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