Three gels of illitic composition having different octahedral substitutions were hydrothermally treated in distilled water at 300°C and 500 bars for 7 to 90 days. The gel of the composition K0.7Al2.0(Si3.3Al0.7)O10(OH)2, having no octahedral substitutions, yielded poorly crystalline and randomly interstratified illite/smectite (I/S) having possibly 30% or more expandability. These I/S particles occurred predominantly as foliated or compact lamellar aggregates having morphological features characteristic of common smectite. Laths having a morphology similar to that of diagenetic “fibrous illites” were also found in small amounts after the gel had been aged for 35 days. Their chemical composition was similar to that of the foliated aggregates of I/S in the same run.

Illitic gel having the composition K0.7(Al1.7Fe3+0.3)(Si3.3Al0.7)O10(OH)2 having octahedral Fe3+ substitutions yielded more and better crystalline I/S. The reaction products of the 7-35-day runs consisted mainly of randomly interstratified I/S having an expandability of 90 to 65%. The I/S particles occurred as foliated or compact lamellar aggregates having a smectite-like morphology, as described above. The Fe3+-bearing gel yielded, after 90 days of ageing, an Rl-ordered I/S having 25% expandability and a morphology of ribbons and hexagonal platelets.

Illitic gel of composition K0.8Al1.6Mg0.4)(Si3.6Al0.4)O10(OH)2 having octahedral Mg2+ substitutions produced no illite even after 90 days of ageing, but a pure smectite in the form of foliated aggregates.

To obtain Cu-kaolinites with a controlled range of chemical compositions, syntheses were performed by hydrothermally ageing gels with kaolinite stoichiometric compositions. Gels were prepared with sodium metasilicate and nitrates of octahedral cations. Temperature of synthesis was 250°C with a corresponding equilibrium water pressure of 38 bars.

Three samples with copper contents ranging from 0.1 to 7% and another one with the chemical composition of the Cu end-member were synthesized. While this fourth sample led to tenorite after the hydrothermal treatment, the three others crystallized well into kaolinite.

Up to almost 1% CuO was measured by TEM in some isolated ‘clean’ and hexagonal kaolinite particles. EPR and XPS spectroscopies were consistent with an octahedral position of Cu2+. In IR spectra, vAl-OH-Cu absorption bands were not observed, but vAl2OH bands appeared more and more blurred when Cu content of samples increased. Weak bands situated at 868 cm−1 and 840 cm−1 are tentatively attributed to δAlCuOH. By differential thermal analysis, a downward shift of 20°C in temperature of the endothermic peak from the less Cu-rich sample to the most Cu-rich one, argued for the existence of some Al-OH-Cu bonds, whose binding energies are presumed to be less than the Al-OH-Al ones.

In view of these results, Cu2+ appears incorporated in the octahedral sheet of kaolinite. Moreover, this incorporation is made without major perturbation of the kaolinite structure.

Fourier transform infrared (FTIR) spectroscopy investigations in the near infrared (N1R) region of synthetic and natural kaolinites with various octahedral substitutions have been carried out in order to elucidate the relationships between the substituted cations and specific features of the NIR spectra. The combination modes of the OH stretching and bending vibrations characterizing Fe(III), Ga(III) and Cr(III) octahedral substitutions are identified in the NIR region at 4466, 4498 and 4474 cm-1, respectively, and the first overtones of the OH stretching vibrations at 7018, 7018 and 6986 cm-1, respectively. As far as we know, the bands of kaolinites containing Ga(III) or Cr(III) have not been reported yet. For both Ga(III) and Cr(III), the NIR observations explain why the bending vibration bands of AlGaOH and AlCrOH groups are not observed in the middle infrared (MIR) region.