WHAT IS A K-BENTONITE?

Questions frequently arise as to whether a particular clay-rich bed might be an altered volcanic ash fall in the form of a bentonite or K-bentonite. These beds are often datable using fission track and U/Pb dating of zircons, K/Ar, and Ar/Ar of amphibole, biotite and sanidine. Due to their unique composition, they provide an indispensable tool when correlating sections. The criteria for recognizing such beds are varied, but fall into two broad categories, field criteria and laboratory criteria. Ideally, one would want both, but often that is not possible. However, there are key features to look for in each case that can aid in reliable identification and we present a summary of them here. Field Criteria: K-bentonites can be different colors when wet (blue, green, red, yellow) but are characteristically yellow when weathered. Due to their clay rich nature, they will feel slippery and waxy when wet. Some K-bentonites contain euhedral to anhedral volcanogenic biotite, quartz, feldspar, amphibole, zircon and apatite. The typical appearance of a K-bentonite bed in outcrop is that of a fine-grained clay-rich band ranging between 1 mm – 2m in thickness that has been deformed by static load from the enclosing siliciclastic or carbonate sequence. Accelerated weathering of K-bentonites causes them to be recessed into the outcrop face. For thicker K-bentonites there is often a zone of nodular or bedded chert in the adjacent strata at both the base and the top of the bed. Laboratory Criteria: Most bentonites and K-bentonites are smectite- or illite/smectite-rich, although some may contain a considerable amount of kaolinite, and those that have undergone low-grade metamorphism may be dominated by R3 I/S and/or sericite plus chlorite/smectite (corrensite) and/or chlorite. So initial steps should begin with separation and XRD analysis of the clay fraction. Wet sieving the sample is important to separate the clay portion from the volcanic crystals that could possibly be present in the sample. Bentonites may contain volcanic phenocrysts and volcanic glass. Study of the non-clay fraction under a high quality optical microscope is satisfactory to determine what types of crystals are present in the sample. Thin section study may also be used. We illustrate with seven examples that were thought to be bentonites. Only one was positively confirmed.