Abstract
Infrared (IR) spectra of well-ordered kaolinites generally show four OH-stretching frequencies, near 3697, 3670, 3652 and 3620 cm (super -1) . Raman spectra of the same kaolinites mostly show an additional band near 3686 cm (super -1) , which, in the coarsely crystalline Keokuk kaolinite, largely replaces the 3695 cm (super -1) band. It is shown that the 3686 cm (super -1) band can be ascribed to a transverse optical crystal vibration involving the in-phase stretching vibration of the three inner-surface hydroxyl groups in the unit-cell. Raman exciting radiation, commonly of wavelength near 500 nm, can excite this crystal vibration in crystals of comparable and greater thickness. Infrared studies are usually made on clay-size (
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