Talk:Mineral redox buffer
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Are there comments to aid in an attempt to clarify this article and to include other oxygen buffers? Some thoughts about changes follow.
The Mineralogical Society of America Reviews in Mineralogy volume 25 provides documentation in support of possible changes, and a diagram based on that source, Oxygen_Buffers.jpg, has been uploaded into "Geological Diagrams" in Wikipedia Commons in preparation for revisions. A complementary entry should be created on "oxygen fugacity" or some such title to also utilize that diagram.
The introduction should note that, although the buffers provide a framework for discussion of rocks, the oxygen buffer reactions themselves are rarely useful in fixing oxygen fugacity within rocks (see treatment of "Some Common Misconceptions" in Frost's introduction to the MSA volume mentioned above). The wustite-magnetite buffer would be better presented without the complications of carbon and CO, and such more complicated reactions could be better discussed in an entry on oxygen fugacity. It would be good to emphasize that Fe2/Fe3 in rocks is dependent upon the mineralogy as well as on redox state -- an example is the stability of the pyroxene endmember NaFeSi2O6 (all iron ferric) in the presence of native iron.
The section "Redox effects on sulfur fugacity" could be revised, but regardless, it would be better in a separate entry devoted to oxidation of sulfides to sulfates. As discussed in the entry for pyrite, iron in pyrite is nominally ferrous, as is much but not all of the iron in pyrrhotite. One can write a reaction between pyrite, pyrrhotite, and sulfur to constrain sulfur fugacity, but it has little to do with redox reactions, as oxygen is involved only because of the small and variable ferric iron in pyrrhotite. Indeed, pyrrhotite is oxidized more easily than pyrite, perhaps for kinetic reasons related to vacancies in the structure, and both native sulfur and marcasite are common products of the oxidation, but not pyrite, according to Jambor (2003, in Mineral Association of Canada Short Course Series volume 31). A separate entry for sulfur fugacity and sulfide-sulfate reactions not only would be appropriate, but also it would be valuable because of environmental problems and ore deposits. Omphacite 23:03, 21 December 2006 (UTC)
