Abstract--The charge of the expandable interlayers in a series of fourteen diagenetic illite/smectites (I/ S) from lower Paleozoic K-bentonites was determined by the alkylammonium ion exchange method. The magnitude (<0.50 equivalents per half formula unit) and characteristic heterogeneous distribution of interlayer charges in eight samples with expandabilities from 70% to about 15% confirm the smectitic character of the expandable interlayers in this range. This result coupled with the lack of a correlation between expandability and interlayer charge is consistent with the hypothesis of a layer-by-layer transformation from a precursor smectite to highly illitic I/S clays during K-bentonite diagenesis. The charge of the expandable interlayers in I/S samples with about 10% or less expandabilities have been inferred to be vermiculitic rather than smectitic. The K-fixed interlayers and expandable interlayers in these samples appear to be similar in charge. The significantly higher charges inferred for the highly illitic samples can be consistent both with a layer-by-layer transformation and the neoformation mechanisms proposed in the literature for the formation of illite.

--High resolution transmission electron microscopy (HRTEM) have been performed on dispersed portions of one R > 1 and two R3 illite/smectite (I/S) samples from Silurian K-bentonites. R > 1 sample was studied by HRTEM before and after alkylammonium ion treatment and R3 samples were studied only after alkylammonium ion treatment. The HRTEM images of the chemically untreated R > 1 sample were predominated by lattice fringe contrast with 20-40/~ periods, interpreted to represent various ordered I/S units. HRTEM images of the three alkylammonium-treated samples displayed very small, dispersed particles composed of iilite packets separated by alkylammonium expanded interlayers. In the R > 1 sample, iUite packets were mostly 20/~ to 40 A thick whereas in R3 samples they were predominantly over 40 A. Although a good degree of dispersion of the bulk samples was achieved, dispersed particles recorded on images were thicker than the fundamental particles postulated by Nadeau and coworkers. Alkylammonium ion-expanded interlayer thicknesses point out a trend toward a higher charge in the expandable interlayers (i.e., illite particle surfaces) with increasing illite content from the R > 1 sample to the R3 samples. In the R3 samples, the interlayer charge is sufficiently high to be vermiculitic.