Mineral redox buffer
In geology, a redox buffer is an assemblage of minerals or compounds that constrains oxygen fugacity as a function of temperature. Knowledge of the redox conditions (or equivalently, oxygen fugacities) at which a rock forms and evolves can be important for interpreting the rock history. Iron, sulfur, and manganese are three of the relatively abundant elements in the Earth's crust that occur in more than one oxidation state. For instance, iron, the fourth most abundant element in the crust, exists as native iron, ferrous iron (Fe2+), and ferric iron (Fe3+). The redox state of a rock affects the relative proportions of the oxidation states of these elements and hence may determine both the minerals present and their compositions. 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.
For other rocks with suitable minerals, oxygen fugacities can be calculated, and the redox conditions can be obtained by comparison to the fugacity-temperature diagram.
Common redox buffers
Redox buffers were developed in part to control oxygen fugacities in laboratory experiments to investigate mineral stabilities and rock histories. Each of the curves plotted in the fugacity-temperature diagram is for an oxidation reaction occurring in a buffer. 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. As long as all the pure minerals (or compounds) are present in a buffer assemblage, the oxidizing conditions are fixed on the curve for that buffer. Pressure has only a minor influence on these buffer curves for conditions in the Earth's crust.
4 Fe3O4 + O2 = 6 Fe2O3
NiNiO nickel-nickel oxide
2 Ni + O2 = 2 NiO
3 Fe2SiO4 +O2 = 2 Fe3O4 + 3 SiO2
3 Fe1−xO + O2 ~ Fe3O4
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