Analysis of cation distribution in the octahedral sheet of dioctahedral 2 : 1 phyllosilicates by using inverse Monte Carlo methods

An inverse Monte Carlo (MC) method was developed to determine the distribut ion of octahedral cations (Al3+, Fe3+, and Mg2+) in bentonite illite-smecti te (I-S) samples (dioctahedral 2:1 phyllosilicates) using FT-IR and Al-27 M AS NMR spectroscopies. FTIR allows determination of the nature and proporti on of different cation pairs bound to OH groups measuring the intensities o f OH-bending bands. Al MAS NMR data provide information about cation config uration because Al-27 MAS NMR intensity depends on Fe distribution. MC calc ulations based on FT-IR data alone show Fe segregation by short-range order ing (Fe clusters within 9 to 15 Angstrom from a given Fe atom). Fe segregat ion increases with illite proportion. MC calculations based on IR and Al-27 NMR simultaneously yield similar configurations in which Fe clusters are s maller. The latter calculations fail to build appropriate cation distributi ons for those samples with higher number of illite layers and significant F e content, which is indicative of long-range Fe ordering that cannot be det ected by FT-IR and Al-27 MAS NMR. The proportion of Mg-Mg pairs is negligib le in all samples, and calculations, in which the number of Mg atoms, as se cond neighbours, is minimised, create appropriate configurations.