An optical absorption study of the origin of colour and pleochroism in pink and brown tourmalines

Simple ligand-field theory is used in assigning absorption bands of polarized tourmaline spectra to specific transition-metal ions in specific environments. The pleochroic properties of the bands are correlated with some structural properties of the crystal. It is suggested that the colour of pink and brown tourmalines is due to d-d electronic transitions in octahedrally-bonded Mn (super 3+) and Ti ions, respectively. Pink tourmalines have an absorption band at 19,000 cm (super -1) (520 mmu ) and brown tourmalines at 22,000 cm (super -1) (450 mmu ). The bands in both systems have maximum intensity when the electric vector of incident light is vibrating in the 001 plane; the probable causes of the pleochroism are discussed. It is also concluded that the transition metal ions are located mainly in the trigonal brucite units.