Structural characterization of the clay mineral illite-1M

Illite is one of the major clay minerals found on the Earth’s surface, constituent of the soils, and araw material used for a variety of industrial applications. It is an Al-K mica-like, non-expanding,dioctahedral mineral that crystallizes in the monoclinic system. Its structure is very similar to that of2:1 mica, where two tetrahedral sheets sandwich an octahedral one, to build up the T-O-T sheet. Katoms, and possibly water molecules are hosted in the interlayer region.This work presents the results of the structural characterization of illite-1M from Northern Hungarywith the first attempt to refine the structure model and locate the interlayer water molecules. Thestructural characterization was accomplished using current state of the art analytical methodsavailable for the structural characterization of clays. The results illustrate the status of techniquesfor clay structure determination, as well as a structural model for illite.A chemical formula for the illite-1M under investigation can be written as:K0.78Ca0.02Na0.02(Mg0.34Al1.69Fe’’’0.02)[Si3.35Al0.65]O10(OH)2×nH2O.Structure simulations with WILDFIRE yielded a model with 30% of cis-vacant layers andexpandability percentage of 10 %. The value of the percentage of expandability was confirmed withNEWMOD whose best simulation was obtained with 90% of di-octahedral mica, 10% ofexpandable layers, and K = 0.8 in the interlayer region. The best structure simulation obtained withDIFFaX was obtained with a population of the K atoms of 80%. To obtain the best fit, 6 cells alongc (in agreement with the results of the TEM study) and an average dimension of the particles in thea-b plane of 300 nm were used.Besides the determination of the basic structure unit (the results are consistent with those obtainedwith the local information provided by the fit of the PDF data) and the model of disorder, therefinement with DIFFaX+ made it possible attempt to locate the interlayer water molecule and torefine its site population. Although physically sound, both the observed tetrahedral layercorrugation and the location of the water molecule need further experimental support, because thefinal fit of the observed pattern is still imperfect. The reasons for this misfit are thoroughlydiscussed.