Investigation of clay mineralogy in a temperate acidic soil of a forest using X-ray diffraction profile modeling: Beyond the HIS and HIV description

Abstract

The crystal structures as well as the amount of hydroxy-interlayered clay minerals resulting from the Al-hydroxylation process in temperate acidic soil developed in a forest were studied by modeling experimental X-ray diffraction (XRD) patterns. The <2μm clay fraction of this soil was separated into four sub-fractions of different particle sizes (<0.05, 0.05–0.1, 0.1–0.2, and 0.2–2μm), and XRD patterns were obtained after different treatments of the samples (i.e., calcium saturation and air-drying, ethylene-glycol solvation, or potassium saturation and heating at 110°C). The qualitative analysis of the experimental XRD patterns revealed the presence of hydroxy-interlayered-smectite (HIS) and hydroxy-interlayered-vermiculite (HIV) phases commonly considered in the description of clay mineralogy in acidic soil environments. Quantitative information derived from the fitting procedure for the different sub-fractions allowed for the determination of the complex mineralogy of the <2μm clay fraction through the identification and quantification of ten clay phases. The results revealed that the HIS and HIV clay phases considered so far in the literature actually consist of three-component randomly interstratified mixed-layer minerals (MLMs) with contrasting relative proportions of smectite, hydroxy-interlayers (HI) and illite layers. Moreover, the quantitative results derived from this study showed that these two MLMs account for less than 50% of the total HI layer abundance. In that regard, most HI layers are present in illite-rich MLMs with poorly resolved XRD reflections. The contrasting behaviors of smectite and HI layers in the coarsest 0.2–2μm sub-fraction compared to those in the finer sub-fraction indicated the possible increase of the layer charge of the expandable layers in the coarsest sub-fraction. Finally, it was shown that the detailed structural analysis derived from the methodology used in the present study is conducive for estimating the degree of expandable layer transformation in natural soil samples while reconciling the specific behaviors of expandable and HI layers from natural soil with those described by laboratory experiments performed on monomineralic samples.