Encyclopedia of Glass Science, Technology, History, and Culture. Группа авторов. Читать онлайн. Newlib. NEWLIB.NET

Автор: Группа авторов
Издательство: John Wiley & Sons Limited
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Жанр произведения: Техническая литература
Год издания: 0
isbn: 9781118799499
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      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 Al3+ in tetrahedral coordination.

      5.1 Redox Relations of Iron

      Equilibria between Fe3+ and Fe2+ in silicate melt and glass include interaction with oxygen in the structure. Conversely, variations in redox behavior of iron oxides affect the silicate melt structure. From the simple relationship,

      (2)equation

Bar charts depict the distribution of redox ratio of iron among various common rock types. Examples of average compositions of basalt, andesite, and rhyolite are given in Figures 5 and 6.