Crystal Field Splitting and Covalent Bonding in Fe++ Silicate Garnets

Abstract

A point‐ion model and Slater free‐ion wave functions have been used to calculate the crystal field splitting of the Fe⁺⁺ ion ⁵D ground term in Fe₃Al₂(SiO₄)₃ garnet for various assumed charge distributions within the (SiO₄)^4— ion. The best fit with Clark's experimental data on absorption spectra is obtained with about 50% covalent bonding between Si and O, and with an effective nuclear charge for the Fe⁺⁺ ion about 10% less than that given by Slater's rules. This estimate of the degree of covalency is consistent with that given by Pauling. The calculated pressure dependence of the optical absorption is in satisfactory agreement with the experiments of Balchan and Drickamer.