Abstract: The layer structure of kaolinite from Twiggs, Georgia and fire-clay type kaolinite (Frantex B, from France), particle size separates 2–0·2 µm was studied by high resolution electron microscopy after embedment in Spurr low-viscosity Epoxy media and thin sectioning normal to the (001) planes by an ultramicrotome. Images of the (001) planes (viewed edge-on) of both kaolinites were spaced at 7 Å and generally aligned in parallel, with occasional bending into more widely spaced images of about 10 Å interval. Some of the 10 Å images converged to 7 Å at one or both ends, forming ellipse-shaped islands 80 to 130 Å thick and 300 to 500 Å long. The island areas and interleaved 10 Å layers between 7 Å layers may represent a residue of incomplete weathering of mica to kaolinite.
The proportions of micaccous occlusions were too small and the layer sequences too irregular to be detected by X-ray diffraction. The lateral continuity of the layers through the 7-10-7 Å sequence in a kaolinite particle would partially interrupt or prevent expansion in dimethyl sulfoxide (DMSO) and other kaolinite intercalating media. Discrete mica particles were also observed with parallel images at 10 Å, as impurities in both kaolinites. The small K content of the chemical analyses of the kaolinite samples is accounted for as interlayer K, not only in discrete mica particles but also in the micaceous occlusions.