Visible spectrum of hornblende.

Picture of a hornblende crystal in thin section showing the pleochroism from green to blue to yellow depending on the section orientation. The two images were photographed with a polarizer rotation by an angle of 90.

After rotation of the polarizer, the section changes its color.

Spectra of the blue and yellow-green sections. The large band in the visible spectrum of hornblende is attributed to a charge transfer between Fe++ and Fe+++. The position of this band is orientation dependant so the observed pleochroism. Two small additional spin forbidden bands are also visible at lower wavelengths.

Crystal orientation with light green - dark green pleochroism. 

 

Spectra of the crystal above, in both orientations. The pleochroism is due to a large change in the charge transfer band with crystal orientation.

View of another crystal orientation. Polarizer direction is close to X axis for the yellow crystals and close to Z axis for the blue ones.
 

Spectra of the elongated blue-yellow hornblende crystals. The dark blue orientation is close to Z axis. The charge transfer band is the strongest. In the perpendicular orientation, the light vibration direction is close to X axis. In this case, the charge transfer band Fe++ ---> Fe+++ almost disappears. The yellow color is due to the end of a strong UV band.

 

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