Encyclopedia of Astrobiology

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| 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
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Keywords

Mineral redox buffer Redox reactions Redox zonations 

Definition

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.

Overview

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:
$$ {\mathrm{CO}}_2+2{\mathrm{H}}_2\mathrm{O}\iff {\mathrm{O}}_2+{\mathrm{CH}}_4 $$
(1)
The status of this reaction can be evaluated by writing the equilibrium equation:
$$ \frac{P_{{\mathrm{O}}_2}{P}_{{\mathrm{CH}}_4}}{P_{{\mathrm{CO}}_2}{P}_{{\mathrm{H}}_2\mathrm{O}}^2}={K}_1(T) $$
(2)
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+:
$$ 2{\mathrm{Fe}}_3{\mathrm{O}}_4+3{\mathrm{SiO}}_2\iff 3{\mathrm{Fe}}_2{\mathrm{SiO}}_4+{\mathrm{O}}_2 $$
(3)
For ideal solids and gas:
$$ \frac{P_{{\mathrm{O}}_2}\left[{\mathrm{Fe}}_2{\mathrm{SiO}}_4\right]}{\left[{\mathrm{Fe}}_3{\mathrm{O}}_4\right]\left[{\mathrm{SiO}}_2\right]}={K}_2(T) $$
(4)
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:
$$ \ln {f}_{\mathrm{O}2}= \ln {K}_2(T)=\frac{\Delta {H}_2}{RT}+\mathrm{Const} $$
(5)
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.

See Also

Copyright information

© Springer-Verlag Berlin Heidelberg 2014

Authors and Affiliations

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