Cryptomelane formation from nanocrystalline vernadite precursor: a high energy X-ray scattering and transmission electron microscopy perspective on reaction mechanisms.
BACKGROUND
Vernadite is a nanocrystalline and turbostratic phyllomanganate which is ubiquitous in the environment. 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 temperature and ambient pressure, in the dark) and over a time scale of ~10 years was monitored using high-energy X-ray scattering (with both Bragg-rod and pair distribution function formalisms) and transmission electron microscopy.


RESULTS
Migration of Mn(3+) from layer to interlayer to release strains and their subsequent sorption above newly formed vacancy in a triple-corner sharing configuration initiate the reaction. Reaction proceeds with preferential growth to form needle-like crystals that subsequently aggregate. Finally, the resulting lath-shaped crystals stack, with n × 120° (n = 1 or 2) rotations between crystals. Resulting cryptomelane crystal sizes are ~50-150 nm in the ab plane and ~10-50 nm along c*, that is a tenfold increase compared to fresh samples.


CONCLUSIONS
The presently observed transformation mechanism is analogous to that observed in other studies that used higher temperatures and (or) pressure, and resulting tectomanganate crystals have a number of morphological characteristics similar to natural ones. This pleads for the relevance of the proposed mechanism to environmental conditions.
BRGM, 3 Avenue Guillemin, 45060 Orléans Cedex 2, France.
Sylvain
S
Grangeon
Univ. Grenoble Alpes, ISTerre, 38041 Grenoble, France ; CNRS, ISTerre, 38041 Grenoble, France.
Alejandro
A
Fernandez-Martinez
ICMN-CNRS-Université D'Orléans, 1b rue de la Férollerie, 45071 Orléans Cedex 2, France.
Fabienne
F
Warmont
Laboratoire de Physique des Solides, Université Paris-Sud, CNRS, UMR 8502, 91405 Orsay Cedex, France.
Alexandre
A
Gloter
BRGM, 3 Avenue Guillemin, 45060 Orléans Cedex 2, France.
Nicolas
N
Marty
ESRF-The European Synchrotron, 71 avenue des Martyrs, Grenoble, France.
Agnieszka
A
Poulain
Univ. Grenoble Alpes, ISTerre, 38041 Grenoble, France ; CNRS, ISTerre, 38041 Grenoble, France.
Bruno
B
Lanson
Bragg rod; Cryptomelane; High-energy X-ray scattering; Pair Distribution Function; Phyllomanganate; Tectomanganate; Transmission electron microscopy; Vernadite; X-ray diffraction; δ-MnO2