GREEN KEY TO THE PAST: A COMPARATIVE STUDY OF THE PALEOENVIRONMENTAL IMPLICATIONS OF OPTICAL VS. CHEMICAL PROPERTIES OF GLAUCONITE

Glauconite is an authigenic green clay mineral enriched in potassium and iron, and has long been employed as a paleoenvironmental and stratigraphic indicator and for geochronology. Previous research has shown that glauconite occurs in a variety of Phanerozoic depositional settings with a range of formative conditions. However, since most studies are tied to particular localities within specific times, they are not representative of the full range of morphological and chemical variability of glauconite. To better constrain glauconite as a tool for paleoenvironmental analysis by relating optical properties of varied glauconitic minerals to their chemical compositions, this study undertakes a spatiotemporal investigation into the different morphologies and compositions of glauconite and their associated paleoenvironments.

Utilizing samples from the Paleozoic of the Midwest, USA, and the Mesozoic of Madagascar, preliminary petrographic and chemical analyses reveal a greater range of glauconitic compositions and contexts than previously documented within single studies. The defining oxides of glauconite each apparently vary independently, and the following bounded ranges of concentrations were determined via semi-quantitative SEM analyses from a varied suite of glauconitic grains: 11.3-38.4% Fe2O3, 3.2-20.4% Al2O3, and 2.7-10.0% K2O. Although the percentages of defining oxides within the sample set varied, XRD spectra across variable grain compositions and contexts remained consistent.

Several paleoenvironmental factors are shown to be sufficient, but not strictly necessary, to form glauconite. These include the presence of fecal pellets, iron-rich volcanic input, siliciclastic sediment starvation , or high-nutrient conditions. Each of these is shown to be associated with glauconite development, though no single factor should be considered to absolutely yield glauconite, particularly as alternative scenarios combining other environmental factors can also result in glauconite formation.

Preliminary results support that optical characteristics of glauconite do relate to chemical compositions, though key variations can be subtle, and each case studied should be calibrated based on the unique combination of paleoenvironmental factors present.