Influence of crystal structure defects on the small-angle neutron scattering/diffraction patterns of clay-rich porous media

Analysing the structure and microstructure of compacted swelling clay minerals is important because of the applications of these minerals in engineering and environmental sciences. Given the typical sub-micrometre size of the particles and pores in clays, small-angle scattering techniques are well suited for such analysis. Interpretation of the intensity patterns, however, remains complex, especially in the intermediate region between the first Bragg peak and the small-angle range. In this study, theoretical small-angle neutron scattering and neutron diffraction patterns are calculated for three-dimensional virtual porous media representative of packed swelling clay particles (i.e. 0.1-0.2 mu m size fraction of vermiculite). This packing represents the distribution of the size, shape and particle orientation of a bulk vermiculite sample, for which experimental scattering/diffraction patterns were also collected. It was found that a good fit between the experimental and calculated scattering/diffraction profiles can be obtained only if the presence of crystal-structure defects in the particles is considered. The existence of such defects was supported by transmission electron microscopy analysis. Their influence on power law exponents extracted from intensity profiles is assessed in detail. The analysis is further extended to the influence of mineral dehydration and particle orientation on the intensity profiles. This work shows that using virtual porous media as toy models makes it possible to evaluate the roles of different microstructural parameters in the extent of variation of power law exponents. Such knowledge can be used for better interpretation of small-angle scattering data of natural compacted swelling clay-rich media.