Glaucophane Spectrum

In this page and next one, I have recorded a few spectra on a blue schist thin section containing pleochroic glaucophane crystal. Those crystals have a very low 2V optic axes angle and a negative optic sign. The pleochroism is violet (g), blue (b) and colorless (a). This glaucophane is in fact the crossite variety.

This section in the prism has an index between a  and b. It has a pale blue color.

Same section with polarizer rotated by 90.  Index close to g with a strong violet color.

The spectra of the crystal above is shown here. The large band moving from 18100 cm-1 to 16000 cm-1 is responsible for the color violet to pale blue when the polarizer is rotated. The large bands come from the charge transfer between Fe++ and Fe+++ ions. The small band at approximately 22800 cm-1 arises from crystal field transition in Fe+++ ions.

Another crossite section nearly parallel to the crystal prism with a pleochroism violet to nearly colorless. This section is oriented with index close to g  in horizontal position. 
Polarizer is rotated by 90 with its principal direction now vertically oriented. The crystal has still a very pale violet color.
Spectra of the prismatic section above.

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