Encyclopedia of Glass Science, Technology, History, and Culture. Группа авторов

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7 Abundance evolution (mol %) of Q 2, Q 3, and Q 4 species in alkali silicate glasses as a function of their NBO/Si‐values of compositions as indicated in diagrams. For alkali silicate glasses, the metal/silicon ratio equals the NBO/Si, provided that all Si4+ is in tetrahedral coordination. The ionization potential, Z/r 2, of K+ and Li+ is 0.46 and 1.49, respectively, assuming sixfold coordination of oxygen around the alkali metal. The curves for Na2O─SiO2 (Z/r 2 of Na+: 0.8) fall in between those of Li2O─SiO2 and K2O─SiO2.

Graph depicts the enthalpy change, ∆H, for the disproportionation equilibrium, 2Qn ⇌ Qn - 1 + Qn + 1, n = 3, as a function of ionization potential of alkali cation for two series of alkali metal silicate compositions. In these systems, M/Si = NBO/Si assuming all Si4+ is in tetrahedral coordination in the glasses. The ∆H is derived from temperature-dependent equilibrium constant at temperatures above the glass transition and assuming that mol fraction of Qn-species equals their activity.

      Characterization of the structural roles of Al3+ in aluminosilicate compositions is central to understanding their properties. The role of Al3+ can vary from simple substitution for Si4+ in the network of interconnected aluminosilicate tetrahedra to more complex environments where Al, Si substitution is restricted to only some of the Q n ‐species. The latter feature is important because most chemically complex compositions are depolymerized (NBO/T > 0) so that multiple Q n ‐species will coexist.

      4.1 Al3+ and Qn‐Species

      Where there is excess alkali metal or alkaline earths over that needed for charge‐balance of Al3+ in tetrahedral coordination, Al3+ may be distributed among the various Q n ‐species. However, this distribution is not random and Al3+ dominantly is in Q 4‐species. Coexisting, less polymerized Q n ‐species are essentially devoid of Al3+. This situation reflects the tendency of Al3+ to substitute for Si4+ in SiO4 tetrahedra with the smallest intertetrahedral angle, which is found in Q 4‐species.

      It follows that the equilibrium constant for the reaction, 2 Q 3Q 2 + Q 4, is positively correlated with Al/(Al + Si) of the glass and melt. For peralkaline alkali aluminosilicate melts (see Figure 1) at temperatures above their glass transition, the temperature‐dependent equilibrium constant yields ΔH‐values increasing from near 0 to about 40 kJ/mol in the Al/(Al + Si) = 0–0.4 range. Fewer experimental data exist for peralkaline alkaline earth aluminosilicate glasses.

Graph depicts the activation energy of viscous flow of aluminosilicate melts along the NaAlO2-SiO2 join and Na2Si2O5-Na2(NaAl)2O5 calculated with the assumption of Arrhenian viscosity of the melts.

      The compressibility and thermal expansion of alkali aluminosilicate melts are also correlated with Al/(Al + Si) because the intertetrahedral (Si,Al)─O─(Si,Al) angle is itself more compressible and expandable with increasing Al/(Al + Si). In alkaline earth aluminosilicate systems, however, the opposite relations exist because Ca‐charge‐balanced Al3+ causes the (Si,Al)─O─(Si,Al) bonds to stiffen. In other words, the compressibility and thermal expansion of such aluminosilicate melts decreases with increasing Al/(Al + Si).

      Among the main groups of natural magmatic liquids (basalt, andesite, and rhyolite), the proportion of alkaline earths/alkali metals in charge‐balancing roles decreases in the same order. This means that basaltic melts are less compressible and show smaller thermal expansion than more silica‐rich melts such as rhyolite. Furthermore, the viscosity of basalt melt is less sensitive to Al/(Al + Si) than more silica‐rich magma, which have a higher proportion of alkali‐charge‐balanced

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