[j_juarez]

Question:  Iron formations that originally contained fayalite but have been oxidized and metamorphosed at high pressure and temperature can contain the olivine, laihunite, with iron that is fully oxidized.  What is the exchange reaction that must occur and the composition of laihunite?

Here is what I have:

Fe⁺⁺2SiO₄(aq)(fayalite) + Fe⁺⁺Fe⁺⁺⁺2(SiO₄)₂(laihunite) = Fe⁺⁺2SiO₄ (fayalite) + Fe⁺⁺Fe⁺⁺⁺2(SiO₄)₂(aq)(laihunite)

[hmx9123]

So, you're basically saying that the aqueous solution of fayalite precipitates fayalite and dissolves laihunite?  Are you sure that the fayalite being oxidized and metamorphosed at high temperatures isn't a reaction with oxygen?  I would begin by looking up the actual formulae of olivine and laihunite to make sure you know what you're dealing with.  Although I'm not a geologist, I'd suspect that these are mixed-valence state iron silicates, or all iron (III) silicates.

[j_juarez]

I suspected that the reaction may be between fayalite and iron oxide.  Olivine's formula is Mg2Sio4 with the magnesium having 2+ charge.  Fayalite is Fe2Sio4 iron is 2+.  Laihunite is FeFe2(SiO4)2 with the first iron being 2+ and the second iron 3+.