New Insight into the Relationships Between Structural and Ftir Spectroscopic Features of Kaolinites

Abstract To resolve the existing ambiguities in the interpretation of the OH-stretching vibrations of kaolinites, relationships were, for the first time, established between the structural and Fourier-transform infrared (FTIR) spectroscopic features for a set of kaolinite samples which differed in terms of their relative amounts of coexisting high- and low-ordered phases. For this purpose, a representative collection of kaolinites differing in origin, particle size, and degree of disorder was studied by powder X-ray diffraction (XRD) and FTIR spectroscopy. Modeling of the experimental XRD patterns based on the orthogonal layer unit cell having a mirror plane showed each sample to be a mixture of nearly defect-free high-ordered (HOK) and low-ordered (LOK) kaolinite phases, with HOK varying from 86 to 4%. The wavenumbers, heights, areas, and full widths at half-maximum (FWHM) were determined for the OH-stretching bands at ~3697 ( ν 1 ), ~3670 ( ν 2 ), ~3652 ( ν 3 ), and 3620 cm –1 ( ν 4 ) by decomposition and fitting of the FTIR spectra. The FWHM(ν 1 )/FWHM(ν 4 ) and FWHM(ν 3 )/FWHM(ν 2 ) values were related linearly to the HOK content, which may be associated with the in-phase and out-of-phase character of the corresponding pairs of vibrations, respectively. A novel interpretation was suggested for the variations in the relative integrated intensities of the OH bands with the amount of the HOK phase. The intensity distribution of the ν 2 and ν 3 bands is controlled by the triclinic structure symmetry in the defect-free kaolinite and the mirror symmetry of the layers in low-ordered structures, in agreement with the observed evolution of the corresponding band intensities. The ν 1 and ν 2 band positions for the low-ordered samples are within the wavenumber range for the high-ordered samples. In contrast, the ν 3 and ν 4 band positions for the low-ordered samples are shifted toward higher wavenumbers, indicating that some of the low-ordered kaolinites should contain dickite-like structural fragments distributed among kaolinite layers.