Cryptomelane formation from nanocristalline vernadite precursor.

International audienceVernadite is a nanocristalline and turbostratic phyllomanganate which is ubiquitous in the environment. Its layers, built of MnO6 octahedra connected through their edges, contain vacancies and (or) isomorphic substitutions, both creating a layer charge deficit that can exceed 1 valence unit per layer octahedron. In addition, vernadite has a high affinity for many trace metals (e.g., Co, Ni and Zn) and frequently contain heterovalent Mn cations which provides this mineral with the capacity to oxidize redox-sensitive trace elements (e.g., As, Se) and organic pollutants. As a result of these exceptional properties, vernadite controls the fate of many trace elements in soils and sediments. In the environment, vernadite is often found associated with tectomanganates (“tunnel”-like structures) such as cryptomelane, of which it is thought to be the precursor. A sound description of the vernadite-to-cryptomelane transformation, at the atomic scale, is mandatory to be able to understand and thus model the fate of metals initially present in vernadite structure. To contribute to a better understanding of this transformation, we have synthesized vernadite samples having various Mn4+/Mn3+ ratios (and thus various layer charge) and we have monitored their transformation, under conditions analogous to those prevailing in soils (dry state and ambient conditions, in the dark) over a time scale of ~10 years [1-2]. Initial samples were characterized using a combination of chemistry, thermogravimetric analyses and powder X-ray diffraction. Samples structural formula ranged between Na+0.06(H2O)0.30Mn3+0.19[Mn3+0.12Mn4+0.71Vac0.17O2] (where species under brackets form the layer – “Vac” stands for “layer vacancies”, and species on the left are in the interlayer space) and Na+0.27(H2O)0.30Mn3+0.10[Mn3+0.10Mn4+0.76Vac0.14O2]. Transformation was monitored using high-energy X-ray scattering (with both Bragg-rod and pair distribution function formalisms) and transmission electron microscopy (TEM and STEM). With time, layer Mn3+ was found to migrate in the interlayer, probably to reduce strains induced by its Jahn-Teller distorted coordination sphere. When the abundance of interlayer Mn3+ reached ~0.3 per layer octahedron, interlayer Mn3+ from adjacent layers were found to share their hydration sphere and to form cryptomelane domains