MINERALOGY AND PETROLOGY OF SOME ORDOVICIAN K-BENTONITES AND RELATED LIMESTONES

Abstract

Twenty-two samples of Ordovician K-bentonite were examined by the use of size analyses, heavy- and light-mineral separates, thin sections, differential thermal curves, x-ray diffraction patterns, chemical analyses, and electron micrographs. The less than one micron fraction of two typical samples was studied in detail. The K-bentonite consists of randomly interstratified expanded and nonexpanded 2:1 layers in the ratio of 1:4. In addition, many of the samples contain packets of chlorite, the cause of an endothermal peak on the differential thermal curves formerly believed due to illite. Chemical analyses of the two samples indicate a direct correlation between the percentage of K present and the percentage of nonexpanded layers (illite). The nature of the heavy and light minerals shows that the clay altered from a volcanic glass. It is believed the clay formed as an expanded 2:1 mineral (montmorillonite) and later adsorbed K which caused 80 per cent of the layers to become nonexpanded (illite). Twenty-seven samples of the limestone on either side of a series of K-bentonite beds were examined in detail. The heavy and light minerals indicate that the insoluble residue is composed of both volcanic and nonvolcanic material. The nonvolcanic material is largely a nonexpanded dioctahedral 2:1 clay (illite); the volcanic material altered to form chlorite. The chlorite and volcanic heavy minerals increase in abundance as the K-bentonites are approached. It is believed that the ash altered to chlorite because of the availability of Mg, which was probably enhanced by the presence of the calcite ooze. The “Tioga Bentonite”, a thin clay bed in the Middle Devonian, is nearly identical to the Ordovician K-bentonite.