CHAPTER 1 has reviewed the regular or ordered structures of layer silicates determined mainly by single crystal, X-ray diffraction methods. The present chapter and the two following chapters are concerned with irregular and disordered structures mainly in the 0.1–10 μm particle size range. X-ray powder diffraction methods are commonly used in studying these clay-grade materials, but single crystal, electron diffraction analysis furnishes additional important information (see Gard, 1971; Zvyagin, 1967).

Structural disorder is so prevalent in clay minerals that its recognition and evaluation are important aspects of the identification process. Some acquaintance with the fundamental concepts of diffraction by disordered systems is very helpful in understanding the phenomena involved. A full treatment would go far beyond what can be given in the present monograph, but from the simple treatment presented it is hoped that the main features of powder patterns from disordered layer structures will be more clearly recognized and interpreted than would be possible from a purely descriptive treatment.

The theoretical discussion is presented in Sections 3, 4, 5 and 6 and its application is given in Sections 7 and 8. Inevitably there will be some overlap between the theoretical sections and their application to different mineral groups.

It may be remarked that macro-crystalline minerals are not devoid of structural disorders, but in single crystal methods of structure analysis the investigator can usually select “good” crystals with a minimum of disorder. In studying clay minerals it is usually necessary to study them as formed in Nature and without the option of selecting “good” materials.