Abstract: Electron microscope studies show that most of the clay minerals have morphological characteristics which can be effectively used to aid in their identification. In addition, detailed studies at high magnification have provided structural information not obtainable by other means.
In the kaolinite group each of the common minerals has a clear-cut and diagnostic morphology. However, much more work is needed on flint clays and the kaolins in soils before the same can be said of these varieties. Continued study of halloysite reveals significant details pertinent to the form and structure of the tubular crystals.
Illites from different sources show interesting differences in form which reflect variation in the degree of crystallinity. An illite from the Ordovician Oswego formation shows an unusual development of narrow to broad lath-shaped crystals. Except for lower content of iron and magnesium the material has all the characteristics of other illites. The thinnest laths are of the same order of thickness as the flakes of more common illites. Certain mixed-layer minerals are identical in morphology with some varieties of illite.
The minerals of the montmorillonite group have been the most difficult of the clay minerals to characterize on the basis of morphology. The lath-shaped crystals of certain beidellites, sauconite, nontronite, and hectorite are diagnostic. The metal shadowing technique shows that many of the laths approach one unit cell in thickness.
The morphology of many montmorillonites is dependent upon the mode of sample preparation. The replica method offers a means of studying clay particles in their natural state but this technique is still in the early stages of development as it relates to the clay minerals.