Dissolution experiments of Mn and Fe under natural conditions from fresh basalt, weathered basalt and Mn laterite by different organic acids show that Mn is highly enriched over Fe in solutions from the weathered rocks but that more Fe than Mn is dissolved from the fresh basalt. The enrichment of Mn is caused by sparingly soluble Fe-oxides and hydroxides and more soluble Mn-oxides. In addition from the weathered rocks the Mn concentration dissolved by the organic acids is up to 1000 times higher than in inorganic solutions. Mn enrichment is caused by acid attack, organic reduction of Mn4+ to Mn2+ and complexing by the organic acids. The complexed Mn is not attacked as easily by oxidation as free Mn ions. Higher concentrations of manganese in the organic dissolved stage can therefore be transported by rivers over greater distances.
Organic complexed Mn, derived from lateritic weathered rocks may therefore contribute to the formation of low iron marine sedimentary Mn deposits.