First Petrographic and X-ray Diffraction Investigation of a Glauconite Lithology Associated with the Ordovician Drakes Formation and Silurian Upper Brassfield Formation

Abstract

The Cuckoo road cut is 3.5 miles northwest of the Serpent Mound State Memorial site in South Central Ohio. It exposes the Ordovician Drakes Formation and Silurian Upper Brassfield Formation, which are 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 are observed. Abundant and complex calcite cements formed before later-stage quartz cements and have textures consistent with chalcedony. The observation of green, rounded, and elongated glauconite grains consists through-out the sample. The grains exhibited moderate pleochroism and glauconite grains were measured to be approximately 500µm to 600µm in diameter. Complex microtextures exist within the glauconite grains and fine-grained glauconite material is dispersed in or through surrounding grains and cement material, suggesting some degree of mechanical mobilization or chemical reprecipitation. However, reprecipitation is supported by the late-stage quartz cements. Bulk basic powder X-ray Diffraction (XRD) indicated that calcite and quartz are the primary minerals present in the limestones. 0.1 M hydrochloric acid (HCl), and 0.3 M HCl 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). Using NEWMOD II software, modeling is consistent with glauconite having a composition of 1.5 octahedral Fe and 0.9 interlayer K. This initial analysis indicates that the glauconite is more complex than expected. Furtherwork with these samples with scanning electron microscopy (SEM)  will indicate more details on depositional environment, alteration history, and grain morphology of the glauconite samples.