Clay Minerals (1985) 20, 499-514 THE PHYSICAL DIMENSIONS OF FUNDAMENTAL CLAY PARTICLES P. H. NADEAU Dept. of Mineral Soils The Macaulay Institute for Soil Research, Aberdeen, Scotland AB9 2QJ Received 14 June 1985; revised 28 August 1985) A B S T R A C T : The thickness, length, width, area and perimeter of 575 particles from 16 aqueously dispersed samples of a variety of interstratified clays, smectites and illite have been recorded using TEM techniques. Complete dispersion of the clay material was achieved by saturating the clay with either Na § or Li+, removal of excess ions by dialysis, and isolation of the <0.1 or <0.2/~m fraction by centrifugation. The samples have mean maximum dimensions of 1900 to 90 nm and the dispersed system can be considered as colloidal in nature. The mean thickness of the clays is about 1 nm for smectites, corresponding to that of elementary 2:1 silicate layers, from 1.9 to 4.9 nm for the interstratified clays, and 9 nm for illite. From these data the volume, total surface area and other parameters have been calculated and compared with independent determinations of surface area and CEC. The total surface area by TEM, assuming a density of 2.6 g/cm3, varies from ~675 m2/g for smectites to 86 mZ/gfor illite,and is inversely proportional to the mean particle thickness. The charge density of monovalent cation exchange sites on the surface of the particles as determined for nine of the samples varies from 0.54 to 1.16 nm2/site. The particle-thickness distribution data can be used to calculate interstratified XRD layer sequence probabilities and composition parameters, and agree with XRD data for interstratified clay with <60% illite layers. The thickness data also provide a rationale for the interpretation of TEM lattice-fringeimages. Relationships between the particle area, length, thickness and volume are shown to be potentially useful in assessing the mechanism(s)of crystal growth of these extremely small phyllosilicate particles. Recent quantitative transmission electron microscopic (TEM) studies of completely dispersed smectites and interstratified clays have shown that these materials are composed of fundamental particles which are commonly 1-3 nm (10-30 A) thick (Nadeau et al., 1984a,b). This work has led to a re-evaluation of the X-ray diffraction (XRD) character of such materials, termed interparticle diffraction, and to the proposal of a new conceptual model for the nature and origin of diagenetic interstratified illite-smectite clays. Completely dispersed clay particles yield single-crystal patterns when examined by electron diffraction (ED). From a colloidal viewpoint these would be called primary particles (Yariv & Cross, 1979), but to avoid confusion with the XRD usage of primary particle--the volume which gives rise to coherent interference in the scattering of X-rays, i.e, a crystallite (Mering & Oberlin, 1971)--the term fundamental particle was selected instead. When a fundamental particle corresponds to the thinnest particle unit possible for a mineral, it may be referred to as an elementary particle, as with an elementary smectite particle 1 nm thick. In aggregates, fundamental particles adopt a high degree of face-to-face association, and (c) The Macaulay Institute for Soil Research, 1985