Transport and deposition of iron

The migration of trivalent iron in dissolved form is extremely limited. 

Ionic form of transport. This is possible only in very acid solutions (pH 3), the long existence of which is not likely at the Earth s surface due to interaction with rocks, due to dilution by surface waters, and due to the buffer effect of carbonate and silicate equilibria controlling the pH in the ocean. 

The appearance of acid solutions in the weathered layer cannot be substantiated physicochemically even if a high content of carbon dioxide in the atmosphere is assumed with =1 bar the pH of the saturated aqueous solution (pure water with no impurities of other anions and cations) is only 3.9—a value clearly insufficient for intensive removal of ferric iron in the form of stable Fe FeOH, or Fe(OH)2. Moreover, when pure (rain) carbonic acid waters react with rocks the pH value increases as the saturation with dissolved cations (mainly Mg, Na , K, and Ca ) increases. The constant presence of other volatiles of the acid fume group in the Precambrian atmosphere is not very likely, due to their high solubility 

Acid volcanic waters occur everywhere in recent volcanic areas, and surely they were just as common in the Precambrian. However, these waters contain mainly ferrous iron, not ferric. Mixing with oxygen-bearing surface waters leads to dilution, neutralization, oxidation of Fe , and subsequent deposition of Fe in the form of hydroxide. 

Colloidal form of transport of ferric iron in the Precambrian, apparently, was less important than in later geologic epochs. On the basis of an examination of the known methods of formation of colloidal solutions, it can be assumed that condensation phenomena predominate in the geochemical processes, and dispersion and peptization play a secondary role. Consequently one of the conditions for obtaining colloidal solutions is the presence of ionic solutions as an obligatory intermediate stage in the cycle weathering transport deposition. 

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Crerar, D.A., Means, J.L., Yuretich, R., Borcsik, M., Amster, J., and Hastings, D., 1981, Hydrochemistry of the NJ Coastal Plain II transport and deposition of iron, aluminum, dissolved organic matter Chem. Geol., v. 31, pp. 23-44. 

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