Raman spectroscopy of kaolinite hydroxyls between 25 °C and 500 °C

Abstract

Raman spectra of the hydroxyl-stretching region of a low defect kaolinite were obtained at temperatures between ambient and the predehydroxylation temperatures of 500°C, using a Raman microprobe equipped with a thermal stage. At 25°C, Raman bands of the kaolinite hydroxyl stretching region were observed at 3691, 3683, 3670, 3652 and 3621 cm. Upon heating the kaolinite, the band at 3621 cm moved to higher wavenumbers and is observed at 3629 cm at 500 °C. The other four bands attributed to the inner surface hydroxyl groups of kaolinite shift to lower wavenumbers. The 3691 cm band ascribed to the longitudinal optic vibration shifts to 3674 cm at the predehydroxylation temperature of 500°C. The 3683 cm band ascribed to the transverse optic vibration is observed at 3667 cm at 500°C. Upon cooling the Raman spectrum of the kaolinite differed from the initial 25 °C spectrum, suggesting the thermal treatment caused some stacking disorder. The changes in the band position are determined by the strength of the hydrogen bonds formed between the inner surface hydroxyls and the oxygen of the next adjacent layer. Copyright (C) 2000 John Wiley and Sons, Ltd.