⁵⁷Fe Mössbauer spectral analysis of the sodic clinopyroxenes

Iron-bearing clinopyroxenes were synthesized under hydrothermal, controlled oxygen fugacity conditions at 20 mole% intervals along six binary joins; diopside (CaMgSi₂O₆)–hedenbergite (CaFeSi₂O₆), diopside–acmite (NaFeSi₂O₆), hedenbergite–acmite, acmite–NaCrSi₂O₆, hedenbergite–NaCrSi₂O₆, and acmite–LiFeSi₂O₆.

⁵⁷Fe Mössbauer spectra of these, and of naturally occurring specimens of related compositions, reveal several different types of spectral change as a function of composition. In isovalent solid solutions only small to moderate changes in quadrupole splitting of normal, narrow-line ferrous or ferric doublets are observed. In solid solutions where atoms with different valences are mixed on the same sites, substitution results in marked changes involving line broadening, strong changes in quadrupole splitting and even break-up of single peaks into temperature-dependent, irregularly shaped absorption bands. Lack of a simple one doublet–one site correspondence requires use of more complicated fitting models. The spectra can be fit with multiple doublets having large quadrupole splitting differences. These doublets can be interpreted as either due to different near-neighbor configurations or to a combination of temperature and less specifically localized compositional effects. However, considering the lack of a theoretical basis, the larger number of variables and the generally broadened character of the spectra, neither present interpretation can be considered as firmly established.