Optical (diffuse reflectance) and Moessbauer spectroscopic study of nontronite and related Fe-bearing smectites

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 ⁴T_lg(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 ²A_2g and ²T_1g 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.