A fifth OH-stretching band in IR spectra of kaolinites

Transmission infrared spectra of different kaolinites were studied by curve fitting. These spectra generally exhibit four hydroxyl stretching bands. In this article we show that a fifth OH band (already identified in Raman and photoacoustic IR spectra of kaolinites) is also observed in transmission IR spectra of hydrothermal and authigenic kaolinites, which have a high degree of crystallinity. This additional band is weak or undetectable for kaolinites with a low degree of crystallinity.