining data at higher pressures to determine the pressure dependence for the generation of these species.
DE: 3924 High-pressure behavior
DE: 3929 NMR, Mossbauer spectroscopy, and other magnetic techniques
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting