Temperature dependence of the polarized electronic absorption spectra of olivines. Part II?Cobalt-containing olivines

Polarized electronic absorption spectra of single crystalline Co2[SiO4] and (Co0.64Mg0.36)2[SiO4] (E|| a (|| Z), E || b (|| X), E || c (|| Y)) have been studied in the temperature range 293 ≤ T/K ≤ 1273. The three polarized spectra show a total of 15 bands. Five bands are caused by spin-allowed transitions in Co2+ ions at M1 sites which appear in all polarization directions. Seven polarization-dependent bands can be ascribed to spin-allowed transitions in Co2+ ions at M2 sites and three bands may be assigned to spin-forbidden transitions. The assignment of bands due to Co2+ ions at M1 and M2 sites has been made on the basis of transition energies and intensity ratios. Further arguments have been derived from the comparison of spectra of crystals with different cobalt content, from the analysis of the polarization dependence of the spectra, and from the evolution of band intensities with temperature.