Polymerization of silicate and aluminate tetrahedra in glasses, melts, and aqueous solutions—I. Electronic structure of H 6Si 2O 7, H 6AlSiO 71−, and H 6Al 2O 72−

Abstract

As part of a study of the effect of geologically common network modifiers on polymerization in silicate melts, glasses, and silica-rich aqueous solutions, we have studied the energies, electronic structures, and inferred chemical properties of IVT-O- IVT linkages in the tetrahedral dimers H 6,Si 2O 7, H 6AlSiO 7 1−, and H 6Al 2O 7 2− using semi-empirical molecular orbital theory (CNDO/2). Our results indicate that the electron donating character of the bridging oxygen, O(br), linking two tetrahedra increases with increasing T-O(br) bond length but decreases with decreasing T-O(br)-T angles and increasing O-T-O(br) angles. This increase or decrease of the donor character of O(br) coincides with an increase or decrease of the affinity of O(br) for hard acceptors. The calculated electronic structure for the H 6Si 2O 7 molecule is compared with the observed X-ray emission, absorption, and photoelectron spectra of quartz and vitreous silica; the reasonable match between calculated and observed oxygen Kα emission spectra of vitreous silica supports our assertion that non-bonded O(br) electron density energetically at the top of the valence band controls the chemical reactivity of IVT-O- IVT linkages in polymerized tetrahedral environments.