The OH-stretching region in curve-fitted micro-Raman, photoacoustic and transmission IR spectra of St. Claire dickite was investigated. Polarized Raman spectra recorded from th. (001) an. (010) faces of the dickite crystal displayed six prominent OH bands. The relative intensities depend strongly on both the orientation of the crystallographic axes and the direction of the electric vector of the laser beam. Four out-of-plane vibrations, AA, Az, CA and Cz, at ~3710, 3706, 3654 and 3643 cm-1 respectively, predominate when the electric vector is perpendicular to the dickite plates. Two in-plane vibrations, Dz and DA at 3627 and 3623 cm-1, intensify when the electric vector is parallel to the plane. The relationship between band intensity and crystal orientation was interpreted in terms of longitudinal optic (LO) and transverse optic (TO) crystal vibration modes. These LO and TO crystal modes were also observed in curve-fitted photoacoustic and transmission IR spectra of coarse, non-oriented crystals of the dickite.
