Quantitative determination of the interlamellar volume in an interstratified mica-smectite soil clay

Abstract

Two well-characterized soils containing substantial amounts of interstratified mica-smectite have been examined by a combination of techniques in an attempt to correlate the characteristics of the interlamellar surfaces with other properties. The techniques used included XRD and chemical analyses, determination of surface area by nitrogen sorption and by desorption of water and ethylene glycol monoethyl ether vapour, and determination of cation exchange capacity. Reasonable agreement was found between the results obtained for the proportion of expanding or smectite-like surfaces, although estimates by XRD, especially when coarser material ( > 2/~m) was included, were generally lower than those obtained by the other methods. The mean density of charge on the surfaces of the soil clays appeared to be similar to that of smectite. The decrease in porosity observed for both soils dried by the critical point method was largely accounted for by the decrease in volume associated with the loss of interlamellar water from the smectite-like interlamellar surfaces included within the clay fraction.Many soil clays which show substantial swelling and shrinkage in response to changes in moisture content contain smectites in the clay fraction. Commonly the smectite is interstratified or mixed with other 2:1 lattice clays. It is sometimes suggested that the swelling behaviour of the soil is due to adsorption of water in the interlamellar regions of the clay crystals. This may be true in some sodic soils, but where Ca 2+ is the dominant exchangeable cation Aylmore & Quirk (1962) and others have established that the interlamellar regions make little or no contribution to the swelling of soils observed in field conditions. However, if the absorption and loss of water is to be related to the swelling behaviour of soil clays over a wider range of water contents it is essential that the contribution of the interlamellar surfaces be known quantitatively.Mooney et al. (1952) showed that at relative water vapour pressures <0.05, Camontmorillonite adsorbed one layer of water molecules only in the interlamellar region (d001 ,-~ 12.5 A); at higher vapour pressures two layers of water molecules were adsorbed (d001 ,-~ 15 A), and above P/Po~0.90, three layers of water molecules were present (d001 ~ 19 ~). However, little information is available to relate this work to the swelling behaviour of soil-clays, where the interlamellar charge density may differ considerably from that of the montmorillonite studied by Mooney et al. (1952).The present paper reports work conducted to determine quantitatively the interlamellar water sorption of two soil clays, whose overall swelling behaviour has already been studied in some detail (Lawrence et al. 1979).

Charges on the surfaces of two chlorites

“…The polar nature of water means that it is not uniformly distributed over the surface and there is no monolayer (Greenland &Mott, 1978). Nevertheless, the use of water for the determination of surface area has been found useful to explore the internal surface of clays and good agreement has been found elsewhere between the BET and SD methods (Jones & Greenland, 1980;Quirk, 1955). The reason for the consistent difference between the results for the two methods found here is not understood, but while the BET method is more reliable the SD method is much quicker and was determinations of the chlorites treated with acid.…”

The soil clays of Great Britain: I. England and Wales

“…The Denchworth and Evesham soils have a mineralogy dominated by mica-smectite with much smaller amounts of kaolin and, occasionally, chlorite. The mica-smectite is randomly interstratified with between 50 and 70% smectite layers (Weir & Rayner, 1974;Jones & Greenland, 1980;unpublished). K20 contents are generally <2.5% and CEC values >50 mEq/100 g, i.e.…”

Water sorption on Ca-saturated clays: I. Multilayer sorption and microporosity in some illites

“…In UK soils the clays of commonest occurrence are mixed-layer minerals, often mica-smectites (Weir & Rayner, 1974;Avery & Bullock, 1977;Jones & Greenland, 1980). The structure of such mixed-layer minerals is complex and their interaction with water has been little studied.…”