Optical spectra of Co2+in three synthetic silicate minerals

Abstract

Polarized optical absorption spectra of three synthetic Co-bearing silicates, orthopyroxene, olivine and beryl, were studied at room and liquid nitrogen temperatures. In all three matrices, Co enters octahedral structural sites as the Co 2+ ion. In accordance with the d 7 electronic energy level diagram, the spectra show three distinct band systems which are assumed to originate from three spin-allowed dd transitions of Co 2+ , 4 T 1g ( 4 F) → 4 T 2g ( 4 F), 4 T 1g ( 4 F) → 4 A 2g ( 4 F), and 4 T 1g ( 4 F) → 4 T 1g ( 4 P). The number of bands is different in different matrices and is regulated by the splitting of electronic states, symmetry selection rules for electronic transitions between them, and Co 2+ ion distribution between non-equivalent structural sites. Due to a strong preference of Co 2+ for the M2 site and the greater intensities (oscillator strengths) of electronic dd transitions for Co 2+ in the M2 site, M2 Co 2+ bands prevail in the (Mg,Co)SiO 3 orthopyroxene spectra. In olivines, the number of bands is larger than in orthopyroxene due to the combined contribution of both M1 Co 2+ and M2 Co 2+ ions. By comparison of the (Mg,Co)2SiO 4 and Co 2 SiO 4 olivine spectra, the bands caused by M1 Co 2+ and M2 Co 2+ were distinguished. The spectra of Co-bearing beryls, Be 3 Al 2 [Si 6 O 18 ], prepared by both flux and hydrothermal growth, indicate that Co enters the structure mostly as Co 2+ in the octahedral Alsite.