Study of the occurrence of titanium in kaolinites by micro-Raman spectroscopy

Variations in the far infrared region of the Raman spectra of kaolinites are related to the incorporation of titanium in the kaolinite lattice and to the presence of accessory anatase. The spectra of titanium-free kaolinites exhibit weak kaolinite bands. An exceptional intensity of a kaolinite band at 130 cm^-1 in anatase-free kaolinites is characteristic of titanium-bearing kaolinites. The incorporation of titanium in the octahedral sheet increases the intensity of the (Al,Ti)O₆ vibration due to the easily polarizable character of the titanium atoms in the kaolinite lattice. The spectra of anatase-rich kaolinites are dominated by strong bands of anatase. The extreme Raman sensitivity of the main anatase band at 144 cm^-1 is explained by the easily polarizable character of titanium atoms in the anatase lattice and the high symmetry of the O-Ti-O vibration. In anatase-poor kaolinites the band at 144 is accompanied by the band at 130 cm^-1, whereas in anatase-rich kaolinites the former band overlays the latter band.