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
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
Spectra of the crystal above, in both orientations. The
pleochroism is due to a large change in the charge transfer band with
View of another crystal orientation. Polarizer direction is
close to X axis for the yellow crystals and close to Z axis for the blue
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.