Optical absorption measurements show that pleochroism of erythrite is due to variations in the main absorption envelope in the 400 to 600-mu region as oriented sections are rotated in linearly polarized light. The absorption envelope arises from the 4 T 1 (F) 4 T 1 (P) transition, with contributions from transitions derived from 2 G and 2 H levels, in octahedrally coordinated Co (super 2+) ions, of which there are two kinds (Co I and Co II ) in erythrite. Pleochroism is related to interaction of the electric vector of light with electrons located in specific t2 g orbital lobes of ions in Co I and Co II sites. Validity of the interpretation rests on assumptions that erythrite is isostructural with vivianite and symplesite and that principal optical directions are essentially coincident with 'molecular axes' of the Co (super 2+) -bearing octahedra.

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