Optical Properties and Electronic Structure of Mantle Silicates

Abstract

Unpolarized specular reflectance spectra at near-normal incidence were measured in the silicates olivine, pyroxene, spinel and Al-garnet in the photon energy range 4–14 eV at 300°K, and the optical and dielectric functions were obtained from the reflectance by Kramers-Kronig analysis. The spectra indicate that these minerals, with a variety of divalent cation substitutions, have wide valence and conduction bands separated by an energy gap (Eg) of 7.5–9.5 eV that is marked by a prominent absorption peak of an exciton associated with this gap. Eg increases with the ratio x = Mg/(Mg+Fe+Mn+Ni+Ca) and in olivine it varies from 7.8 eV in fayalite to 8.8 eV in forsterite. For a given x, Eg increases in the order spinel, olivine, pyroxene, garnet over a range of about 1.5 eV. While the pure Mg-silicates are transparent at photon energies below the exciton peak, transition metal substitutions result in strong absorption beginning about 1–2 eV below Eg, presumably due to electronic transitions between the wide bands and d states localized on the transition metal cations. The large magnitude of Eg rules out the possibility that electronic band conduction due to valence-conduction band excitations is an important conduction mechanism in mantle silicates.