Hydroxyl Deformation in Kaolins

Ray L. Frost
Centre for Instrumental and Developmental Chemistry, Queensland University of Technology, 2 George Street, GPO Box 2434, Brisbane Qld. 4001, Australia

Abstract: The hydroxyl deformation modes of kaolins have been studied by Fourier transform (FT) Raman spectroscopy. Kaolinites showed well-resolved bands at 959, 938 and 915 cm−1 and an additional band at 923 cm−1. For dickites, well-resolved bands were observed at 955, 936.5, 915 and 903 cm−1. Halloysites showed less-resolved Raman bands at 950, 938, 923, 913 and 895 cm−1. The first 3 bands were assigned to the librational modes of the 3 inner-surface hydroxyl groups, and the 915-cm−1 band was assigned to the libration of the inner hydroxyl group. The band in the 905 to 895 cm−1 range was attributed to “free” or non-hydrogen-bonded inner-surface hydroxyl groups. The 915-cm−1 band contributed ∼65% of the total spectral profile and was a sharp band with a bandwidth of 11.8 cm−1 for dickite, 14.0 cm−1 for kaolinites and 17.6 cm−1 for halloysites. Such small bandwidths suggest that the rotation of the inner hydroxyl group is severely restricted. For the inner-surface hydroxyl groups, it is proposed that the hydroxyl deformation modes are not coupled and that the 3 inner-surface deformation modes are attributable to the three OH2–4 hydroxyls of the kaolinite structure. For intercalates of kaolinite and halloysite with urea, a new intense band at ∼903 cm−1 was observed with concomitant loss in intensity of the bands at 959, 938 and 923 cm−1 bands. This band was assigned to the non-hydrogen-bonded hydroxyl libration of the kaolinite-urea intercalate. Infrared reflectance (IR) spectroscopy confirms these band assignments.

Key Words: Deformation Mode • Dickite • FT Raman • Halloysite • Hydroxyl • Infrared • Intercalation • Kaolinite • Libration • Reflectance

Clays and Clay Minerals; June 1998 v. 46; no. 3; p. 280-289; DOI: 10.1346/CCMN.1998.0460307
© 1998, The Clay Minerals Society
Clay Minerals Society (www.clays.org)