45-8 Redox Driven Fe and S Speciation in a Kaolinite-Illite Unconsolidated Sedimentary System

Session: Surface Processes Across the Solar System (Posters)

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Massive clay formations functioning as aquifer confining units are exposed in coastal areas on the North Shore of Long Island, NY, USA.  Clays were sampled at three sites and analyzed using x-ray diffraction (XRD), Attenuated Total Reflectance Fourier-Transform Infrared (ATR-FTIR) spectroscopy, and x-ray absorption near edge structure (XANES) spectroscopy.  Results show that chemical weathering processes produced variations in mineral speciation, ratios, and oxides.  The co-existence of kaolinite and illite indicates that variations in clay mineral speciation are the result of changing conditions during the cycles of weathering, transport, and deposition.  Each species has its origin in the geochemical alteration of the gneissic bedrock through dissolution-hydrolysis-precipitation reactions, especially alteration of the K-feldspar component of the parent rock that is the likely source of the potassium cation (K⁺) that provided cohesion for the illite lattice crystal structure.  During different time periods, near-neutral to slightly alkaline reducing conditions formed illite and more acidic leaching conditions produced kaolinite.  Approximate kaolinite/illite ratios range from 0.5:1 to 2.5:1.  Redox conditions that are potentially biologically mediated contributed to ferrous iron (Fe²⁺), ferric iron (Fe³⁺), and sulfate (SO₄^2-) content.  Total Fe and SO₄^2- content follow the same trend across the sampling group.  The samples with the highest abundance in Fe also have the highest abundance in SO₄^2-.  ATR-FTIR midinfrared (MIR) spectra reveal three stretching modes in the ~3600-3750 cm^-1 region indicative of the octahedral layer’s structural hydroxyl (OH) groups; features in the silica (SiO₂) abundance area between 800-1200 cm^-1 indicate high silicate content; and silicon-oxygen (Si-O) stretching of octahedral to tetrahedral bonding is expressed in the 400-550 cm^-1 region.  Although the sampled sites span an area of ~20 km, the bulk mineral characteristics are virtually identical, with variations in mineral ratios and oxide abundances. 

Geological Society of America Abstracts with Programs. Vol. 58, No. 2, 2026

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