Vernadite is a nanocrystalline and turbostratic phyllomanganate which is ubiquitous in the environment. Similar to birnessite, its layers are built of (MnO6)8− octahedra connected through their edges and frequently contain vacancies and (or) isomorphic substitutions. Both create a layer charge deficit that can exceed 1 valence unit per layer octahedron and thus induces a strong chemical reactivity. In addition, vernadite has a high affinity for many trace elements and possesses a redox potential that allows for the oxidation of redox-sensitive elements or molecules. As a result, vernadite acts as a sink for many trace metal elements. The present talk will report on the evolution of vernadite crystal chemistry as a function of pH using a variety of analytical techniques including modelling of powder X-ray diffraction patterns. In particular, the number of vacant layer sites, that of layer and interlayer Mn3+, the resulting average Mn oxidation degree, the Na/Mn ratio, unit-cell parameters, and crystallite size will be determined as a function of equilibration pH. Reactivity of structurally contrasted samples with respect to metal sorption or to their conversion to tunnel structures will be reported next.

Evolution of vernadite crystal structure as a function of pH: Implications for reactivity

255th National Meeting (2018)

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