Network cation coordination in aluminoborosilicate and Mg- aluminosilicate glasses: pressure effects in recovered structural changes and densification

Abstract:

In this study, we compare the aluminum and boron coordination of glass samples recovered from piston-cylinder experiments carried out at 1 to 3 GPa and near to their ambient glass transition temperature (Tg), which we have found gives a more accurate picture of high pressure structural changes than experiments involving quenching from above the liquidus, as large pressure drops can occur in the latter. Aluminoborosilicate glasses with excess modifier (Ca, La and Y- aluminoborosilicate) quenched from melts at 1-3 GPa were studied with B-11 and Al-27 MAS NMR to assess relative effects on two different network cations. Structural changes in the Y-aluminoborosilicate are dramatic, going from mostly AlO4 at low pressure to mostly AlO5 and AlO6 at 3 GPa. Large increases in BO4 (vs. BO3) are also seen. Mg-aluminosilicate glasses, both tectosilicate (Mg2Al4Si6O20) and with excess modifier composition (Mg3Al2Si6O18) quenched from melts at 1-3 GPa pressure were studied with Al-27 MAS NMR. In contrast to our previous study (Bista et al., Am. Min., in press) of jadeite glass, where only 0.5% of fivefold aluminum was seen in glass recovered from 3 GPa, five and six fold aluminum species increase significantly with increasing pressure in both Mg aluminosilicate glass compositions studied here. We observe that the tectosilicate Mg aluminosilicate glass has more higher coordinated aluminum than the excess modifier containing composition in the pressure range in our study. In the previous study (Bista et al., in press) of jadeite and calcium aluminosilicate (Ca3Al2Si6O18) glasses, 6-8% densification was observed in glasses recovered from 3 GPa. In this study of Mg aluminosilicate glasses, we observe 12% densification in glasses recovered from 3 GPa. Both types of observation confirm that structural and density changes with pressure are enhanced by higher field strength modifier cations, and will be especially important in Mg- and Fe-rich mantle melts.