Optical absorption spectra of transition zone minerals and implications for radiative heat transport

• Published in: Physics and chemistry of the earth Vol. 22; no. 1-2; pp. 113 - 118
• Main Author: Ross, N.L.
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 1997
• ISSN: 0079-1946; 1879-3568
• DOI: 10.1016/S0079-1946(97)00086-4

Optical and near-infrared absorption spectra of synthetic crystals of wadsleyite of bulk composition (Mg0.80Fe0.20)2SiO4 and polycrystalline samples of majorite garnet with composition Mg0.85Fe0.15SiO3 were measured from 4,000 to 25,000 cm−1. Unpolarised spectra obtained from wadsleyite display a continuous band of absorption peaks from the near-infrared through most of the visible region. A minimum of four Gaussian components is required to reproduce the spectra. Two of these bands, at 8,902 cm−1 and 11,136 cm−1, are assigned to crystal field transitions 5T2g→5Eg of Fe2+ in the octahedral sites, and two peaks centred at 13,384 cm−1 and 15,921 cm−1 are attributed to Fe2+→Fe3+ intervalence charge transfer facilitated by the edge-sharing octahedra in the structure. The optical absorption spectrum of majorite also has contributions from crystal field transitions and intervalence charge transfer bands. The peaks at 5,561, 6,294, and 8,439 cm−1 are assigned to 5Eg→5T2g transitions of Fe2+ in the dodecahedral site, and the peak at 9,428 cm−1 is due to 5T2g→5Eg transition of Fe2+ in the octahedral site. The peak at 22,663 cm−1 is assigned to the 6A1g→4A1g (4Eg) transition of Fe3+, probably in an octahedral site. The broad peaks at 16,844 and 20,225 cm−1 are attributed to Fe2+→Fe3+ intervalence charge transfer between edge-shared dodecahedral and octahedral sites in the structure. These results indicate that radiative transport of heat in the Earth's transition zone will be effectively blocked by the strong absorption bands of wadsleyite and majorite that cover the near-infrared and visible range. On the other hand, if the total iron content in the Earth's interior is high enough to generate a continuous network of Fe2+ and Fe3+ polyhedra in these phases, then Fe2+→Fe3+ charge transfer could provide a mechanism increasing the electrical conductivity of the Earth's transition zone.