LAYER CHARGE OF THE EXPANDABLE COMPONENT OF ILLITE SMECTITE IN K-BENTONITE AS DETERMINED BY ALKYLAMMONIUM ION-EXCHANGE/

The charge of the expandable interlayers in a series of fourteen diage netic illite/smectites (I/S) from lower Paleozoic K-bentonites was det ermined by the alkylammonium ion exchange method. The magnitude (< 0.5 0 equivalents per half formula unit) and characteristic heterogeneous distribution of interlayer charges in eight samples with expandabiliti es from 70% to about 15% confirm the smectitic character of the expand able 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 precurs or 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 highe r charges inferred for the highly illitic samples can be consistent bo th with a layer-by-layer transformation and the neoformation mechanism s proposed in the literature for the formation of illite.