Assessment of structural order indices in kaolinites: a multi-technique study including EXAFS

Abstract

This work presents a study of the structural properties on four representative kaolinite samples. Scanning Electron Microscopy (SEM), X-ray Diffraction (XRD), Thermochemical Analyses (DTA/TGA), and X-ray Near Edge Structure (XANES) and Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy measurements were carried out, and different empirical indices that quantify the structural order were assessed. Also, XRD patterns and EXAFS signals were compared to those corresponding to idealized defect-free structures calculated theoretically, to analyze the effects related to the structural disorder on real samples. The structural order at different scales (crystallographic, superficial, and local) was comprehensively examined for the four samples, and correlations between the order indices were performed and discussed, including the Debye-Waller factor obtained from Si K-edge EXAFS measurements. The proposed methodology shows that EXAFS is a suitable complementary technique that can provide valuable information about the Si local environments in natural kaolinites.