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It is sometimes assumed that Al³⁺ and Fe³⁺ occupy similar structural positions in silicate melts and glasses because of their common nominal charge and somewhat similar ionic radii. But this assumption is not necessarily warranted because Al³⁺ is dominantly in fourfold coordination in silicate crystals, whereas for the most part Fe³⁺ is in sixfold coordination with oxygen although there are exceptions to this general statement.

In silicate glasses and melts oxygen coordination numbers vary with bulk chemical composition, total iron content, temperature, and redox conditions that existed during precursor melting. The Fe³⁺─O bond distance of ferrisilicate glass is often used as an indicator of oxygen coordination number. For example, increasing iron content in Al‐free silicates results in increasing Fe³⁺─O distance, which may be consistent with a transformation from fourfold to sixfold coordination.

Figure 11 Activity coefficient ratio of Fe²⁺ and Fe³⁺ in CaO─SiO₂ glasses formed by quenching from melt after equilibration at 1600 °C with different oxygen fugacity. The NBO/Si‐values in this figure were calculated from the Ca/Si ratio. From the relationship to oxygen fugacity, any deviation of the concentration ratio, Fe²⁺/Fe³⁺, was ascribed to changes in the activity coefficient ratio, g_Fe2+/g_Fe3+, because (g_Fe2+/g_Fe3+)(Fe²⁺/Fe³⁺) = 0.25.

Mössbauer spectroscopy of glasses is an analytical tool with which both redox ratio of iron and coordination of Fe³⁺ and Fe²⁺ can be determined (Chapter 2.2). In alkali ferrisilicate melts equilibrated with air, Fe³⁺ typically is in fourfold coordination with oxygen. However, by replacing Na⁺ with more electronegative metals, the oxygen tetrahedra surrounding Fe³⁺ become increasingly distorted with an eventual changes to higher oxygen coordination numbers. As a result, in complex aluminosilicate compositions containing both alkalis and alkaline earths it is not unusual that Fe³⁺ exists in more than one coordination state.

Most evidence suggests that Fe³⁺ in fourfold coordination forms oxygen tetrahedra that are isolated from those of Si⁴⁺ and Al³⁺. Furthermore, when both alkali and alkaline earths are potential charge‐balancing cations in complex systems, alkali metals tend to associate with Al³⁺, whereas alkaline earths serve to charge‐balance Fe³⁺ in tetrahedral coordination with oxygen. This means, for example, that if a rhyolite and a basalt melt equilibrated at the same oxygen fugacity, temperature, and pressure, the iron would be more oxidized in the rhyolite than in the basalt melt.

At least in FeO─SiO₂ systems the Fe²⁺─O distances are consistent with sixfold coordination although it has also been suggested that the oxygen coordination number of Fe²⁺ might be closer to 4 than to 6. Results from ⁵⁷Fe Mössbauer resonant absorption spectroscopy of iron‐bearing glasses offer additional aid to distinguish between possible oxygen coordination numbers of ferrous iron (4, 5, and 6).