RAMAN-SPECTROSCOPY OF THE KAOLINITE HYDROXYLS AT 77 K

Raman spectroscopy of two types of kaolinites has been obtained at liq uid nitrogen temperature (77 Ii) with the use of a Raman microprobe an d a thermal stage. The Raman spectrum is characterized by the combinat ion of the frequencies of the inner hydroxyl and the inner surface hyd roxyl groups. The inner hydroxyl frequency is reduced, and the outer h ydroxyl frequencies move to higher frequencies upon cooling to 77 Ii. The inner hydroxyl frequency shifts from 3620 cm(-1) at 298 Ii to 3615 cm(-1) at 77 Ii. The two in-phase inner surface hydroxyl frequencies move from 3684 and 3689 cm(-1) at 298 Ii to 3690 and 3699 cm(-1) at 77 Ii. The two out-of-phase vibrations shift from 3650 and 3668 cm(-1) t o 3656 and 3675 cm(-1), The bandwidth of the inner hydroxyl frequency decreases from 3.7 to 2.1 cm(-1) at 77 Ii. The bandwidth of the inner surface hydroxyl frequency (nu(1)) increases upon cooling from 17.4 to 19.2 cm-(-1). It is proposed that the increased resolution at low tem perature enabled an additional inner surface hydroxyl frequency to be observed.