This work uses nuclear magnetic resonance spectroscopy (NMR) to study questions of cation ordering in ambient pressure aluminosilicate glasses and minerals as well as the pressure-induced structural changes within aluminosilicate glasses. We start by determining the 17O NMR parameters for the minerals jadeite, pyrope, grossular, and mullite for use as crystalline model compounds to aid interpretation of high pressure glass spectra. Next, along the pyrope-grossular join, 17O NMR shows that Ca and Mg distribute randomly over the different oxygen sites and there are two different intermediate or mixed (Ca+Mg) sites with different cation-oxygen bond lengths. Similarly, ambient pressure glasses along the same join were studied using 17O NMR and 27Al NMR. We are able to quantify the Al and observe that the Ca is preferentially associated with the bridging oxygen (BO) and the Mg with the non-bridging oxygen (NBO), and this preference increases with increasing Al/Si. In the first high pressure study, we use both NMR and La-EXAFS to observe Li-, La-, Fe-, and Sc-containing aluminosilicate glasses. The Li-containing glasses have Al coordinations and densities that increase as predicted by cation field strength, however the La glasses have significantly lower Al coordinations, likely related to an increase in the La coordination. 17O NMR reveals that the Li-containing glasses are consistent with a mechanism that generates [5,6]Al at the expense of the NBO, while the La-containing glasses have virtually unchanging oxygen spectra with increasing pressure. The final high pressure study provides the first direct evidence for both highly coordinated Si and Al present in the same glass. We observe that both the average Si and Al coordination numbers decrease with increasing Al/Si, however the combined (Si+Al) coordination number increases.
|School Location:||United States -- California|
|Source:||DAI-B 70/01, Dissertation Abstracts International|
|Keywords:||Aluminosilicate glasses, Cation ordering, Coordination changes|
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