Cryptomelane formation from nanocrystalline vernadite precursor: a high energy X-ray scattering and transmission electron microscopy perspective on reaction mechanisms

Sylvain Grangeon (1) , Alejandro Fernandez-Martinez (2) , Fabienne Warmont (3) , Alexandre Gloter (4) , Nicolas C.M. Marty (1) , Agnieszka Poulain (5) , Bruno Lanson (2)

1 BRGM - Bureau de Recherches Géologiques et Minières (BRGM)
2 ISTerre - Institut des Sciences de la Terre
3 ICMN - Interfaces, Confinement, Matériaux et Nanostructures
4 LPS - Laboratoire de Physique des Solides
5 ESRF - European Synchrotron Radiation Facility

Background: Vernadite is a nanocrystalline and turbostratic phyllomanganate which is ubiquitous in the environ‑ ment. 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 (e.g., Co, Ni, and Zn) and possesses a redox potential that allows for the oxidation of redox sensitive elements (e.g., As, Cr, Tl). As a result, vernadite acts as a sink for many trace metal elements. In the environment, vernadite is often found associated with tectomanganates (e.g., todorokite and cryptomelane) of which it is thought to be the precursor. The transformation mechanism is not yet fully understood however and the fate of metals initially contained in vernadite structure during this transformation is still debated. In the present work, the transformation of synthetic vernadite (δ‑MnO2) to synthetic cryptomelane under conditions analogous to those prevailing in soils (dry state, room tempera‑ ture and ambient pressure, in the dark) and over a time scale of ~10years was monitored using high‑energy X‑ray scattering (with both Bragg‑rod and pair distribution function formalisms) and transmission electron microscopy.

Domaines: Géochimie

Sylvain Grangeon, Alejandro Fernandez-Martinez, Fabienne Warmont, Alexandre Gloter, Nicolas C.M. Marty, et al.. Cryptomelane formation from nanocrystalline vernadite precursor: a high energy X-ray scattering and transmission electron microscopy perspective on reaction mechanisms. Geochemical Transactions, 2015, 16 (1), 16 p. ⟨10.1186/s12932-015-0028-y⟩. ⟨hal-01356808⟩