Analysis of Fe segregation in the octahedral sheet of bentonitic illite-smectite by means of FTIR, Al-27 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 calculatio ns by reverse Monte Carlo simulations to study cation distribution in the o ctahedral sheet, FTIR was used to determine the proportion of different cat ion pairs bound to OH groups utilizing the OH bending bands. The proportion s of cation pairs were used to generate Monte Carlo distributions of the oc tahedral cations in the layers. The calculated distributions were then anal yzed in comparison with the experimental Al-27 NMR results. All samples hav e shown a high dispersion of the Mg cations and a tendency for Fe segregati on which increased with progressing illitization. The illite-rich samples s howed 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.