The polarized optical absorption spectra of tourmaline (blue-green), cordierite, chlori-toid and vivianite have been measured in the 7,000-30,000 cm-1 (1430-330 nm) region.

In the near-infrared region each spectrum exhibits a d-dband system assignable to the 5T25E transition of Fe2+ in pseudo-octahedral sites. This system is split into two components by the dynamic Jahn-Teller mechanism in the spectra of vivianite and chloritoid.

All spectra show a number of minor features in the visible region that are probably due to spin-forbidden transitions of octahedrally-bonded Fe2+.

The marked visible pleochroism of cordierite, vivianite and to a lesser extent of the tourmalines, is due to a strongly pleochroic band that is considered to be due to the electronic interaction of adjacent Fe2+ and Fe3+ ions so located in the crystal lattice that certain of their d-orbital lobes overlap, in many cases, along specific crystallographic directions. It is suggested that the Fe2+→Fe3+ interaction is among the more important and widespread causes of pleochroism in ferromagnesian silicates.

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