On the origin of colour and pleochroism of astrophyllite and brown clintonite

In a spectral study, sections of both minerals cut parallel and perpendicular to the 001 (cleavage) plane were examined in polarized light. Both show a non-pleochroic two-band system in the near-infrared region that is assigned to d-d transitions in octahedrally-bonded Fe (super 2+) , and exhibit a strong pleochroic absorption band in the blue end of the spectrum that is the obvious cause of color and pleochroism of the minerals. The band is observed as 23,000 cm (super -1) in astrophyllite, and has maximum intensity when E coincides with the orientation of Ti-O-Ti units. The band is observed at 22,000 cm (super -1) in clintonite with maximum intensity when E coincides with the plane of d-orbital overlap between neighbouring cations in the octahedral layer. (E represents the vibration direction of the electric vector of incident light). In both minerals, the band is assigned to Ti (super 3+) -Ti (super 4+) electronic interaction.