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Crystal structure of Ni-sorbed synthetic vernadite: a powder X-ray diffraction study

Mineralogy and Environments Group, LGCA, Maison des Géosciences, BP53, Université Joseph Fourier-CNRS, 38041 Grenoble Cedex 9, France

^* E-mail: Sylvain.Grangeon{at}obs.ujf-grenoble.fr

Vernadite is a nanocrystalline turbostratic phyllomanganate containing Ni, and is widespread in surface environments and oceanic sediments. To improve our understanding of Ni uptake in this mineral, two series of analogues of vernadite (-MnO₂) were prepared with Ni/Mn atomic ratios of 0.002-0.105 at pH4 and 0.002-0.177 at pH 7. Their structures were characterized using X-ray powder diffraction (XRD). The -MnO₂ nano-crystals are essentially monolayers with coherent scattering domains sizes of 10 Å perpendicular to the layering and 55 Å within the layer plane. For Ni/Mn < 0.01, the layer charge deficit is apparently balanced entirely by interlayer Mn, Na and protons. At higher Ni/Mn, Ni occupies the same site as interlayer Mn above and below vacant sites within the MnO₂ layer and at sites along the edges of the layer. However, the layer charge is balanced differently at the two pH values. At pH 4, Ni uptake is accompanied by a reduction in structural Na and protons, whereas interlayer Mn remains strongly bound to the layers. At pH 7, interlayer Mn is less strongly bound and is partially replaced by Ni. The results of this study also suggest that the number of vacant octahedral sites and multi-valent charge-copmpensating interlayer species are underestimated by the currently used structure models of -MnO₂.

KEYWORDS: -MnO₂, vernadite, birnessite, Mn oxide, turbostratic structure, XRD, X-ray diffraction, crystal chemistry, Ni sorption