Interparticle diffraction: a new concept for interstratification of clay minerals

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Abstract
A new conceptual model for interstratified clays is proposed, based on complementary use of the transmission electron microscope (TEM) and X-ray diffraction (XRD). Clays yielding interstratified XRD patterns are shown to be composed of aggregates of fundamental particles (here defined as an individual or free particle which yields a single crystal pattern by electron diffraction) whose interfaces are capable of adsorbing water and organic molecules. In these circumstances, the interstratified XRD character results from an interparticle diffraction phenomenon. Smectites are shown to be primarily composed of particles 10 Å thick corresponding to elementary 2:1 silicate layers. Nearest-neighbour regularly interstratified illitesmectite (K-rectorite) is composed of particles of elementary ‘illite’ 20 Å. thick, corresponding to two silicate layers coordinated by a single plane of potassium ions. Regularly interstratified chlorite-smectite (corrensite) consists of particles 24 Å thick corresponding to two silicate layers coordinated by a single brucitic sheet. The layer sequence examined by XRD is determined by the types and proportions of particles present within the clay material. Thus a sedimented aggregate of 20 Å elementary ‘illite’ particles appears to be a regularly alternating sequence of illite and smectite layers by XRD. Randomly interstratified itlite-smectite and chlorite-smectite can be synthesised by making mixed suspensions of the 50 Å thick. This conceptual model has implications for the chemistry, behaviour and genesis of interstratified clays which are common constituents of soils and sediments.