Alternating mica-like and smectite-like layers of rectorite give rise to periodically varying contrast in 10-Å lattice fringes, yielding a periodicity of 20 Å in a transmission electron microscopic study. Expansion of rectorite using dodecylamine hydrochloride yields a three-layer repeat of thickness 32–35 Å, consisting of a basic 20-Å unit, identical to that in images of collapsed, dehydrated rectorite, and a 12-15-Å thick, intercalated organic layer.

Thin packets of layers derived by grinding the sample are only 20 Å thick, or multiples thereof. Serrated edges of rectorite grains likewise have steps 20 Å in height, implying that mechanical cleavage occurs readily along the weakly bonded, smectite-like interlayers. The “fundamental” 20-Å unit is proposed to be compositionally centered on an interlayer bounded by two identical T-O-T layers, each of which has compositionally different tetrahedral sheets. Such structural considerations suggest that resultant 20-Å units (“rectorite units”) are unique in structure and chemistry relative to true illite. These results further imply that grinding and other treatment of coherent crystals of clay minerals may produce individual unit layers. Moreover, when coupled with size-separation, such treatment may yield X-ray powder diffraction data that reflect reconstituted layer sequences.

Treatment of smectite with laurylamine hydrochloride was verified to cause expansion of d(001) which may be retained and observed in ion-milled samples by transmission electron microscopy. The spacings between layers as observed in lattice fringe images, however, are variable and may be as small as 10 Å. The method therefore produces ambiguities in differentiating between some smectites and illites, similar to those that have been found for untreated samples; e.g., on this basis, expanded layers may be inferred to be smectite, but layers with d-values approaching 10 Å may be either illite or smectite. Expansion also destroys the original rock texture, which, therefore, must be observed using only untreated samples.