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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.