Ca–Mg mixing in aluminosilicate glasses: An investigation using 17O MAS and 3QMAS and 27Al MAS NMR

To better understand non-framework cation mixing in Ca–Mg aluminosilicate glasses, 17O MAS and 3QMAS NMR studies were done on glasses on the Mg3Al2Si3O12–Ca3Al2Si3O12 join as well as several compositions with lower Al/Si ratios. While not all of the oxygen sites are fully resolved, the non-bridging oxygen associated with two Ca and one Mg cations is under-represented relative to that predicted by a model assuming random Ca/Mg mixing. Therefore, local non-bridging oxygen environments are rich in Mg, and extra Ca must be associated with charged bridging oxygens such as Al–O–Si. The deviation from random mixing increases as the Al/Si ratio decreases and as XMg approaches 0.50, suggesting a reduction in configurational entropy that may be compensated for by other sources, including mixing of network forming cations. Al–O–Al is detected, and appears to increase with increasing XMg and increasing Al/Si. High field 27Al MAS NMR also shows that these glasses all have substantial concentrations of [5]Al, but no detectable [6]Al (0.5% detection limit). The amount of [5]Al increases non-linearly as XMg increases and very slightly as Al/Si increases.