Encyclopedia of Astrobiology

Living Edition
| Editors: Muriel Gargaud, William M. Irvine, Ricardo Amils, Henderson James Cleaves, Daniele Pinti, José Cernicharo Quintanilla, Michel Viso

Oxygen Fugacity

  • Francis AlbaredeEmail author
Living reference work entry
DOI: https://doi.org/10.1007/978-3-642-27833-4_4021-3


Mineral redox buffer Redox reactions Redox zonations 


Oxygen fugacity is an equivalent of the partial pressure of oxygen in a particular environment (atmosphere, rocks, etc.) corrected for the nonideal character of the gas.


There are multiple ways of characterizing how reducing or how oxidizing an environment is. An example of a redox reaction in an ideal gas mixture of carbon dioxide, methane, water, and oxygen is:
$$ {\mathrm{CO}}_2+2{\mathrm{H}}_2\mathrm{O}\iff {\mathrm{O}}_2+{\mathrm{CH}}_4 $$
The status of this reaction can be evaluated by writing the equilibrium equation:
$$ \frac{P_{{\mathrm{O}}_2}{P}_{{\mathrm{CH}}_4}}{P_{{\mathrm{CO}}_2}{P}_{{\mathrm{H}}_2\mathrm{O}}^2}={K}_1(T) $$
where P i stands for the partial pressure of gas i and K 1(T) is the reaction coefficient at temperature T. For real gases, pressure P i should be replaced by fugacities f i . Fugacity and pressure converge toward each other when the total pressure diminishes.
Likewise, the reduction of magnetite, which contains one Fe2+ and two Fe3+, by silica gives fayalite (ferric olivine Fe2SiO4), which contains two ferrous ions Fe2+:
$$ 2{\mathrm{Fe}}_3{\mathrm{O}}_4+3{\mathrm{SiO}}_2\iff 3{\mathrm{Fe}}_2{\mathrm{SiO}}_4+{\mathrm{O}}_2 $$
For ideal solids and gas:
$$ \frac{P_{{\mathrm{O}}_2}\left[{\mathrm{Fe}}_2{\mathrm{SiO}}_4\right]}{\left[{\mathrm{Fe}}_3{\mathrm{O}}_4\right]\left[{\mathrm{SiO}}_2\right]}={K}_2(T) $$
where concentrations are in bracket. For pure phases, concentrations are equal to 1, while for real gases P O2 is replaced by f O2. Taking the logarithm of both sides, we get:
$$ \ln {f}_{\mathrm{O}2}= \ln {K}_2(T)=\frac{\Delta {H}_2}{RT}+\mathrm{Const} $$
where ΔH 2 is the enthalpy of the reaction and R is the gas constant. At a given temperature, the quartz-fayalite-magnetite (QFM) mineral assemblage (see entry “Mantle, Oxidation of”) imposes or “buffers” a particular value of f O2. For all practical purpose, oxygen fugacity measures the electron trade among minerals and describes the proportions of Fe2+ and Fe3+ coexisting in the rock, rather the other way around. Under most conditions, the f O2 is so low that oxygen is actually fully combined to the mineral phases and is not present as a gas. Oxygen fugacity is a robust estimate of the redox conditions because rocks are electrical insulators. Other buffers of oxygen fugacity exist, such as iron-wüstite (IW), which describes the coexistence of metallic iron Fe0 and Fe2+ in extremely reducing environments like the lunar mantle.

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© Springer-Verlag Berlin Heidelberg 2014

Authors and Affiliations

  1. 1.Ecole Normale Supérieure de LyonLyon Cedex 7France