This section in the prism has an index between a and b. It has a pale blue color.
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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. |
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Same section with polarizer rotated by 90°. Index close to g with a strong violet color. |
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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. |
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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. |
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Polarizer is rotated by 90° with its principal direction now vertically oriented. The crystal has still a very pale violet color. |
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Spectra of the prismatic section above. |
