Most of the structures of clay minerals are based on composite layers of (Si, Al)-O tetrahedra and Al or (Mg, Fe)-O octahedra stacked on one another in various ways. In many cases, however, the details of these structures cannot be established in conventional ways because of the lack of suitable crystals for structure determination, and those of micas to which they are structurally related are frequently referred to for understanding the average structures of clay minerals. Although micas are commonly found in nature as large crystals with well-developed platy habit, in some occurrences they are less well crystallized and they then overlap in many respects the more strictly defined clay minerals.
By means of electron microscopy, the understanding of clay minerals has been extended by direct observation of particle shapes and sizes. Further understanding of their physico-chemical properties has come from investigation of the various imperfections exhibited by crystals of clay minerals. The existence of defects and disorderings in the layer structures of clay minerals has been suggested mainly by X-ray investigations during the past twenty years and the interpretation of these observations has been based mainly on the profiles of X-ray powder reflections. Without single crystals, however, considerable ambiguity is inevitably involved in the interpretation, especially when a natural clay mineral is investigated. More direct observation of the defect structures is possible to some extent by transmission electron microscopy, and the understanding of the averaged properties as observed by the X-ray powder method can be complemented by the electron-microscopic observations.