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Normal biotite has the absorption formula Y>Z>>X with Y reddish brown, Z greenish brown (thick sections only) and X pale brown. Rarely biotite is pleochroic with absorption X>>Y>Z. Blue River, B. C., biotite is of the second ("reverse") type with the pleochroism
especially pronounced in the short-wave portion of the visible spectrum. Brown biotite-phlogopite from Old Chelsea, P.Q. is optically similar. The effect is caused by a broad transmission maximum for light vibrating parallel with the mica sheet and a narrow transmission maximum for light vibrating across the sheet. Green phlogopite from Old Chelsea shows less marked reverse pleochroism. With thick sheets transmission maximum Y is at 6200 A and Z at 5800 A. No absorption maxima were noted in these varieties. Chemical data suggest the reverse pleochroism may be associated with ferric iron in tetrahedral sites.
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