Chapter 11 - The effect of composition, compression, and decompression on the structure of high-pressure aluminosilicate glasses: An investigation utilizing 17O and 27Al NMR

Abstract

This chapter reviews recent work on the determination of the structure of high pressure aluminosilicate glasses and presents new 17O and 27Al magic-angle spinning (MAS) and 17O triple-quantum (3Q) MAS NMR spectra that investigate the structure of K3AlSi3O9 (KAS) and Ca3Al2Si6018 (CAS) glasses quenched at 5 GPa. Comparison of the 27Al MAS spectra for the high-pressure and ambient-pressure CAS and KAS glasses shows that the average Al-coordination increases with increasing pressure and that there is a strong compositional dependence on the amount of high-coordinated Al, as the CAS glass contains significantly more Al than the KAS glasses. The spectra of the 5 GPa and the ambient pressure glasses are compared and it shows that high coordinated Al is formed by the mechanism that is similar to the mechanisms for the generation of Si and Si in high-pressure silicate glasses. Decompression times vary (14 h and 1 s) and longer times are shown to reduce the average Al-coordination. In addition, the reproducibility of glass making in a multi-anvil apparatus is investigated by comparing the 27Al MAS spectra of two glasses made in the same high-pressure experiment, but in separate capsules. This suggests that the reproducibility of the average Al-coordination is about 0.1 (for e.g., 5.0 ± 0.1) for glasses with an Al/(Al + Si) ratio of 0.25. The chapter also concludes that the structure of glasses with drastically different decompression rates is not the same, which suggests that decompression affects the structure of high-pressure aluminosilicate glasses. However, more work remains to determine the reproducibility of high-pressure glass structure in multi-anvil apparatuses, but it seems as though the limitation in determining the structural speciation lies in the reproducibility of the glass structure, not the detection limits of NMR.