Origin of the two quadrupole doublets in NAu-1 nontronite

Mössbauer spectra of octahedral Fe 3+ in NAu-1 nontronite, saturated with Na + , K + , NH4 + , Ca 2+ and La 3+ have been analysed as two quadrupole doublets. The interlayer cations were found to affect the mean isomer shift, the quadrupole splitting and the spectral area. By considering the statistically significant nearest cation neighbour coordination environments, the inner doublet has been assigned to an octahedral Fe 3+ surrounded by three Fe 3+ in the octahedral sheet and eight Si 4+ in the tetrahedral sheets. The outer doublet is due to the same octahedral coordination, but combined with [7Si 4+ , 1Al 3+ ] and [6Si 4+ , 2Al 3+ ]. © 2010 IOP Publishing Ltd.