EXAFS spectroscopy, combined with X‐ray powder diffraction, chemical and thermal analysis, scanning electronic microscopy, 29Si, 27Al, and 19F MAS NMR spectroscopy, are used in the structural analysis of montmorillonites, synthesised in an acidic fluoride medium. Hydrothermal synthesis performed under mild conditions (493 K, autogenous pressure) enables the formation of montmorillonite clays, containing Al and Zn or Mg in the octahedral sheet. It is shown that montmorillonites can only be synthesized in a narrow range of compositions. An accurate value of the Al for Si substitution rate in the tetrahedral sheet is determined by using 27Al MAS NMR spectroscopy performed under definite conditions. Some interatomic distances are determined by EXAFS spectroscopy (RMg−O = 2.11 Å, RZn−O = 2.08 Å, RZn−Al = 2.98 Å, RZn−Zn = 3.11 Å) which reveals strong local distortions in the octahedral sheet with respect to the ideal montmorillonite structure. Lengthening of the out‐of‐plane Zn−Si(Al) distances also shows the swelling of the whole layer in the neighbourhood of Zn. Moreover, the combination of EXAFS and 19F MAS NMR spectroscopic data reveals a clustering of the divalent elements of the octahedral sheet, which is related to a possible local trioctahedral character of these materials.

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Talc, Mg3Si40io(OH)2 is a trioctahedral phyllosilicate with no layer charge. It is macroscopically hydrophobic as it floats naturally. However, water immersion calorimetry measurements reveal that, once outgassed at medium temperatures (100-400 °C), it exhibits a strong affinity towards water molecules. Controlled rate thermal analysis measurements coupled with mass spectrometric analyses show that, upon outgassing, different surface species (water, nitrogen, carbon dioxide, and organic molecules) are released from the talc surface. The behavior of the talc surface toward nitrogen, argon, and water as a function of the outgassing temperature was then studied in detail. The surface presents highly energetic sites for nitrogen and water molecules. These sites are assigned to the hydroxyl groups pointing toward the basal surface of talc through the hexagonal cavity formed by the silica tetrahedra. The adsorption of water and nitrogen on a synthetic fluorinated talc confirms this assignment as the substitution of OH groups by fluorine atoms suppresses the specific interactions. On outgassed talc, the presence of highly hydrophilic sites isolated on a hydrophobic surface controls the adsorption of water which seems to occur through the growth of hydrogen-bonded clusters anchored on the OH groups.

The fluorine route is thoroughly investigated for the hydrothermal synthesis of montmorillonite in the Na2O-MgO-Al2O3-SiO2-H2O system. Using the optimal conditions suggested by Reinholdt et al. (2001) for the crystallization of pure montmorillonites with the formula Na2x(Al2(1-x)Mg2x☐)Si4O10(OH)2, several parameters (x, Mg content, duration of crystallization, F/Si atomic ratio, pH, nature of counterbalance cation) are varied independently from their ideal values. The products are analysed by various techniques (X-ray diffraction, thermogravimetric analysis-differential thermal analysis, 29Si, 27Al and 19F magic angle spinning-nuclear magnetic resonance). It appears that a pure montmorillonite can only be obtained within a narrow x range (0.10 ≤ x ≤ 0.20). The presence of F in the starting hydrogel and the crystallization time also have significant effects on the purity of the final products. It is shown that a small amount of fluorine is needed for the crystallization of pure montmorillonite phyllosilicates.