INTERLAMELLAR complexes of caly minerals are formed by the introduction of inorganic and organic meterials between the structural layers, the relatively weak bonding between the layers as compared with the strong ionic-covalent bonding within the layers facilitating their formation. The most common of the interlayer materials is water which is normally present between the layers of smectites and vermiculites, and in the hydrated form of halloysite. The extent to which inorganic and organic materials enter the silicate structures varies greatly and depends on many factors related to the detailed structure and composition of the layers, and the nature of the materials.

In the early stages of the structural study of these complexes attention was focused principally on the swelling–shrinking behaviour with respect to water, and the complexes formed with simple organic liquids, notably ethylene glycol and glycerol. The use of these liquids became a standard identification test. From these simple beginnings, a wide range of investigations has developed oriented towards the understanding of the formation, structure and properties of the complexes, as well as the refinement of identification procedures and the development of new procedures. Both aspects will be treated in this chapter. As regards research on the complexes themselves, Xray diffraction is the major method of investigation, but other techniques, notably infrared absorption spectroscopy, are being increasingly applied. In keeping with the particular theme of this monograph, X-ray studies will be considered with rare references to other methods.

For purposes of X-ray identification of clay minerals, it will often be necessary to utilize no more than a fraction, possibly a small fraction, of the total knowledge now available on the interlamellar complexes of clay minerals.