Ligand field theory and inter- and intracrystalline cation distribution of transition elements in minerals and related inorganic compounds

Many independent investigations have shown, that the ligand field stabilization energy LFSE has, in addition to other factors such as ionic size, site distortion, covalency, respectively, an important influence on the distribution of transition elements between different coexisting minerals as well as between different, crystallographic sites within the same mineral. A transition element prefers or is enriched in that mineral or that lattice position within a mineral, in which its LFSE has its maximum value. Principally, the LFSE of transition metal ions can be evaluated from the absorption spectra in the visible and near infrared parts of the electromagnetic spectrum. In the present paper, the influence of the LFSE on the polyhedral distortion, the phase stability, the intra-as well as the intercrystalline cation distribution of transition metal ions in some well established examples is presented and discussed. The Gibbs free energies of the exchange reactions can be evaluated from the site occupancies and used as a measure for the site preference. Deviations from a pure ionic model are explained on the basis of MO-theory and covalent bonding and other crystal chemical effects.