Analysis of Fe segregation in the octahedral sheet of bentonitic illite-smectite by means of FTIR, 27Al MAS NMR and reverse Monte Carlo simulations

A series of bentonite illite/smectite with a range of Fe content (0.08-0.95 atoms per half formula unit) and illite composition (3-97 percent illite) has been analyzed by means of FTIR Al-27 MAS NMR and theoretical calculations by reverse Monte Carlo simulations to study cation distribution in the octahedral sheet, FTIR was used to determine the proportion of different cation pairs bound to OH groups utilizing the OH bending bands. The proportions of cation pairs were used to generate Monte Carlo distributions of the octahedral cations in the layers. The calculated distributions were then analyzed in comparison with the experimental Al-27 NMR results. All samples have shown a high dispersion of the Mg cations and a tendency for Fe segregation which increased with progressing illitization. The illite-rich samples showed medium- or long-range ordering of the octahedral cations. The minimum radius for the wipeout sphere of the Al-27 NMR signal around Fe atoms is 7.5 Angstrom, for our working conditions of 11.7 T.