Structural characterization of the clay mineral illite-1M

This work reports the structural characterization of illite-1M from northern Hungary, with the first attempt to refine the structure model and locate the interlayer water molecule. Structural characterization was accomplished using state-of-the-art analytical methods available for clays. The results illustrate the status of techniques for clay structure determination, as well as providing a structural model for illite. The chemical formula for the illite-1M under investigation can be written as K_0.78Ca_0.02Na_0.02(Mg_0.34Al_1.69Fe^III_0.02)[Si_3.35Al_0.65]O₁₀(OH)₂·nH₂O. Structure simulations with WILDFIRE yielded a model with 30% of cis-vacant layers and an expandability percentage of 10%. The value of the percentage of expandability was confirmed with NEWMOD, with which the best simulation was obtained with 90% of di-octahedral mica with K (80% site population) in the interlayer region and 10% of expandable layers. The best structure simulation obtained with DIFFaX was also obtained with a population of K atoms of 80%, six cells along c (in agreement with the results of a transmission electron microscopy study) and an average dimension of the particles in the ab plane of 300 nm. Besides the determination of the basic structure unit (the results are consistent with those obtained with the local information provided by a fit of the pair distribution function data) and the model of disorder, refinement with DIFFaX⁺ allowed the calculation of a possible position for the interlayer water molecule. Although physically sound, both the observed tetrahedral layer corrugation and the location of the water molecule need further experimental evidence, because the final fit of the observed pattern is still imperfect. The reasons for this misfit are discussed.