Near-ultraviolet to near-infrared optical (diffuse reflectance) spectra of several nontronites and related Fe-bearing smectites [(Fe2+,Fe3+)-bearing saponite and (Fe2+,Fe3+)-bearing montmorillonite] are presented and interpreted. Mössbauer spectra at 298 K are also presented to help interpret the optical spectra. The optical spectra of nontronites are dominated by the ligand field transitions of Fe3+ in octahedral coordination sites. Values for the ligand-field-theory parameters 10Dq, B, and C, are 15050 cm−1, 614 cm−1, and 3268 cm−1, respectively. In addition to the ligand field transitions of single Fe3+ cations, a broad absorption band centered near 22000 cm−1 is observed that may be due to the simultaneous excitation of two Fe3+ cations to the 4Tlg(4G) state. Such “pair excitations” have been proposed in the spectra of other Fe3+-bearing minerals in which next-nearest-neighbor Fe3+ cations are magnetically coupled. Alternatively, this band may represent excitations to the 2A2g and 2T1g ligand field states.
For most samples, the amount of tetrahedrally coordinated Fe3+ was below that detectable by Mössbauer spectroscopy (1-3% of total Fe). However, the optical spectra of all of the nontronites show an absorption band near 23000 cm−1. This band is assigned to the 6A1 → 4E,4A1 transition of tetrahedrally coordinated Fe3+.
The optical spectra of mixed-valence Fe-bearing smectites [(Fe2+,Fe3+)-bearing saponite and (Fe2+,Fe3+)-bearing montmorillonite] show a broad absorption band at 14000-15000 cmowing to Fe2+ → Fe3+ charge transfer.