Nature of polymerization and properties of silicate melts and glasses at high pressure

The structures of sodium silicate and aluminosilicate glasses quenched from melts at high pressure (6-10 GPa) with varying degrees of polymerization (fractions of nonbridging oxygen) were explored using solid-state NMR [O-17 and Al-27 triple-quantum magic-angle spinning (3QMAS) NMR]. The bond connectivity in melts among four and highly coordinated network polyhedra, such as Al-[4], Al-[5,Al-6], Si-[4], and Si-[5,Si-6], at high pressure is shown to be significantly different from that at ambient pressure. In particular, in the silicate and aluminosilicate melts, the proportion of nonbridging oxygen (NBO) generally decreases with increasing pressure, leading to the formation of new oxygen clusters that include 5- and 6-coordinated Si and Al in addition to 4-coordinated Al and Si, such as Si-[4]-O-Si-[5,Si-6], Si-[4]-O-Al-[5,Al-6] and Na-O-Si-[5,Si-6]. While the fractions of Al-[5,Al-6] increase with pressure, the magnitude of this increase diminishes with increasing degrees of ambient-pressure polymerization under isobaric conditions. Incorporating the above structural information into models of melt properties reproduces the anomalous pressure-dependence of O2- diffusivity and viscosity often observed in silicate melts. Copyright (C) 2004 Elsevier Ltd.