Transverse and longitudinal crystal modes associated with OH stretching vibrations in single crystals of kaolinite and dickite

Raman spectra of randomly-oriented kaolinite, dickite and nacrite show, for coarsely crystalline material, an extra band in the OH stretching region w hich is absent from the IR spectra of clay-size samples. Oriented single-cr ystal Raman spectra of these minerals provide confirmation for the assignme nt of the extra bands to transverse optical modes involving in-phase couple d vibrations of the layer-surface hydroxyl groups. The corresponding IR ban ds have transition moments nearly perpendicular to the layer surface, and a ppear at the higher frequencies of the longitudinal optical modes of macros copic crystals. (C) 2000 Elsevier Science B.V. All rights reserved.