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

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−1. Upon heating the kaolinite, the band at 3621 cm−1 moved to higher wavenumbers and is observed at 3629 cm−1 at 500 °C. The other four bands attributed to the inner surface hydroxyl groups of kaolinite shift to lower wavenumbers. The 3691 cm−1 band ascribed to the longitudinal optic vibration shifts to 3674 cm−1 at the predehydroxylation temperature of 500 °C. The 3683 cm−1 band ascribed to the transverse optic vibration is observed at 3667 cm−1 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.