Hydroxyl-stretching bands 'A' and 'Z' in Raman and infrared spectra of kaolinites

The high frequency OH-stretching band in micro-Raman spectra of kaolinites consistently exhibits two partly overlapping components with varying relative intensities. These bands, labelled A and Z, are located at ~3700-3690 and 3690-3680 cm^-1, respectively. Band Z displays greater intensity in Raman spectra of highly crystallized kaolinites which have large coherent domains. Both components are also observed in the curve fitted infrared (IR) spectra of kaolinites; band Z is stronger in IR spectra of highly crystallized samples. Data for kaolinites with different particle size support the idea that bands A and Z are the longitudinal optic and transverse optic modes of the high frequency OH-stretching band.