Appendix D: Ferric/Ferrous Iron Content

Perple_X thermodynamic data files typically specify compositions in terms of reduced binary metal oxide components and excess oxygen. Where the excess oxygen component is the amount of oxygen needed to define the actual redox state of the metal represented by the metal oxide components. In this context, metal refers to any element that binds with oxygen to form an oxide (e.g., Fe, H, C) and the reduced oxide components represent the total amount of the metal in the system or a phase.

When there is only a single redox couple operative in a system, then the Perple_X program ctransf can be used to transform the excess oxygen component to a component that directly reflects the redox state of the metal. For example, if the redox couple is ferrous-ferric iron, then the O2 component can be transformed into (replaced by) a ferric iron component

(25)\[\text{FeO}_{1.5} = \text{FeO} + \frac{1}{4} \text{O}_2\]

Caution

This transformation is not recommended when multiple redox couples are operative in a system because the composition of diatomic oxygen becomes \(\text{O}_2 = 4\,\text{FeO}_{1.5} - 4\,\text{FeO}\); thus if carbon is described by the oxide component CO2, the composition of graphite becomes \(\text{C} = \text{CO}_2 - 4\,\text{FeO}_{1.5} + 4\,\text{FeO}\). Not only is such a composition confusing, but it requires specification of the Fe-components in problems involving graphite regardless of whether iron is present in the system.

It is always possible to evaluate the redox state of all elements in a phase from the redox state and fractions of its endmembers. The latter are output by both meemum and werami. Additionally, for more recent 688 format solution models, meemum and werami explicitly output the state of redox sensitive elements. The proportion of an element in a particular redox state in the system can then be computed by summing over all phases. In systems with multiple redox couples there is no shortcut for this process.

This appendix describes the shortcut for systems, or phases, with a single redox couple and, because of its importance, the specific case of computing the ferric and ferrous iron content from a composition defined in terms of the components {FeO, O2}. In this case, the molar amount of the ferric iron species is

(26)\[n_{\text{Fe3+}} = \frac{1}{4} n_{\text{O}_2}\]

the molar amount of the ferrous iron species is

(27)\[n_{\text{Fe2+}} = n_{\text{FeO}} - \frac{1}{4} n_{\text{O}_2}\]

and the molar ferric-ferrous ratio is

(28)\[\frac{n_{\text{Fe3+}}}{n_{\text{Fe2+}}} = \frac{\frac{1}{4} n_{\text{O}_2}}{n_{\text{FeO}} - \frac{1}{4} n_{\text{O}_2}}.\]

In all 3 equations, the amounts on the right-hand side are the molar amounts of the components. In petrological literature, the amount of the FeO component is sometimes written \(\text{FeO}_{\text{total}}\) to distinguish the total iron content (as the FeO component) from the actual ferrous iron species content.