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

      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 ³ ⇌ Q 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.

In chemically complex environments with both alkalis and alkaline earths present, the alkali metals associate with the Al³⁺ in the Q ⁴ species, whereas alkaline earths tend to bond with NBO in depolymerized species [10]. It follows, therefore, that for systems such as natural magmatic liquids, the Q ⁿ ‐distribution is controlled for the most part by the rules that govern alkaline earth compositions.

This can be illustrated with the use of experimental data from ternary aluminosilicates to model the properties of chemically more complex systems. For example, the physicochemical properties of aluminosilicate glasses and melts, which vary with (Si,Al)─O bond energy, can be related qualitatively, and sometimes semi‐quantitatively, to the Al/(Al + Si) ratio and to the electronic properties of charge‐balancing cation because the bond energy varies with these same variables. The viscosity and activation energy of viscous flow of melts along meta‐aluminosilicate joins, which are essentially fully polymerized, follow such simple evolutions within temperature intervals where the temperature‐dependent viscosity is approximately Arrhenian (Figure 9). In contrast, the viscosity of peralkaline aluminosilicate melts does not show this effect of Al/(Al + Si) and actually may have a minimum at intermediate Al/(Al + Si)‐values (Figure 9). This difference is because of a combination of weakening of (Si,Al)─O bonds in Q ⁴‐species with increasing Al/(Al + Si) and changing Q ⁿ ‐species abundance with increasing Al/(Al + Si).

Figure 9 Activation energy of viscous flow of aluminosilicate melts along the NaAlO₂─SiO₂ (nominal NBO/T = 0) (meta‐aluminosilicate) join (closed squares) and Na₂Si₂O₅─Na₂(NaAl)₂O₅ (nominal NBO/T = 0.5) (peralkaline aluminosilicate) calculated with the assumption of Arrhenian viscosity of the melts. As discussed in the text, NaAlO₂─SiO₂ melts and glasses do have a small fraction of a percent nonbridging oxygen, but are essentially pure Q ⁴ with Al‐substitution for Si. Melts and glasses on the Na₂Si₂O₅─Na₂(NaAl)₂O₅ join, on the other hand, comprise Q ², Q ³, and Q ⁴ species with essentially all Al³⁺ in substitution for Si⁴⁺ in the Q ⁴ species thus weakening the (Si,Al)─O bridging oxygen bonds in this specie. This, in turn, lowers the activation energy as for the NaAlO₂─SiO₂ melts and glasses. However, because of Al‐preference, the proportions of Q ⁿ ‐species change so that Q ⁴ abundance increases, thus countering the effect of the Al,Si‐substitution. These two mechanisms give rise to the minimum activation energy values at intermediate Al/(Al + Si) values.

      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 Ferric and Ferrous Iron

Iron is found in three redox states, Fe°, Fe²⁺, and Fe³⁺, but only the last two can enter the structure of silicate compositions in significant amounts. In this environment, the proportions of Fe²⁺ and Fe³⁺ vary with bulk chemical composition of glasses and melts, temperature, pressure, and redox conditions during equilibration of precursor melt. In magmatic liquids, the Fe³⁺/∑Fe ranges from essentially 0 to 1 (Figure 10) in such a way that this redox ratio has been employed to estimate the activity of oxygen and, therefore, oxygen budgets during formation and evolution of earth materials and, indeed, the Earth, itself.

      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)

where O²⁻ is the link to the silicate structure, the relationship between redox ratio and oxygen fugacity provides a measure of the activity coefficient ratio of Fe³⁺ and Fe²⁺, g_Fe3+/g_Fe2+. This ratio often is about 1, but does depend on silicate polymerization (Figure 11). The redox ratio also varies with Al/(Al + Si) and the electronic properties of the metal cations. It increases with Al/(Al + Si) and NBO/T. The redox ratio also increases the more electropositive the network‐modifying cation. This means, for example, that the Fe³⁺/Fe²⁺ of alkali silicate melts is greater than that of alkaline earth silicate melts at the same temperature (pressure) and redox conditions.

Figure 10 Distribution of redox ratio of iron (Fe³⁺/∑Fe) among various common rock types. Database: http://Georock.org. Examples of average compositions of basalt, andesite, and rhyolite are given in Figures 5 and 6.