On the Physical Significance of Oxygen Fugacity

To indicate how small a number this is, remember that at 1 bar and 0°C, 1 mole (or 6 X 10 molecules) of gas occupies 22.7 liters. A pressure of 10 bars would thus be exerted by about 3 molecules of 02(5) in 10 liters or 10 cubic meters of space—a volume considerably larger than that of a sphere having our Solar System s diameter. What kind of significance can this possibly have  

Neither does using oxygen fugacities imply anything about the mechanism of oxidation or reduction in real systems. Evidently, systems such as reducing sediments at ordinary temperatures contain essentially no molecular O2, so that redox couples such as hematite-magnetite will actually react by some other mechanism, such as 

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