Interstratified Illite‐Hydroxy‐Interlayered Smectite—A Disorder Structure Model for Quantification Using Rietveld Refinement

ABSTRACT Background The acidic weathering of soils is known as a problem for decades. It leads to the deterioration of essential soil properties, including soil fertility, water, or cation buffering capacity due to solution and solid storage of Al 3+ by polymerization in the interlayer of (former) swellable clay minerals. These Al 3+ ‐intercalated minerals/phases are known as hydroxy‐interlayered minerals (HIM). X‐ray diffraction, especially in combination with Rietveld refinement, is a powerful tool to quantify HIM and characterize the soil. Disorder structure models for hydroxy‐interlayered smectite (HIS) exist, but in soils, discrete smectite is less common than interstratifications like illite‐smectite. The process of hydroxy‐interlayering turns the latter into illite‐HIS (Ilt‐HIS), an interlayering of micaceous (illitic) layers with HIS layers and still swellable smectitic layers. Aims The aim of the present study was to develop disorder structure models for Ilt‐HIS, taking into account different states (air‐dried or ethylene‐glycol saturated) and different Reichweite values typically present in soils (R0, R1, and R3). Results These models were tested on real Al‐treated samples (ISCz–1 as R1, illitic clay from Király‐hegy near Sárospatak, Hungary as R3) or verified by comparison with independently calculated/simulated diffraction patterns (R0). Physical mixtures of selected R1 and R3 Ilt‐HIS samples with corundum were quantified, and the models validated by results deviating ≤5% w/w from actual values. Where higher deviations were present, these were explained by the presence of x‐ray amorphous phases as detected by Fourier‐transformed infrared spectroscopy and simultaneous thermal analysis. For both R1 and R3 samples, the hk lines did not differ significantly with increasing Al treatment, indicating no significant change of the stacking and ordering. Conclusions The created structure models can be used to quantify Ilt‐HIS samples (R0, R1, and R3) in soil.