Should Hydroxy-interlayering in Soil Smectites Increase their Layer Charge?

Abstract

Hydroxy-interlayering in clays is a common phenomenon in smectitic soils of the Peninsular India. It has been reported that smectites when hydroxy-interlayered (HIS) lose their cation exchange capacity. Logically therefore, the process of interlayering should decrease the layer charge of the original mineral. To validate this observation we carried out an experiment. It involved the determination of layer charges of two selected benchmark Vertisols. These soils represent Amravati district of Maharashtra (Takarkheda) and Anantapur district of Andhra Pradesh (Sollapuram). Fine clay fractions (<0.2 mm) of Takarkheda soils showed the presence of a strong peak around 1.4 nm on K-saturation at room temperature and the peak persisted between 1.4 and 1.0 nm at temperatures of up to 550°C. On the other hand, in Sollapuram soils, these peaks were relatively weak and almost negligible at 550°C. Further, in Takarkheda soils the X-ray peak intensity of the Ca-treated and glycolated sample was less than the Ca-treated sample, whereas in Sollapuram soils the corresponding intensity of the Ca-treated and glycolated sample was more than the Ca-treated sample. These facts suggest the presence of relatively higher degree of hydroxy-interlayering in smectites in Takarkheda than that in Sollapuram soil clays. The clay CEC of these soils is 88 and 93 cmol(p+)kg-1, respectively. When layer charges of these soil clays were determined (by standard method), it was found that the Takarkheda soil clays have more charge (0.46 mol(-)/half unit cell) than Sollapuram (0.38 mol(-)/half unit cell). This is in sharp contrast with the accepted logic that hydroxy-interlayering of clays reduce the layer charge as well as the CEC. The possible explanations for this anomalous behaviour are: (1) The hydroxy-interlayered materials are dominated by positively charged ions mainly by Fe3+/Fe2+ followed by Al3+, Ca2+ and Mg2+. To accommodate the higher positive charge, the hydroxy-interlayers might enforce greater isomorphous substitution in the basic silicate layers of smectites to maintain the crystal stability. (2) There may be isomorphous substitution in the hydroxy-interlayer materials itself such as Fe2+/Mg2+/Ca2+ for Fe3+ or Al3+ which may also initiate isomorphous substitution in the silicate layers. (3) With the increase in the degree of hydroxy-interlayering, a smectite would behave like a chloritised smectite, which has a higher layer charge due to the substitution of Al3+ and occasionally of Fe3+ or Cr3+ for Si4+ of the tetrahedral layer.