Manganese oxides play a pivotal role in redox and adsorption processes in soils, groundwater, oceanic and aquatic chemical systems. In laboratory controlled studies of these processes, synthetically prepared manganese oxides are commonly employed as "model" materials in order to simplify the inherent complexity of natural systems. The buserite/birnessite group of minerals being of mixed manganese valency and generally disordered structure, is one of the most important and has been used as model maganese oxide material. Some recent studies of electron transfer (Crowther et al., 1983; Xyla et al., 1992; Manceau and Charlet, 1992) have been conducted with Na-buserite, however the relatively poor understanding of its structure remains a principle limitation in obtaining a fundamental understanding the mechanisms of these processes. In particular, knowledge of the spatial distribution of lower valence manganese (Mn(III) and Mn(II)), as well as the presence and distribution of lattice cation vacancies, would greatly enhance the understanding of these reactions at the atomic level.
The dehydration of Na-buserite leads to the formation of the better known Na-birn essite phase and is associated with a change in the interlayer spacing from l0Å to 7Å, a difference which is approximately equal to the water molecule diameter (2.8Å). A similar collapse of the interlayer distance occurs upon lowering the pH of Na-buserite suspensions to acidic conditions, typical of the conditions under which adsorption and redox processes are studied. With the exception of the study by Giovanoli et al. (1970) there has been very little investigation of the structure of the low pH form of birnessite (H-birnessite). From a structural point of view the conversion from Na- to H-birnessite is accompanied by a transformation from a monoclinic unit cell at high pH to a hexagonal unit cell at low pH.
The structure of these two forms, and the mechanism of the acidic transformation of Na-birnessite, was investigated by combining several approaches including: chemical studies, X-ray and selected area electron diffraction (XRD, SAED), and powder plus polarized EXAFS experiments. Na-birnessite consists of vacancy-free manganese octahedral layers. The departure from the hexagonal to the monoclinic symmetry is caused by the Jahn-Teller distortion associated to Mn(III) for Mn(IV) substitutions. Mn(III) cations are ordered in rows parallel to  and separated from each other along  by two Mn(IV) rows. The maximum value of the layer negative charge due to Mn(III) for Mn(IV) substitutions is equal to 0.333 v.u per Mn atom and is obtained when Mn(III)-rich rows are free of tetravalent manganese. The structural formula proposed for these Mn(III)-rich Na-birnessite layers is
In Mn(III) rows, tetravalent and trivalent manganese atoms are ordered with a Mn(IV) periodicity of 6b. This periodicity leads to the formation of modulated structures, which are responsible for the appearence of the satellites in the SAED pattems of Na-birnessite.
The structure of H-birnessite consists of hexagonal octahedral layers containing only Mn(IV) and vacancies. The distribution of layer vacancies is inherited from the former Mn(III) distribution in Na-birnessite as attested by the presence of weak super-reflections in SAED patterns. Interlayer Mn(IV) and Mn(II) cations are located either above or below the layer vacant sites, and result from the disproportionation of former Mn(III) layer cations. The driving force of the Na- to H-birnessite transformation is the destabilization of Mn(III) rows at low pH.
It will be shown in a second part of the presentation how our present understanding of the birnessite structure can be used to assist in the interpretation of the adsorption and electron transfer processes which occur on this mineral.
Crowther, D.L., Dillard, J.G. & Murray, J.G., Geochim. Cosmochim. Acta 1399-1403 (1983).
Giovanoli, R., Stahli, E. & Feitnecht, W., Helvetica Chimica Acta 53, 454-464 (1970).
Manceau, A. & Charlet, L., J. Colloid Interface Sci. 148, 443-458 (1992).
Xyla, A.G. et al., Langmuir 8, 95-103 (1992).