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Soil smectites differ from standard bentonite-type smectites in several respects. First, soil smectites generally have a composition varying between montmorillonite and beidellite. They commonly contain >20% octahedral Fe3+ (excluding magnesium) and can be described as iron-rich beidellites. Second, interstratified smectites formed in soils differ from interstratified smectites formed during diagenesis. Thus, interstratified soil smectites commonly form by means of transformation reactions involving relatively large crystals of mica or chlorite, whereas, according to a recently developed conceptual model, diagenetic interstratified clays consist of exceedingly fine particles some tens of Ångstrom units in thickness which are characterized by the phenomenon of interparticle diffraction. Third, soil smectites may be interlayered with non-exchangeable aluminous, organic, or other material, a phenomenon that may produce anomalously high basal spacings on X-ray powder diffraction patterns. Many soil smectites are, therefore, significantly different from bentonite-type smectites; such differences must be fully characterized if the properties and behavior of smectitic soils are to be understood.

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