Electrical Conductivity and the Red Shift of Absorption in Olivine and Spinel at High Pressure

Abstract

Above 100 kilobars the apparent absorption edges (approximately 3 electron volts) of single-crystal and polycrystalline samples of the metastable olivine and stable spinel forms of Fe₂SiO₄ shift rapidly with pressure from the near-ultraviolet into the lower-energy infrared region. Simultaneously, an exponential increase in electrical conductivity occurs. These effects are reversible as pressure is reduced or reapplied and are not accompanied by a first-order phase change in olivine or spinel. These observations relate to fundamental concepts of electrical conductivity and photon absorption in complex transition-metal silicates in that they cannot be readily interpreted in terms of an intrinsic band-gap model. The intensity and energy changes are too great and the effect occurs at too low a pressure to be explained by processes such as spin-pairing and other crystal-field effects. The results suggest that a new mechanism of conduction, perhaps symbiotic and employing an efficient charge-transfer process, is induced at high pressure.