STRUCTURE OF SYNTHETIC MONOCLINIC NA-RICH BIRNESSITE AND HEXAGONAL BIRNESSITE .2. RESULTS FROM CHEMICAL STUDIES AND EXAFS SPECTROSCOPY

Solution chemical techniques were used to study the conversion of synt hetic Na-rich buserite (NaBu) to hexagonal (H+-exchanged) birnessite ( HBi) at low pH. The low-pH reaction is broadly characterized by the ex change of structural Na+ with solution H+ and the partial loss of Mn2 to the aqueous phase. The desorption of Na+ in two temporally distinc t steps indicates the existence of two types of binding sites for this cation. Mn2+ appears to originate from a partial disproportionation o f Mn3+ in the NaBu layers, according to the sequence Mn-layer(3+) + Mn -layer(3+) --> Mn-layer(4+) + Mn-layer(2+) --> Mn-layer(4+) + Vacancy + Mn-aq(2+). EXAFS measurements on Na-rich birnessite (NaBi) show that this mineral is primarily a layered structure formed by edge-sharing MnO6 octahedra, with no evidence for triple-corner (TC) sharing Mn. HB i is significantly different with strong evidence for TC-sharing Mn an d therefore layer vacancies. The relative numbers of edge (E)-sharing and TC-sharing neighbors determined from EXAFS measurements on HBi is consistent with SAED results (Drits et al. 1997), which suggest that t he layer vacancies are restricted to every third row of Mn cations, wi th 50% of the Mn sites along these rows vacant. The density of vacanci es in the entire layer is therefore one in six of layer Mn sites. Pola rized EXAFS measurements on orientated films of NaBi and HBi confirm t he absence of TC-sharing Mn in NaBi and indicate that Mn adsorbed at l ayer vacancy sites in HBi at pH 4 is dominantly Mn3+. The intensity of the TC-sharing contribution to the Mn EXAFS spectra of HBi samples in creases with increasing pH from pH 2 to 5, and supports a mechanism of formation involving both the direct migration of layer Mn3+ to interl ayer TC-sharing positions and re-adsorption of Mn2+ from solution onto layer vacancy sites. The migration of Mn3+ cations into the interlaye r releases the steric strain associated with the Jahn-Teller distortio n of these octahedra. This model of the NaBu-to-HBi conversion explain s the transformation from orthogonal to hexagonal layer symmetry, resp ectively, as reported by Drits et al. (1997). Analysis of the Zn EXAFS spectrum of Zn2+-exchanged birnessite shows that Zn2+ also occupies T C-sharing positions at layer vacancy sites. The results of this study strongly suggest that lattice cation vacancies are of critical importa nce in adsorption and electron transfer processes occurring at the sur face of this mineral.