Thermochemistry and structure of glasses along the join NaAlSi 3O 8-NaBSi 3O 8

Abstract

Glasses along the join NaAlSi 3O 8-NaBSi 3O 8 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 Rd 20Ab 80 to about 29% at Rd 100. 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 Q 4 (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 SiO 4 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.