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 and mineralogy

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.

MH hematite

4 Fe3O4 + O2 = 6 Fe2O3

NiNiO nickel-nickel oxide

2 Ni + O2 = 2 NiO

FMQ quartz

3 Fe2SiO4 +O2 = 2 Fe3O4 + 3 SiO2

WM magnetite

3 Fe1-xO + O2 ~ Fe3O4

IW wustite

2(1-x) Fe + O2 = 2 Fe1-xO

QIF fayalite

2 Fe + SiO2 + O2 = Fe2SiO4

Minerals, rock types and characteristic buffers

Mineralogy and correlations with redox buffer

The ratio of Fe2+ to Fe3+ within a rock determines, in part, the ulvospinel, enlarges the stability field of magnetite. Likewise, at conditions more reducing than the IW (iron-wustite) buffer, minerals such as pyroxene can still contain Fe3+. The redox buffers therefore are only approximate guides to the proportions of Fe2+ and Fe3+ in minerals and rocks.

Igneous Rocks

Terrestrial gabbro in non-arc settings typically record oxygen fugacities from about those of the FMQ buffer to a log unit or so more reducing than that buffer.

Sedimentary rocks

Oxidizing conditions are common in some environments of deposition and diagenesis of sedimentary rocks. The fugacity of oxygen at the MH buffer (shale, are relatively reducing.

Metamorphic rocks

Oxygen fugacities during metamorphism extend to higher values than those in magmatic environments, because of the more oxidizing compositions inherited from some sedimentary rocks. Nearly pure hematite is present in some metamorphosed serpentinites.

Extraterrestrial rocks

Within meteorites, the wüstite redox buffer may be more appropriate for describing the oxygen fugacity of these extraterrestrial systems.

Redox effects and sulfur

Sulfide minerals such as shales. These sulfide minerals form in environments more reducing than that of the Earth's surface. When in contact with oxidizing surface waters, sulfides react: sulfate (SO4--) forms, and the water becomes acidic and charged with a variety of elements, some potentially toxic. Consequences can be environmentally harmful, as discussed in the entry for acid mine drainage.

Sulfur oxidation to sulfate also is important in generating sulfur-rich volcanic eruptions, like those of Pinatubo in 1991 and El Chichon in 1982. These eruptions contributed unusually large quantities of sulfur to the tephra. In contrast, sulfides contain most of the sulfur in magmas more reducing than the FMQ buffer.

References

  • Donald H. Lindsley (editor), Oxide minerals: petrologic and magnetic significance. Mineralogical Society of America Reviews in Mineralogy, Volume 25, 509 pages (1991). ISBN 0-939950-30-8
  • Bruno Scaillet and Bernard W. Evans, The 15 June 1991 Eruption of Mount Pinatubo. I. Phase Equilibria and Pre-eruption P–T–fO2–fH2O Conditions of the Dacite Magma. Journal of Petrology, Volume 40, pages 381-411 (1999).

See also

 
This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Mineral_redox_buffer". A list of authors is available in Wikipedia.