Talk:Wüstite

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[edit] FeO != Wustite

The mantle does have 7.5% FeO as weight percentage. However, this is not present as wustite neccessarily. It is present as FeO components in olivine and pyroxene solid solution series. Rolinator 03:06, 15 August 2007 (UTC)

You sound like you know what you are talking about, but what you are saying does not seem reasonable to me (which does not mean you are wrong). Plagioclase is a solid solution with albite and anorthite endmembers, yet neither albite nor anorthite cease to exist by being components of the solid solution. So why would FeO cease to be wüstite by being a component of olivine and pyroxene solid solution series, unless special definitions are being applied whereby only pure FeO can be called wüstite? --Ben Best 05:01, 15 August 2007 (UTC)

The composition of magnetite is sometimes expressed as FeOFe₂O₃. Yet it is not a solid solution between wustite and hematite, it is a mineral by itself. When quoting a whole rock chemical analyses, the convention is to give percentages of FeO and Fe₂O₃, which gives you an idea of what redox state the rock is in. There will only be wustite as a mineral phase wheren there is no Fe₂O₃ in the rock.

Consider the mantle xenolith on the pyroxene page. It is composed primarily of pyroxene (orthopyroxene and clinopyroxene), olivine, chromite, etc. Pyroxene has a range of compositions, but if you work out the ionic balance of the major cations in the XYSi₂O₆ series, they are +2. Therefore, iron is present in Fe²⁺ state outside of wüstite, and within other minerals. This is also true of olivine, where iron is present in the 2+ valent state. Therefore, analyses which quote FeO 7.5% are quoting it as a chemical composition, not a mineralogical composition (otherwise known as normative mineralogy). Iron is also present in Fe2+ state in spinel minerals such as magnetite, chromite, etc.

Further, in order to have iron present as a pure oxide, outside of a spinel such as chromite, the rock needs to have enough iron and little enough other components in order to prevent forming minerals which contain iron. For example, it has to become enriched in Fe, but not oxygen, in order to convert Fe₂O₃ into FeO. Once enough FeO is created that there is too much to be hosted within olivine, and there is not enough oxygen to form magnetite, then you form wustite. Theoretically, this could happen with as little as 7.5% FeO, however you would need some bizarre chemical composition with lots of calcium, sodium, etc and very little silicon. Therefore, the mantle composition has too much silicon and oxygen to allow wustite to form - which is why it is such a rare mineral. I hope this explains things well enough. You may also wish to check out mineral redox buffer for a thorough explanation.Rolinator 05:38, 15 August 2007 (UTC)

Thank you for taking the trouble to give such a detailed explanation. Hopefully it will be of benefit to others who read this page. I think that I have reasonable grounds for having been confused and I think that others could easily be as afflicted. In the main text of this wüstite entry I see:

1. FeO.Fe₂O₃ + C --> 3FeO + CO

magnetite + graphite/diamond --> wüstite + carbon monoxide

ie, "FeO == wüstite". But in the context of normative mineralogy,

"FeO != wüstite" -- because you aren't really speaking of the mineral FeO. --Ben Best 06:50, 15 August 2007 (UTC)

Again you are confusing the FeO functional group in magnetite FeO.Fe₂O₃ as being "magnetite is made from one part wustite, one part hematite". Magnetite is one part magnetite, not a solid solution between wustite and magnetite.

Remember, a solid solution is notated (X,Y)O. Capisce? Therefore FeO.Fe₂O₃ != FeO (wüstite) + Fe2O3 (hematite).

Secondly, the equation FeO.Fe₂O₃ + C --> 3FeO + CO can also be expressed as the equilibrium equation.

If you understand equilibria, you'll realise that there is a back reaction to the wüstite-creating reaction above;

FeO.Fe₂O₃ + C <-- 3FeO + CO

This is a redox equilibrium, or in other words, oxygen fugacity. And, again, the redox conditions of the mantle generally do not favor wüstite. Again, read the text of the article: "until all Fe³⁺ ions are converted to Fe²⁺ (ie; via reduction of the oxygen fugacity of the system via addition of reducing agents such as native carbon) the oxide mineral assemblage (ie; the normative mineralogy) remains wüstite-magnetite". ie;

You seem seriously confused by the concept of expressing magnetite as FeO.Fe2O3. Maybe if you think of magnetite as Fe₃O₄, that will help? Maybe think of the above reactions as;

where there is an intermediate equilibrium state (a redox buffer assemblage) containing graphite, magnetite and wustite as separate minerals where wustite is NOT within magnetite.
Rolinator 00:57, 16 August 2007 (UTC)

Thanks for what is apparently a frustrated, but patient explanation, Rolinator. But you seem to have misunderstood the simplicity of my comment:

1. FeO.Fe₂O₃ + C --> 3FeO + CO

magnetite + graphite/diamond --> wüstite + carbon monoxide

ie, "FeO == wüstite".

I was referring ENTIRELY to the right side of the equation and to the equation of FeO with wüstite on the right side. It was a conceptual, not a quantitative reference. --Ben Best 03:52, 16 August 2007 (UTC)