In the process of serpentinisation, protons act as oxidising agents for ferrous ions, which results in the production of dihydrogen gas. How is that possible?
Dihydrogen being a strong reductor, I woud expect this reaction spontaneously going the opposite way.
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Jan Doggen
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Chris D
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2Maybe you should consider asking it in http://chemistry.stackexchange.com/ – arkaia May 29 '15 at 13:44
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@aretxabaleta I disagree - this is completely related to earth science! As for the question, this might be extremely relevant: Thermodynamic constraints on hydrogen generation during serpentinization of ultramafic rocks. This might be paywalled though, unless you have an institutional subscription. – Gimelist May 30 '15 at 11:45
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@Michael, I don't think it does not belong here. My suggestion comes from the fact that it has not received as much attention as some other questions and it does not have an answer yet. I know that chemistry currently gets more action than earth science and thus, my suggestion. I think it is fine in this site if the author is ok with it. – arkaia May 31 '15 at 01:17
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Under normal circumstances your suggestion might be correct, but bear in mind that the serpentinization reaction takes place under very negative Eh, and very high pH conditions (pH 12 or more). This abnormal combination is right on the boundary of water-hydrogen stability conditions. In hyperalkaline springs, which result from the reaction water + peridotite = serpentine, there is hardly a free molecule of oxygen left in solution.
Gordon Stanger
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This answer delves a bit more into the reaction mechanism, which involves two parts: the formation of metal hydroxide and the conversion of iron hydroxide to magnetite with the evolution of hydrogen. The key feature is that ferrous ions are really stable only in acidic or neutral conditions, whereas the reactions in serpentization inherently make the environment basic.
Hydroxide formation
Serpentization occurs in mafic rock, which inherently produces basic products when decomposed by water. The calcium, magnesium and iron displaced from the rock emerge as hydroxides. Even where carbon dioxide is present and converts the calcium to relatively insoluble $\text{CaCO}_3$, magnesium hydroxide is more resistant to this process and thus appears as the mineral brucite. This magnesium hydroxide is still a strong enough base to also favor the iron forming a hydroxide rather than remaining in solution as ions, setting the stage for ...
The Schikorr reaction
Iron in mafic rocks is predominantly in the form of iron(II) and thus its hydroxide would be ferrous hydroxide, $\text{Fe(OH)}_2$. Unlike dissolved ferrous ions, this hydroxide can displace hydrogen from water via the Schikorr reaction, forming magnetite ($\text{Fe}_3\text{O}_4$) with evolution of hydrogen:
$\text{3 Fe(OH)}_2 \to \text{Fe}_3\text{O}_4 + \text{H}_2 + 2\text{H}_2\text{O}$
This reaction is well known not only in serpentization but also as a component reaction in the rusting of iron.
Oscar Lanzi
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