Network polymerization and cation coordination environments in boron-bearing rhyolitic melts: Insights from 17O, 11B, and 27Al solid-state NMR of sodium aluminoborosilicate glasses with varying boron content

Despite the geochemical implications for the dissolution behaviors of nuclear waste glasses and the properties of boronbearing rhyolitic melts, a detailed atomic-level understanding of the overall effect of boron content on the structural characteristics of boron-bearing silicate glasses remains elusive. Herein, we explore the effects of B/Al and Si/B ratios on the local configurations around cations and the degree of network polymerization of Na2O-Al2O3-B2O3-SiO2 glasses in nepheline (NaAlSiO4) - malinkoite (NaBSiO4) and albite (NaAlSi3O8) - reedmergnerite (NaBSi3O8) joins using multi-nuclear magnetic resonance (NMR). The B-11 NMR results confirmed an increase in the B coordination numbers from 3 (B-[3]) to 4 (B-[4]) as B/Al and Si/B ratios increase. The Al-27 NMR spectra show that the Al is mainly four coordinated, except in NaAl0.25B0.75SiO4 glasses, with a minor amount of Al-[5] species, revealing a more prominent control of Si over that of B on the Al coordination environments. The O-17 NMR spectra resolve distinct bridging (BO, Si-O-Al, Al-O-Al, Si-O-Si, B-O-B, and Si-O-B) and non-bridging oxygens (NBO, Na-O-Si). In contrast, the absence of B-O-Al species confirmed the separation between B and Al and prevalent melt depolymerization exclusively in silicate-networks. The fractions of B-O-B and Na-NBO increase as B/Al ratio increases, accounting for a drastic enhancement in the dissolution rate of glasses. A dramatic increase also indicates an enhanced connectivity of relatively weaker bonds such as B-O-B and Na-O-Si as composition reached a certain boron content. This is linked to a decrease in the melt viscosity with increasing B2O3 component. Based on the measured fractions of B-[3,B-4] in B-11 NMR spectra with varying composition, the trend of the evolution of boron isotope composition in sodium aluminoborosilicate melts was estimated, confirming the preferential enrichment of B-10 into rhyolitic melts with increasing boron content. The decreased fraction of B-[3] species with increasing Si/B may partly explain the reduction in delta B-11 in boron-bearing volcanic rocks at deeper depths in the subduction zone. The results suggest that composition-induced changes in the atomic structures should be constrained to precisely interpret the melt viscosity, boron isotope composition of arc volcanic lavas, and the dissolution rates of nuclear waste glasses. (C) 2019 Elsevier Ltd. All rights reserved.