The Nature of Polymerization in Silicate Glasses and Melts: Solid State NMR, Modeling and Qauntum Chemicial Calculations.

Silicate melts are among the dominant constituents of the upper mantle and crust. The full understanding of atomic scale disorder is essential to the macroscopic properties of the melts such as viscosity and configurational thermodynamic properties. Recently, we quantified the various aspects of the extent of disorder in ›r­’charge-balanced silicate glasses (non bridging (NBO)/T=0)›r­_ using solid state NMR and theoretical analysis, which allowed the degree of randomness of these systems to be determined in terms of the degree of Al-avoidance and degree of phase separations (Lee and Stebbins, Geochim. Cosmochim. Acta. 66, 303). Quantitative estimation of the framework connectivity and the atomic structures of depolymerized silicate melts (NBO/T>0), however, are still poorly known and framework cations and anions have often been assumed to be randomly distributed. Here, we explore the extent of disorder and the nature of polymerization in several binary and ternary silicate glasses with varying NBO/T using O-17 NMR at varying magnetic fields of 7.1, 9.4 and 14.1 T in conjunction with quantum chemical calculations. We also quantify the extent of intermixing among non-framework cations in mixed cation glasses, and calculate corresponding configurational thermodynamic properties. Non-random distribution among cations is clearly demonstrated from the relative populations of oxygen sites and the variation of distribution of structurally relevant NMR parameters with NBO/T from O-17 3QMAS NMR. The proportion of NBO (Na-O-Si) in Na₂O-SiO₂ glasses increases with NBO/T. Its chemical shift distribution decreases about 18 % from NBO/T of 0.7 to 2, suggesting a reduced configurational disorder around NBO with Na contents. Preferential interactions among framework cations are further manifested in peralkaline Ca- and Na- aluminosilicate glasses where depolymerization of networks selectively occurs at Si rather than Al tetrahedra, forming Na-O-Si or Ca-O-Si. The result is consistent with our quantum chemical calculations based on density functional theory where the silicate chain with Al-NBO has energy penalty of about 108 kJ/mol compared with the cluster with Ca-O-Si. The degree of Al avoidance (Q) among framework units in Na- aluminosilicate is larger than that in Ca-aluminosilicates, as recently observed for fully polymerized glasses. On the other hand, Q varies with NBO/T. The above results support the significant chemical order in silicate glasses that leads to considerable mixing among framework units. The degree of intermixing among non-framework cations in mixed-cation glasses has remained controversial. The population of each Na-NBO in Ca-Na and Ba-Na mixed cation silicate glasses is smaller than the prediction given from random distribution of these cations, and thus supports the preference for dissimilar pairs of cations, which could explain a decrease in diffusivity in these melts. In this study, we provide new insights into the structure of silicate glasses with varying NBO/T, highlighting more complete, atomic-level understanding on the dynamic processes in silicate magmas.