Visible spectrum of hornblende (1).

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.

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