Mineral redox buffer

In geology, a redox buffer is an assemblage of minerals or compounds that constrains oxygen fugacity as a function of temperature.

Iron, sulfur, and manganese are three of the relatively abundant elements in the Earth's crust that occur in more than one oxidation state.

If a rock contains pure minerals that constitute a redox buffer, then the oxygen fugacity of equilibration is defined by one of the curves in the accompanying fugacity-temperature diagram.

Redox buffers were developed in part to control oxygen fugacities in laboratory experiments to investigate mineral stabilities and rock histories.

These redox buffers are listed here in order of decreasing oxygen fugacity at a given temperature—in other words, from more oxidizing to more reducing conditions in the plotted temperature range.

For instance, at redox conditions more oxidizing than the MH (magnetite-hematite) buffer, at least much of the iron is likely to be present as Fe3+ and hematite is a likely mineral in iron-bearing rocks.

Igneous rock emplaced and erupted in island arcs typically record oxygen fugacities 1 or more log units more oxidizing than those of the NiNiO buffer.

Oxygen fugacities during metamorphism extend to higher values than those in magmatic environments, because of the more oxidizing compositions inherited from some sedimentary rocks.