The optical absorption spectra of iron in six-coordinate sites in chlorite, biotite, phlogopite and vivianite; some aspects of pleochroism in the sheet silicates

Electron absorption spectra have been measured in the 7,000-30,000 per cm region. The complex spectra of chlorite and biotite in the visible region show similarities to vivianite, known to contain Fe (super 2+) in octahedral sites. It is proposed that most of the features in the 15,000-30,000 per cm region of these spectra are due to spin-forbidden transitions of Fe (super 2+) . Spectra of all minerals show a double-band system due to spin-allowed transitions of octahedrally-bonded Fe (super 2+) . Splitting of bands is attributed to the dynamic Jahn-Teller mechanism. A relatively intense polarized band at 14,000 per cm, assigned to Fe (super 2+) Fe (super 3+) interaction, occurs in spectra of chlorite, biotite and some phlogopites. This, together with polarized high-energy charge transfer processes, is the cause of pleochroism in sections of these minerals perpendicular to the (001) cleavage.