LO and TO crystal modes of the hydroxyl stretching vibrations in micro-Raman and infrared spectra of nacrite

Three pairs of OH-stretching bands are observed at 3709 and 3701 cm^-1, 3653 and 3644 cm^-1, and 3628 and 3623 cm^-1 in curve-fitted micro-Raman and infrared spectra of nacrite. The pairing is interpreted in terms of longitudinal (LO) and transverse (TO) modes of crystal vibrations and labelled A_LO and A_TO, C_LO and C_TO, and D_LO and D_TO, respectively. The relative intensities of the OH-stretching bands in the polarized micro-Raman spectrum depend strongly on both the orientation of the crystallographic axes and the direction of the electric vector of the laser beam. The inner hydroxyl bands D_LO and D_TO, which arise from nearly in-plane vibrations, are intense in the c(aa)c, c(bb)c and b(aa)b spectra, in which the incident laser is polarized along the crystallographic a or b axes parallel to the nacrite plates. Due to nearly out-of-plane vibrations, the inner-surface hydroxyl bands C_LO and C_TO, as well as A_LO and A_TO are intensified in the b(cc)b spectrum, where the incident laser is polarized along the c axis perpendicular to the nacrite plates.