Interstratified clays: The hidden actors of “kaolinitic” Vertisols

Abstract

The abundance of clay, presence of vertic horizon and shrink-swell cracks characterize the Vertisols. The presence of expansive 2:1 phyllosilicates is common in these soils. Vertisols with a predominance of kaolinite, differ from the modal mineralogy described for this soil class. The existence of kaolinitic assemblages in Vertisols suggests the possible occurrence of interstratified phases composed of layers of expansive 2:1 phyllosilicates. This study aimed to investigate the mineral assemblage of a Vertisol, supposedly kaolinitic. One soil profile was analyzed by the combination of analytical techniques, together with the computational modeling of oriented X-ray diffraction (XRD) patterns, to verify the presence of interstratified layers and to better understand the relationship between the presence of kaolinite and vertic features in the soil. The soil displayed common morphological, physical and micromorphological attributes of Vertisols. The XRD pattern of the parent material presented carbonates and traces of quartz and some mineral from the mica group. In the clay fraction of soil, the presence of a kaolinite mineral was recorded. The d001-value, with a maximum at 0.74 nm, and the asymmetry of these reflections in the low angle region indicated that kaolinite does not occur as a discrete phase. The XRD pattern of oriented clay aggregates modeling procedure confirmed the existence of irregular interstratified (R0) kaolinite-smectite (K-S). There is a K-S phase rich in kaolinite layers (∼ 90 % K). The COLEstd. expansibility test values were moderated (0.05 cm cm⁻¹), compared to typically smectite-rich Vertisols. The presence of K-S phases suggests a sequential process of gradual mineral transformation from smectite to kaolinite via K-S, influenced by the past and current climate conditions.