Mineral redox buffer
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In geology, a mineral redox buffer is a mineral assemblage which constrains a particular range of oxygen fugacity or sulfur fugacity within a naturally occurring rock composition.
In the oxygen fugacity system there are two redox buffers generally applied to terrestrial rocks; the wüstite-magnetite redox buffer (often abbreviated the W-M buffer) and the quartz-fayalite-magnetite redox buffer (or QFM Buffer). Within meteorites, the iron-wüstite redox buffer may be more appropriate for describing the oxygen fugacity of these extraterrestrial systems.
Sulfur fugacities are constrained primarily by the pyrrhotite-pyrite-sulfate redox sulfide buffer, which describes the sulfur oxidation state of a system and the sulfur saturation of the system. Toward the pyrite-sulfate end of this compositional space this system, in nature, is at least in part controlled by the QFM redox buffer for oxygen, given the necessary oxidation of sulfur into sulfate compounds.
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[edit] Redox effects upon silicate minerals
Main article: Normative mineralogy
The ratio of Fe2+ to Fe3+ within a rock determines, in part, the silicate mineral assemblage of the rock. Within a rock of a given chemical composition, iron enters minerals based on the bulk chemical composition and the mineral phases which are stable at that temperature and pressure. Iron may only enter minerals such as pyroxene and olivine if it is present as Fe2+; Fe3+ cannot enter the lattice of fayalite olivine and thus for every two Fe3+ ions, one Fe2+ is used and one molecule of magnetite is created.
[edit] Redox effects on sulfur fugacity
As alluded to above, excess free oxygen (high oxidation state) is required to drive rocks from the pyrrhotite-pyrite stability field into the pyrite-sulfate field. Pyrrhotite and sulfate are not a stable equilibrium assemblage, and thus before sulfate can be produced all pyrrhotite must be oxidised to pyrite.
It is usually the case that in sulfur saturated systems oxidation state is controlled by the pyrite-pyrrhotite buffer line, and in sulfur poor systems it is usually controlled by the QFM redox buffer of oxygen. In some compositions poor in both iron and sulfide, oxygen and sulfur fugacity is not controlled, and this can result in significant deviations from the normal oxidation states experienced in terrestrial rocks. Such rocks tend to be rich in micas, rare earth elements, incompatible elements and refractory elements such as boron, fluorine, etcetera. In these cases, unusual mineral compositions, for instance Fe-rich orthoclase, may be created due to the extreme oxidation states.
[edit] Wüstite Redox Buffer
Wüstite, in geochemistry, defines a redox buffer of oxidation within rocks at which point the rock is so reduced that Fe3+ and thus hematite is absent.
As the redox state of a rock is further reduced, magnetite is converted to wüstite. This occurs by conversion of the Fe3+ ions in magnetite to Fe2+ ions. An example reaction is presented below;
- FeO.Fe2O3 + C --> 3FeO + CO
magnetite + graphite/diamond --> wüstite + carbon monoxide
This is termed a redox buffer because until all Fe3+ magnetite is converted to Fe2+ the oxide mineral assemblage of iron remains wüstite-magnetite, and furthermore the redox state of the rock remains at the same level of oxygen fugacity. This is similar to buffering in the H+/OH- acid-base system of water.
Once the Fe3+ is consumed, then oxygen must be stripped from the system to further reduce it and wüstite is converted to native iron. The oxide mineral equilibrium assemblage of the rock becomes wüstite-magnetite-iron.
In nature, the only natural systems which are chemically reduced enough to even attain a wüstite-magnetite composition are rare, including carbonate-rich skarns, meteorites and perhaps the mantle where reduced carbon is present, exemplified by the presence of diamond and/or graphite.
In chemically reduced rocks, magnetite may be absent due to the propensity of iron to enter olivine, and wüstite may only be present if there is an excess of iron above what can be used by silica. Thus, wüstite may only be found in silica-undersaturated compositions which are also heavily chemically reduced, satisfying both the need to remove all Fe3+ and to maintain iron outside of silicate minerals.
In nature, carbonate rocks, potentially carbonatite, kimberlites, carbonate-bearing melilitic rocks and other rare alkaline rocks may satisfy these criteria. However, wüstite is not reported in most of these rocks in nature, potentially because the redox state necessary to drive magnetite to wüstite is so rare.
[edit] Quartz-Fayalite-Magnetite Redox Buffer
The QFM redox buffer occurs when all wüstite is converted to magnetite at the expense of oxygen, and then magnetite begins to be converted to hematite at the expense of FeO functional groups within magnetite.
The QFM buffer is defined by the redox condition of an artificial composition of quartz, fayalite (Fe olivine) and magnetite. This mineral assemblage will persist until the redox state is changed to either consume magnetite or quartz via the following equilibrium reaction;
- Fe2SiO4 + O2 <--> FeOFe2O3 + SiO2
Once all magnetite is consumed by quartz to produce fayalite, the redox potential of the system can rise past the buffered state, and magnetite will be converted to hematite. If all fayalite is consumed to create magnetite, the redox potential can fall to produce wüstite.
