Reference Work Entry

Encyclopedia of Astrobiology

pp 1196-1196

Oxygen Fugacity

  • Francis AlbarèdeAffiliated withEcole Normale Supérieure de Lyon Email author 


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:

$$ {\text{C}}{{\text{O}}_2} + {2}{{\text{H}}_2}{\text{O}} \Leftrightarrow {{\text{O}}_2} + {\text{C}}{{\text{H}}_4} $$

The status of this reaction can be evaluated by writing the equilibrium equation:

$$ \frac{{{P_{{{\text{O}}_2}}}{P_{{\text{C}}{{\text{H}}_4}}}}}{{{P_{{\text{C}}{{\text{O}}_2}}}P_{{{\text{H}}_2}{\text{O}}}^2}} = {K_1}(T) $$
where Pi stands for the partial pressure of gas i and K1(T) is the reaction coefficient at temperature T. For real gases, pressure Pi should be replaced by fugacities fi. Fugacity and press ...

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