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A Deep-Water Glauconitization Process on the Ivory Coast—Ghana Marginal Ridge (ODP Site 959): Determination of Fe3+-Rich Montmorillonite in Green Grains
- A. Wiewióra, P. Giresse, S. Petit, A. Wilamowski
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- Journal:
- Clays and Clay Minerals / Volume 49 / Issue 6 / December 2001
- Published online by Cambridge University Press:
- 28 February 2024, pp. 540-558
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- Article
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The mineral and chemical composition of green glauconitic grains from ODP Site 959 (2100 m water depth) located on the northern flank of the Ivory Coast—Ghana Marginal Ridge was studied. Recurrent winnowing of a 20 m thick Pleistocene succession resulted in a low accumulation rate and stratigraphic hiatuses. The green clay material typically occurs as fillings in the chambers of pelagic foraminifers. The amount of green clay present in sediments older than 1 Ma is small, and greater in younger material. Mud composed of smectite, kaolinite, traces of mica, calcite and quartz was the precursor material that filled the chambers of the foraminifers. Processes at the water-sediment interface slowly modified this composition. Kaolinite was dissolved; smectite lost Al but gained Fe, K and layer charge. In that matrix, the nanocrystals of neoformed smectite are observed. The infrared (IR) spectra showed OH-stretching and bending vibrations due to groups incorporating Fe3+. The spectra are in agreement with the crystallochemical formulae of Fe3+-rich montmorillonite as determined by point-by-point analyses on the neoformed crystallites and on the surrounding matrix. The layer charge in this Fe3+-rich montmorillonite is almost wholly octahedral as shown in crystallochemical formulae and documented independently by a new IR method. The tetrahedral charge appeared when the Fe content increased by > 1.2 Fe per formula unit. With the maturation process, the increased role of the closed layers is observed, with the color of grains becoming greener. We have documented for the first time glauconitization proceeding at a depth of 2100 m at a temperature near 3°C. The most important factors of the process are: accumulation of terrigenous clayey material in the foraminiferal chambers, Fe supply from a nearby continent, and a lengthy residence at the water-sediment interface in the zone of the winnowing and low sediment accumulation rate.
An Experimental Study on Transforming Montmorillonite to Glauconite: Implications for the Process of Glauconitization
- Xiaoke Zhang, Yuanfeng Cai, Dongmei Jiang, Yang Zhang, Yuguan Pan, Lijuan Bai
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- Journal:
- Clays and Clay Minerals / Volume 65 / Issue 6 / December 2017
- Published online by Cambridge University Press:
- 01 January 2024, pp. 431-448
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The objective of this study was to explore the geological origin of glauconite, which is believed to precipitate and mature very slowly (~1 Myr) in neritic environments (shallow water, oceanic coastal zones, at water depths of 100–200 m) with very low sedimentation rates. A series of simulation experiments was designed and carried out in sealed tubes placed in an oven and heated to a constant temperature of 50°C (±2°C) for 60 or 150 d. The parent materials used for these experiments were two low-Fe montmorillonites with different crystallinities. The montmorillonites were introduced to solutions with concentrations of 0.02–0.1 mol/L Fe3+ and 0.05–0.2 mol/L K+ with various values of pH and Eh. The products were analyzed using X-ray powder diffraction (XRD), Fourier-transform infrared (FTIR) spectrometry, electron spin resonance (ESR) spectrometry, scanning electron microscopy (SEM), and Mössbauer spectroscopy. The morphological changes from parent material to product were observed under SEM, which revealed the formation of a flaky mineral (e.g. a product formed in the interstitial spaces between montmorillonite crystals). The formation of a flaky mineral indicates that the product is a layer silicate. Qualitative analysis of XRD patterns revealed that the main product phase was a mica group mineral and the d060 value was consistent with the presence of glauconite (0.152 nm) and/or Fe-illite (0.150 nm). A glauconite and Fe-illite mineral assemblage formed in a weakly acidic solution, while Fe-illite, mixed-layer Fe-illite, and montmorillonite formed in neutral and alkaline solutions. Stretching vibrations of Fe(III)Fe(III)OH-AlFe(II)OH and/or MgFe(III)OH were observed in FTIR spectra (3550–3562 cm−1) of the products formed in acidic solutions, which along with the g = 1.978 ESR signal indicated that Fe(III) entered octahedral positions in the tetrahedral/octahedral/tetrahedral layer (TOT) platelets. The AlFe(II)OH-MgFe(III)OH (3550–3562 cm−1) and AlFe(III)OH (870 cm−1) vibrations were only observed in products formed in neutral and alkaline solutions. Analysis of the Mössbauer spectra showed that Fe(III) substituted for Al and Mg in the cis octahedral sites of montmorillonite. The simulation experiments demonstrated that the pH and redox conditions (Eh) of the environment controlled the nature of the product mineral species. Results of the present study revealed that glauconitization and illitization occurred under different conditions, where glauconitization preferentially occurred in an acidic environment and illitization preferentially occurred in a nearly neutral to alkaline environment.