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High-temperature geochemists use oxygen fugacity to characterize the oxidation state of systems. Consequently, we want to write redox reactions that contain O₂. Thus, equilibrium between magnetite and hematite would be written as:

(3.122)

(or alternatively, as we wrote in eqn. 3.101) rather than the way we expressed it in eqn. 3.120. We note, however, there is negligible molecular oxygen in magmatic systems, and other species are often responsible for transfer of electrons and O^2–. For example, the equilibrium between magnetite and hematite may be mediated by water:

(3.123)

The above two reactions are thermodynamically equivalent in terms of magnetite oxidation. The first reaction is simpler, of course, and hence preferred, but it may sometimes be necessary to consider the proportions of the actual gas species present.

If we can regard magnetite and hematite as pure phases, then their activities are equal to one and the equilibrium constant for reaction 3.122 is the inverse of the oxygen fugacity:

(3.124)

We can rewrite eqn. 3.86 as:

(3.125)

and taking the standard state as 1000 K and 1 bar, we can write:

Thus, oxygen fugacity can be calculated directly from the difference in the free energy of formation of magnetite and hematite at the appropriate T and P. Substituting appropriate values into this equation yields a value for log of −10.86.

It is important to understand that the oxygen fugacity is fixed at this level (though the exact level at which it is fixed is still disputed because of uncertainties in the thermodynamic data) simply by the equilibrium coexistence of magnetite and hematite. The oxygen fugacity does not depend on the proportion of these minerals. For this reason, it is appropriately called a buffer. To understand how this works, imagine some amount of magnetite, hematite and oxygen present in a magma. If the oxygen fugacity is increased by the addition of oxygen to the system, equilibrium in the reaction in eqn. 3.121 is driven to the right until the log of the oxygen fugacity returns to a value of −10.86. Only when all magnetite is converted to hematite can the oxygen fugacity rise. A drop in oxygen fugacity would be buffered in exactly the opposite way until all hematite were gone. A number of other buffers can be constructed based on reactions such as:

and

These can be used to construct the oxygen buffer curves in Figure 3.22.