Abstract
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.
Highlights
Glauconite is a series name of Fe-rich interlayer-deficient dioctahedral micas with a representative formula of K0.8 R3+ 1.33 R2+ 0.67 Al0.13 Si3.87 O10 (OH)2 [1], in which the ratioVI R2+ /(VI R2+ + VI R3+ ) is greater than or equal to 0.15, the ratio VI Al/(VI Al + VI Fe3+ ) is lesser than or equal to 0.5, and the total interlayer cations comprise between 0.60 and0.85 atoms per formula unit
It is generally agreed that glauconitization is an authigenic process that typically develops in marine settings, on the outer margins of continental shelves and adjacent slope areas with low sedimentation rates [6], ancient glaucony has been reported in shallow-marine depositional environments, including estuaries and coastlines [7]
We report a comprehensive study focusing on the process of glauconitization of fecal pellets and foraminiferal tests in a passive-margin condensed section of the lower Guadalquivir Basin (SW Spain), by integrating X-ray diffraction (XRD), thermo-gravimetric analysis (TGA)-differential scanning calorimetric (DSC), SEM-energy-dispersive X-ray detectors (EDS), and electron probe microanalysis (EPMA) data
Summary
Glauconite is a series name of Fe-rich interlayer-deficient dioctahedral micas with a representative formula of K0.8 R3+ 1.33 R2+ 0.67 Al0.13 Si3.87 O10 (OH)2 [1], in which the ratio. Glauconite has been described as a mica-rich mica-smectite (R3 ordered) mixed layer mineral, with pure end-member glauconitic mica having up to 0.8 apfu of K+ in the interlayer space [2]. It is generally agreed that glauconitization is an authigenic process that typically develops in marine settings, on the outer margins of continental shelves and adjacent slope areas with low sedimentation rates [6], ancient glaucony has been reported in shallow-marine depositional environments, including estuaries and coastlines [7]. The glauconitization process has been revisited by many authors, and a variety of models have been postulated to explain the origin and compositional evolution of glauconite, including: (1) the “layer lattice” theory [8,9], which assumes a conversion of degraded 2:1 clay mineral to a newly formed Fe- and K-rich glauconitic mineral, under reducing conditions, with the simultaneous increase of K and Fe contents;
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