MINERALOGICAL AND COMPOSITIONAL VARIATION OF GLAUCONITE

Glauconite as a mineral species is broadly defined in the literature, with wide-ranging compositional and clay mineralogical content. Here we present data on 9 glauconite samples, ranging in age from Cambrian to Eocene. Samples were analyzed by powder X-ray diffraction (XRD), scanning electron microscopy (SEM), Mössbauer spectroscopy with a 57Co/Rh source, and wavelength dispersion X-ray fluorescence (XRF). Samples were first prepped using the citrate-bicarbonate-dithionite (CBD) extraction method (from Mehra and Jackson, 1960) to remove free iron oxide and were subsequently concentrated by magnetic separation. XRD patterns were interpreted using NEWMOD. Clay mineralogy ranges from R=1 to R=3 illite-smectite, with trace amounts of chlorite. No kaolinite was found. SEM images reveal morphologies ranging from packed pellets to granular aggregates. Fine-scale resolution shows stacked packets of sub-parallel sheets for all samples. The Mössbauer spectra illustrate isomer shift and quadrupole splitting characteristics that reveal strong Fe3+ resonance in all samples. Dominant Fe3+ occupancy and 15-20% Fe2+ appear to be only in octahedral coordination. Results are consistent with primary cis-OH site occupancy, reported by previous authors. Structural formulas obtained from the XRF data show all samples to be dioctahedral, iron-rich, high potassium bearing layer silicates, with tetrahedral silica ranging from 3.6-4.0, based on half-cell formula notation. In all samples, the majority of Fe2+ in the proto-glauconite formative stage has been oxidized to Fe3+. There is no time-dependent diagenetic pattern in the oxidation of Fe2+ to Fe3+. Thus, we concluded that oxidation of iron in glauconite is an early diagenetic phenomenon.