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Interpretation of the nontronite-dehydroxylate Mössbauer spectrum using EFG calculations

¹ Geological Institute of the Russian Academy of Science, Pyjzhevsky Street 7, 119017 Moscow, Russia
² Moscow State University, Department of Physics, Vorobyovy Gory, 119899, Moscow, Russia

^* Corresponding author, e-mail: dainyak{at}ginras.ru

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, , 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 should be assigned to the former trans-octahedra whereas the doublet with larger 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.

Key-words: Mössbauer spectroscopy, EFG calculations, nontronite-dehydroxylate, structural modelling.

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