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