Particle size distribution of four clays, from Mexico, Wyoming, Texas, and Puerto Rico, ranging in cation composition from predominantly Na+ to Ca++, were obtained. Fractions ranged from 0.020 to +40 microns in size. X-ray diffraction patterns of each fraction are discussed in regard to mineral composition and degree of particle dispersion. A new concept is presented on clay particle size which includes influence of degree of hydration, time, and mechanical shear.

Naturally occurring clays are heterogeneous, presenting structural and compositional differences not only from one deposit to the next, but often within the same deposit. The clays chosen for this study represented a wide range of possible variations in montmorillonites. The results show that clays containing mono- and bivalent exchange cations will divide into two phases upon fractionation, the coarse fractions containing the bivalent exchange cations, and the finer fractions containing monovalent cations. It appears that Na+ bentonites disperse into single-layered particles, each comprising two silica tetrahedral sheets with an aluminum octahedral sheet in between and resist coagulation. Ca++ clay particles cluster immediately upon cessation of agitation into aggregates in the form of loose, irregular particle flocks. Gel properties of drilling fluids were found to be directly controlled by particle size.


Although considerable advances have been made in the study of clays in recent years by use of such techniques as differential thermal analysis, x-ray diffraction, electron diffraction, and electron microscopy, there remains considerable uncertainty regarding details of clay structure. The difficulty stems from the fact that clay particles are extremely small and offer no possibility for single crystal studies. The currently accepted structure follows the proposal of Hofmann, Endell, and Wilm. According to this concept, the structure is made up of two sheets of silicon-oxygen tetrahedrons with an aluminum-hydroxyl octahedral sheet in between. The atoms common to both the silicon and aluminum sheets are oxygen instead of hydroxy groups. A structure comprising two silica and one alumina sheet is called a layer, and more specifically, it is called a "unit layer" in this work. Unit layers are continuous in a and b axial directions and are stacked above each other in the c direction.

A fundamental characteristic of the clays of the montmorillonite group is the tendency to swell when placed in water. This swelling, a result of hydration, depends on availability of water and nature of the base exchange cations on the clays. Exchangeable cations occur between silica sheets, and are held there to satisfy unbalanced lattice charges resulting from isomorphous substitutions in octahedral and tetrahedral sheets. The most common substitutions are Mg++ and/or Fe++ for Al+++ in octahedrons, and Al+++ for Si+++ in tetrahedrons. In both cases, a clay particle bearing a net negative charge results. The edges of such a particle have, nevertheless, a slight positive charge since all broken edge bonds expose positive ions. Consequently, free cations in the environment, such as Na+, Mg++, or are repelled from edges, but attracted to the basal surfaces of unit layers. The cations thus act as binding agents between layers which normally repel each other due to their net negative charge.

In the stacking of clay particles, a sheet of oxygen is against another sheet of oxygen of the neighboring unit layer. These sites are then weak in bond and will readily cleave to receive water in between. Entry of water will swell the particles, affecting separation of unit layers from each other, thus, influencing sub-division of clay particles.

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