Thermally modulated absorption of Fe2+ions in micas, vivianite and olivine

Abstract

The thermally modulated absorption (TMA) spectra of Fe ions in micas, vivianite and olivine have been measured in the visible and near infrared region at room temperature and they agree with the corresponding static absorption spectra. The micas show a strong negative TMA peak, located between 12000 cm^-1 and 14000 cm^-1, corresponding to Fe²⁺ to Fe³⁺ intervalence charge transfer and two negative-going peaks at 11000 cm^-1 and 9000 cm^-1 for spin-allowed absorption of Fe²⁺ ions in the crystal field. The TMA spectrum of vivianite is essentially similar to that of micas. The olivine shows a strong negative-going peak at 9700 cm^-1 for the spin allowed absorption of Fe²⁺. The positive background is consistent with the phonon-assisted transition of Fe ions in the near infrared region.