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+, gFe3+/gFe2+. 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.
5.2 Structural Roles of Fe3+ and Fe2+
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, gFe2+/gFe3+, because (gFe2+/gFe3+)(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).
5.3 Structure–Property Relations
The physical properties of iron‐bearing silicate melts and glasses are less well known than for iron‐free materials. Viscosity and volume data can nonetheless be rationalized in structural terms.
The partial molar volumes of FeO and Fe2O3,
