Differing effects of particle size and in the infrared and Raman spectra kaolinite shape of

A B S T R ACT : Infrared (IR) spectra of well-ordered kaolinites generally show four OH-stretching frequencies, near 3697, 3670, 3652 and 3620 cm i. Raman spectra of the same kaolinites mostly show an additional band near 3686 cm-~, which, in the coarsely crystalline Keokuk kaolinite, largely replaces the 3695 cm-I band. It is shown that the 3686 cm ~ 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 (<2 ~tm) particles, which have mean thicknesses of 30-150 nm. Infrared radiation of wavelength near 2.7 gm can only excite whole crystal vibrations perpendicular to the plates. The in-phase vibration of inner-surface OH groups in the whole-crystal mode lies at 3697 cm-1, which is the same frequency as that of the longitudinal optical mode in macroscopic crystals. Infrared spectroscopy is a powerful and widely applicable tool for the characterization of both inorganic and organic materials, but requires some care in the preparation of samples for examination. Raman spectroscopy provides information similar but complementary to that provided by IR spectro-scopy, and can often be applied non-destructively to bulk samples. However, it is difficult to obtain satisfactory Raman spectra from very fine powders. Recent technical improvements have made it easier to obtain Raman spectra of mineral powders, and a nmnber of papers have explored the Raman spectra of various kaolinites in order to assess the value of Raman for the characterization of kaolinite. The results have presented an apparent anomaly, in that the OH-stretching region in Raman spectra often shows a band near 3686 cm-~, in addition to the four bands observed in IR spectra, near 3697, 3670, 3652 and 3620 cm-1. The relative intensities of the 3697 and 3686 cm-I bands vary inversely. The 3686 cm-I band predominates in the Raman spectrum of the coarsely crystalline Keokuk kaolin-ite (Wiewiora et al., 1979), but is absent or very weak in the spectra of specimens published by Pajcini & Dhamelincourt (1994), Frost (1995) and Frost et al. (1996). Spectra showing a wide range in relative intensities of the two bands have been published by Johnston et al. As kaolinite has a triclinic structure with no symmetry other than translation, all …