X-RAY-DIFFRACTION CHARACTERISTICS AND RELATED PROPERTIES OF SMECTITESIN SOME CANADIAN SOILS

The X-ray diffraction (XRD) peak intensities of smectites in Chernozem ic and related soils of Western Canada are generally low, in contrast to the high peak intensities of smectites in Podzolic soils of Eastern Canada and those of standard smectite samples. Consequently, X-ray qu antitative analysis based on standard smectite samples may underestima te the amount of smectite and overestimate the amount of noncrystallin e material in western Canadian soils. This study was undertaken to fin d the reasons for the weak XRD peak intensities of western soil smecti tes in terms of their purity and crystallinity. The Tiron dissolution method extracted only small amounts of noncrystalline material from th e western soil clays and had little effect on XRD characteristics. The cation exchange capacity (CEC) hysteresis (or pH dependent CEC) betwe en pH 3.5 and 11.0 of the western soil clays was also relatively small which confirmed the absence of significant amounts of noncrystalline material in these soil clays. Observed deviations of XRD positions fro m true basal spacings indicated that the western soil smectite particl es were consistently thinner than the eastern soil smectite and Wyomin g montmorillonite particles. Electron microscope observations supporte d these results. Because the diffraction intensity is proportional to the square of the particle thickness, the thinness of the western soil smectite particles appears to be a major factor in reducing their pea k intensities. Thus, to obtain comparable peak intensity data from dif ferent soil smectites, particle thickness should be taken into account . Since it is often not practical to measure particle thickness, a pro cedure for quantifying smectite in soils that avoids this measurement is proposed.