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HYDROTHERMAL SYNTHESIS, BETWEEN 75 AND 150°C, OF HIGH-CHARGE, FERRIC NONTRONITES

¹ Université de Poitiers, UMR 6532 HydrASA CNRS/INSU, 40 Av. Recteur Pineau, F86022 Poitiers cedex, France
² ERT 1074 CNRS Géomatériaux, LMTG-OMP-UPS-IRD-CNRS, 14 Avenue Edouard Belin, F31400 Toulouse, France
³ USM 201 – UMR.CNRS 7160, Muséum National d’Histoire Naturelle, 61 rue Buffon, F75005 Paris, France
⁴ Université de Limoges, UMR 6532 HydrASA CNRS/INSU, 123 Av. A. Thomas, F87060 Limoges cedex, France

^* E-mail address of corresponding author: alain.decarreau{at}hydrasa.univ-poitiers.fr

High-charge nontronites were synthesized at 75, 90, 100, 110, 125, and 150°C from a silico-ferrous starting gel with Si₂FeNa₂O₆.nH₂O composition. This gel was oxidized in contact with air and then hydrothermally treated, for a period of 4 weeks, under equilibrium water pressure. The synthesized nontronites were similar to each other, regardless of the synthesis temperature. Their structural formula, obtained from chemical analysis, X-ray diffraction (XRD), and Fourier transform infrared (FTIR), Mössbauer, and X-ray absorption fine structure spectroscopies is: (Si_3.25Fe³⁺_0.75)Fe₂³⁺O₁₀(OH)₂Na_0.75. A strictly ferric end-member of the nontronite series was therefore synthesized for the first time. The uncommon chemistry of the synthesized nontronites, notably the high level of Fe-for-Si substitution, induced particular XRD, FTIR, and differential thermal analysis-thermogravimetric analysis data. The ethylene glycol expandability of the synthetic nontronites was linked to their crystallinity and depended on the nature of the interlayer cation, moving from smectite to vermiculite-like behavior. As the synthesis temperature increased, the crystallinity of the synthesized clays increased. The nontronite obtained at 150°C had the ‘best crystallinity’, which cannot be improved by increasing synthesis time or temperature.

Key Words: Clay Synthesis • FTIR Spectroscopy • High-charge Nontronite • Mössbauer Spectroscopy • XAFS

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