INTERPRETATION OF THE NONTRONITE-DEHYDROXYLATE MöSSBAUER SPECTRUM USING EFG CALCULATIONS

Abstract

Dehydroxylated Garfield nontronite has been studied using Mössbauer spectroscopy. According to literature, in dehydroxylated Fe3+-rich dioctahedral 2:1 phyllosilicates (celadonites, glauconites and nontronites), the octahedral cations migrate from cis- into trans-sites with the formation of five-fold coordination of the former cis- and trans-octahedra. Therefore, the two main fitted doublets with quadrupole splittings, Δ, of 0.906 and 1.392 mm/s are supposed to be related to Fe3+ in the former cis- and trans-octahedra. To assign these doublets to this or that positions, analysis of Pauling's bond strength (PBS), structural modeling and EFG calculations were performed. The calculated quadrupole splittings for Fe3+ located in the former cis- and trans-octahedra in the nontronite-dehydroxylate structure are equal to 1.295 and 1.026 mm/s, respectively. On the basis of the calculation results, the quadrupole doublet with smaller Δ should be assigned to the former trans-octahedra whereas the doublet with larger Δ should be assigned to the former cis-octahedra. The calculated EFG parameters proved to be independent of the mode in the layer stacking, which confirms the major role of the Fe3+ nearest environment in the formation of the different EFGs at Fe nuclei. Possible disruptions of two-dimensional continuity of dehydroxylated octahedral sheets that may be a reason for superparamagnetic effects in nontronite-dehydroxylates at low temperatures are discussed in terms of the structural model of dehydroxylated Fe-rich dioctahedral 2:1 phyllosilicates. © 2006 E. Schweizerbart'sche Verlagsbuchhandlung, D-70176 Stuttgart.