Mineralogical and Crystal-Chemical Constraints on the Glauconite-Forming Process in Neogene Sediments of the Lower Guadalquivir Basin (SW Spain)

Glaucony is a significant green marine facies in the northwestern passive margin of Guadalquivir Basin (Spain), where glauconite formed authigenically on sediment-starved continental shelf, with fecal pellets and benthic foraminiferal tests being main glauconitized substrates. Results from study using XRD, TGA-DSC, SEM-EDS, EPMA have revealed that remarkably heterogeneous mineral composition chemical maturity, even single grain, reflecting complex interaction micro-environmental factors, substrate influences post-depositional alterations. In its early stage, glauconitization process consistent slow precipitation Fe-rich smectite phase, most likely intergrade between nontronite Fe-montmorillonite end-members, which evolved to regularly interstratified glauconite-smectite (Gl/S). The Fe-smectite-to-Gl/S transformation interpreted as diffusion-controlled reaction, involving sufficient Fe availability pore water constant diffusive transport seawater K+ Mg2+ ions towards substrate. pelletal actually highly Gl/S consisting almost totally mica layers, 0.74 ± 0.05 apfu interlayer, while occurring replacements contains mean 7% expandable layers walls 16% chamber fillings, due rate-limited ion diffusion.