FIRST PETROGRAPHIC AND X-RAY DIFFRACTION INVESTIGATION OF A GLAUCONITE LITHOLOGY ASSOCIATED WITH THE ORDOVICIAN DRAKES FORMATION AND SILURIAN UPPER BRASSFIELD FORMATION.

The Cuckoo road cut is 3.5 miles northwest of Serpent Mound State Memorial site in south central Ohio and exposes the Ordovician Drakes Formation and Silurian Upper Brassfield Formation. This location is reported to contain a glauconite-bearing lithology. The outcrop and three stratigraphic sections were investigated for glauconite collection. Previous work indicates glauconite grains were identified in unit one of four defined stratigraphic sections, but no detailed petrographic or mineralogical analysis was done. Initial observations of the glauconite are from polarized light microscopy (PLM) of a limestone unit containing 15 to 20% glauconite grains; however, some areas of the thin section material were damaged in preparation. The limestone is composed of poorly preserved and micritized fossils with echinoderm plates and pelecypod fragments being observed. Abundant and complex calcite cements formed before later-stage quartz cement that has some textures consistent with chalcedony. The appearance of green rounded and elongated glauconite grains were present throughout the sample. The grains exhibited moderate pleochroism and glauconite containing grains were measured to be approximately 500 to 1000µm in diameter. Complex microtextures exist within the glauconite grains and fine grained glauconite material is dispersed in or through some grains and cement material, suggesting some degree of either mechanical mobilization or chemical reprecipitation. Some degree of reprecipitation is supported by the late-stage quartz cements. Bulk basic powder X-ray diffraction indicated that calcite and quartz are the primary minerals present in the limestones. Mild acetic acid, 0.1 M, and 0.3 M hydrochloric acid treatments were used to extract the glauconite clay fraction from the limestone. X-ray diffraction data on initial sets of oriented clay mineral slides indicate a diffraction pattern with a (001) of glauconite greater in intensity than the (002). Modeling using NEWMOD II software is consistent with glauconite with a modeled composition of 1.5 octahedral Fe and 0.9 interlayer K. The initial analysis indicates that the glauconite is more complex than expected. Further work with these rocks will include scanning electron microscopy and transmission electron microscopy.