Transformation of the phyllomanganate vernadite to tectomanganates with small tunnel sizes : Favorable geochemical conditions and fate of associated Co
Résumé
The present work uncovers the geochemical control on the nature (tunnel size) of the tectomanganates formed from layered precursors, and thus provides insights into the formation of Mn oxides in natural environments. Large tunnel sizes are favored under circum-neutral conditions, whereas low pH conditions favor the formation of tectomanganates with smaller tunnel sizes. Both the increased proportions of Mn(III) in vernadite/birnessite layers resulting from low pH conditions and the subsequent enhancement of Mn(III) disproportionation during subsequent transformation contribute to the formation of tectomanganates with smaller tunnel sizes. The fate of foreign elements during the phyllomanganate-to-tectomanganate mineral transformation is another important aspect of this mineral transformation, together with the impact of these elements on the transformation. Layered and tunnel Mn oxides have indeed a pivotal influence on the geochemical cycling of transition metals, including Co, that possess a strong affinity for these mineral species. The present experimental work shows that the formation of todorokite (3 × 3 tunnel size), hollandite (2 × 2), or nsutite (intergrown 1 × 1 and 1 × 2 fragments) is essentially unaffected by limited Co-enrichment (≤5 at.%) of the initial phyllomanganate structure. Higher Co contents reduce the content of Jahn-Teller distorted Mn(III) octahedra in layered precursor and hamper the phyllomanaganate-to-tectomanganate transformation. Finally, Co is retained in the structure of todorokite and hollandite during their formation under circum-neutral conditions whereas part (∼20%) of the Co present in layered precursors is expelled out of the framework and/or sorbed to nsutite formed under acidic conditions. This effect is induced by the reduced stability of Co(III) octahedra when the relative proportion of corner-sharing linkages increases. In turn, this effect influences Co structural incorporation in different Mn oxides and its potential release to solution.
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