Crystal field and charge transfer spectrum of (Mg, Fe)SiO₃ majorite

Abstract

Majorite of bulk composition Mg_0.86Fe_0.15SiO₃ was synthesized at 19 GPa and 1900 °C at an oxygen fugacity close to the Re/ReO₂ buffer. Optical absorption spectra of polycrystalline samples were measured from 4000 to 25000cm⁻¹. The following features were observed: (1) Three bands at 4554, 6005 and 8093 cm⁻¹ due to the ⁵E_g → ⁵T_2g transition of Fe²⁺ in a distorted dodecahedral site. (2) A band at 9340 cm⁻¹ due to the transition ⁵T_2g → ⁵E_g of octahedral Fe²⁺. (3) A band at 22784 cm⁻¹ resulting from Fe³⁺, probably in an octahedral site (⁶A_1g → ⁴A_1g, ⁴E_g). (4) A very intense system of Fe²⁺ → Fe³⁺ intervalence charge transfer bands which can be modelled by two Gaussian components centered at 16542 and 20128 cm⁻¹. The existence of two components in the charge transfer spectrum could be related to the fact that the tetragonal majorite structure may contain Fe³⁺ in two different octahedral sites. The crystal field splitting Δ of Fe²⁺ in dodecahedral coordination is 5717 cm⁻¹. If a splitting of the ground state in the order of 1000 cm⁻¹ is assumed, this yields a crystal field stabilization energy (CFSE) of 3930 cm⁻¹, comparable to the CFSE of Fe²⁺ in pyrope-rich garnet. However, the splitting of ⁵T_2g is significantly higher than in pyrope. This would be consistent with Fe²⁺ preferentially occupying the more distorted one of the two dodecahedral sites in the majorite structure. For octahedral Fe²⁺, Δ= 9340 cm⁻¹ and CFSE=3736 cm⁻¹, assuming negligible splitting of the ground state.