On the origin of colour and pleochroism of kyanite

Although the colour and pleochroism of blue kyanite has been attributed to the presence of Ti (super 3+) by previous workers, the present study of the polarized absorption spectra of such kyanite indicates that Fe (super 2+) Fe (super 3+) charge transfer accounts more readily for the optical properties. Experimental evidence is given for the presence of both Fe (super 2+) and Fe (super 3+) in blue kyanite and proposals are made for the manner in which these ions interact, across octahedral edges, to give rise to a pleochroic absorption band at 16500-1700 cm (super -1) . The direction of maximum intensity of this band correlates directly with the principal optical direction, Z, of kyanite.