Thermochemistry and structure of glasses along the join NaAlSi₃O₈-NaBSi₃O₈

Glasses along the join NaAlSi₃O₈-NaBSi₃O₈ and the end-member crystalline phases albite and reedmergnerite were investigated using ¹¹B, ²³Na, ²⁷Al and ²⁹Si MAS NMR spectroscopy and oxide-melt solution calorimetry. The glasses have substantial positive enthalpies of mixing described by the equation ΔH_mix (kcal · mol ^{t ̄1}) = 4.4(±1.0)X(1 - X), where X is the mole fraction Rd component. ¹¹B MAS NMR spectra show that boron occurs in both trigonal and tetrahedral coordination in the glasses, with the percentage of B in trigonal coordination decreasing from about 60% at Rd₂₀Ab₈₀ to about 29% at Rd₁₀₀. Thus, the glasses are structurally more complex than the end-member crystals which are tetrahedral framework minerals. The NMR data suggest that all or most of the Si, Al and tetrahedral B occur in Q⁴ (framework) sites. They are also consistent with the ideas that most of the tetrahedral B does not extensively copolymerize with trigonal boron, that at least some of the non-bridging oxygens formed by the presence of trigonal boron are located on SiO₄ tetrahedra, and that Si and trigonal B do not occur as next-nearest neighbors. Both the NMR and the Δ H_mix results indicate the presence of clustering. Such clustering might involve separation into framework regions and regions dominated by trigonal boron and/or into B-rich and Al-rich framework regions.