DIAGENESIS AND POLYTYPISM OF ILLITES IN SOME K-BENTONITES AND SHALES FROM THE MIDDLE ORDOVICIAN UTICA SHALE, MOHAWK VALLEY, NEW YORK

K/Ar dating of illites and K-bentonites is dependent on a thorough understanding of the processes responsible for illite formation, deposition, and subsequent diagenesis. Prior to obtaining K/Ar dates for K-bentonites from the Middle Ordovician Utica Shale, Mohawk Valley of New York, it was necessary to examine the possibility of detrital illite contamination in volcanic ash beds. Presence of detrital illite would affect the resulting K/Ar age, making the ash appear older. Previous studies suggest that detrital illite, being a product of mica and feldspar weathering, is typically coarser (0.2 to 2 micron fraction), and comprised of the 2M illite polytype. Diagenetic illite, formed through either burial or hydrothermal diagenesis, is found in the finer grain size (less than 0.2 micron fraction), and is comprised of the 1M illite polytype.

To determine the polytype present, both oriented slides and random, freeze dried samples were analyzed by X-ray powder diffraction. Preliminary X-ray diffraction analysis of the Paradise Ash Bed from the Utica Shale shows that there is exclusively the 1M polytype. Furthermore, a certain degree of structural disorder is present in the different size fractions in this K-bentonite sample. These observations suggest that detrital contamination in Utica Shale K-bentonites is not a major factor, making these K-bentonites appropriate for K/Ar dating. The aim of further research of these materials is to confirm preliminary results concerning illite polytype distribution as well as the relationship between different size fractions and illite disorder in K-bentonites.