| Abstract
| - Soda-lime silicate glass is the fundamental base glass for many technologically important oxide glasses, andit has been used as window glass since the Roman Empire. Mixed-cation silicates also are useful models ofthe structure and dynamics of basaltic magmas and mantle melts. The diffusivity of Na+ in silicate melts andits variation with composition play key roles in melting behavior. This property also depends strongly on thecomposition and framework structures of glasses and melts and on the presence of other types of nearbycations that often impede Na motion; this is known as a mixed cation effect. Structure-dependent silica activityalso controls the composition of melts in equilibrium with mantle peridotite. Despite its importance, little isknown about the detailed atomic structure and the degree of cation mixing in Ca-Na silicate glasses andmelts with varying composition. Most modeling efforts assume a random distribution of these cations. Here,we use 17O magic angle spinning (MAS) and triple quantum magic angle spinning (3QMAS) NMR to shownonrandom distributions of the network-modifying cations Na and Ca in Ca-Na silicate glasses, by probingthe atomic configurations around the nonbridging oxygens. Nonrandomness in Na−Ca mixing, in particularthe prevalence of Na-Ca pairs in this system, was clearly observed in 17O MAS and 3QMAS NMR whereseveral nonbridging oxygen peaks such as Na−O−[4]Si, and mixed peaks ({Na,Ca}−O−[4]Si) are partiallyresolved. The observed fractions of Na−O−[4]Si are smaller than those predicted by random distributions ofNa and Ca, suggesting preference to dissimilar pairs. There are also considerable interactions between bridgingoxygens and charge-modifying cations, which supports rather homogeneous distribution of such cations. 23NaMAS NMR spectra at high field (14.1 T) provide information on chemical shift distributions with relativelysmall perturbations from quadrupolar broadening and provide further support for significant Na−Ca mixing.The results given here partly account for reduced Na diffusion when mixed with Ca, and together withnonrandom distributions of nonbridging oxygens, lead to negative deviation in silica activity. This alsocontributes to atomistic models of several magmatic processes.
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