Characterization of octahedral substitutions in kaolinites using near infrared spectroscopy

Fourier transform infrared (FTIR) spectroscopy investigations in the near i nfrared (NIR) region of synthetic and natural kaolinites with various octah edral substitutions have been carried out in order to elucidate the relatio nships between the substituted cations and specific features of the NIR spe ctra. The combination modes of the OH stretching and bending vibrations cha racterizing Fe(III), Ga(III) and Cr(III) octahedral substitutions are ident ified in the NIR region at 4466, 4498 and 4474 cm(-1), respectively, and th e 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 G a(III) or Cr(In) 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.