Asymmetric Zonation of a Thick Ordovician K-Bentonite Bed at Kinnekulle, Sweden

Ann Marie Brusewitz
Geological Survey of Sweden, Box 670 S-75128 Uppsala, Sweden

Abstract: The clay fraction of a ∼2-m-thick K-bentonite at Kinnekulle in southwestern Sweden has been studied by X-ray powder diffraction, chemical analyses, and cation-exchange capacity determinations. Previous investigations of the upper half of this bed (B-bed) have shown the clay mineral to be illite/smectite (I/S), containing about 50% illite layers at the center of the bed and 65% illite layers at the upper contact. New data on six samples from the lower part of the bed show no zonation, the I/S containing about 50% illite layers in all samples. The bentonite occurs in limestones and shales overlying a 40-m-thick limestone bed and is overlain by limestones, marls, and shales (∼90 m), which in turn are overlain by a 40-m-thick diabase. The limestones are silicified in contact with the bentonite, particularly beneath the B-bed. The increased K content towards the upper contact may therefore have resulted from diffusion of K-bearing solutions from overlying shales, a process that was impeded in the lower part of the bed, probably by the silicification.

Key Words: Bentonite • Diagenesis • Illite/smectite • Potassium • Shale • X-ray powder diffraction

Clays and Clay Minerals; August 1988 v. 36; no. 4; p. 349-353; DOI: 10.1346/CCMN.1988.0360409
© 1988, The Clay Minerals Society
Clay Minerals Society (www.clays.org)