||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.