Hydroxy-interlayering in soil clay smectites: An indicator of climate change

Abstract

Hydroxy-interlayerig (HI) in soil clay smectites of Vertisols in Peninsular India is a common phenomenon. Presence of small to moderate amounts of HI also affects the properties of soils. Earlier the view was that pH higher than 7.5 in soils may be the reason for HI. However, this explanation did not work in soil clay smectites of sodic Vertisols of Anantapur district of Andhra Pradesh and Purna valley of Maharashtra as the smectites in these areas are not hydroxy-interlayered. This demanded investigation for possible source and reason of HI in the soil clay smectites. Clay smectites of some Vertisols from Maharashtra (Takarkheda, Seloo and Saikhindi series) had small (about 3-5% in Seloo and Saikhindi) to moderate (about 5-15% in Takarkheda) HI. Mineralogical analysis by X-ray diffraction of soil clays indicated presence of HI, that were identified by (1) the nature of the graphs of K-saturated samples heated from 110° to 550°C; the peak at 1.0 nm was greatly reinforced with tailing on its low angle sides. In absence of chlorite, the 1.4 nm peak indicated hydroxy-interlayered smectite (HIS), (2) intensity of Ca-treated samples were higher than the Ca-EG treated samples which indicated presence of HIS and (3) persistence of 1.4 nm peak (after Greene-Kelly test) confirmed presence of HIS. The hydroxy-interlayered materials in hydroxy-interlayered smectities (HIS) are positively charged hydroxy-cations such as [Al3(OH)4]5+, [Al6(OH)15]3+, [Fe3(OH)6]3+, [Mg2Al(OH)6]+, etc. The positively charged hydroxy-cations are usually formed under acid weathering conditions in a humid tropical climate where substantial amount of Fe3+, Al3+, Mg2+ are available. Hence, the transformation of smectite to HIS by inclusion of hydroxy-cations would require acid weathering under humid tropical environment. In contrast, the study area where the Vertisols are formed have a semi-arid climate and a moderately to strongly alkaline conditions. This suggests that the presence of HIS in the study area representing semi-arid climate and having alkaline conditions is unlikely to favour the formation of HIS. Thus, the reactions involving hydroxy-interlayering is not a contemporary pedogenic process under semi-arid climatic conditions. The HIS which has been produced in the humid tropical area may have been transported through rivers and streams. After this depositional episode in wetter climate, soils became calcareous due to the change of climate to semi-arid. Thus, the HIS remained as a signature of climate change phenomenon.