Minerals of the kaolin group were among the first materials to be subjected to study in the early days of electron microscopy when easily obtained, previously studied, morphologically appropriate specimens were sought as much to test the capabilities of the new instruments as to reveal previously unresolved features of the minerals themselves. Light microscope work such as that included in the studies of Ross and Kerr (1930, 1935) had successfully established the characteristic shapes and habits of resolvable single crystals and crystal aggregates of nacrite, dickite, and kaolinite; and had indicated that whereas halloysite often possesses birefringence probably “due to a partly oriented aggregate of birefracting mineral grains of submicroscopic size” (1935, p. 129), allophane is light microscope and X-ray amorphous. Unanswered were questions as to the morphology of halloysite and allophane, and the size range exhibited by the various minerals of the group; and it was for the answers to these and similar questions that clay and soil mineralogists first turned to the electron microscope.
The period 1938 to 1942 produced a significant number of papers describing the results of several groups of workers in Germany and one group in the United States. In the kaolin group of minerals the pseudo-hexagonal morphology of various kaolinite samples and of dickite was confirmed, but it was also revealed (see, for example, Humbert, 1942)