Crystal-Field and Mössbauer Applications to the Study of Site Distribution and Electronic Properties of Ferrous Iron in Minerals with Emphasis on Calcic Amphiboles, Orthopyoxene and Cordierite

The electronic absorption spectroscopy of ferrous iron is sensitive to the geometry of the coordination site in which it resides. This sensitivity enables ferrous iron in multiple sites in a mineral to be distinguished. The spectra of ferrous iron in the M(2) site in orthopyroxene, (Mg,Fe)SiO₃, are used as a model for the spectroscopic properties of iron in a distorted site. The splitting of the ⁵T_(2G) ground state is observed to be 2350 cm⁻¹ enabling a theoretical point-charge model to be developed using C_(2V) symmetry. The intensity of the near infrared bands due to the splitting of the ⁵E_G state are found to linearly correlate with the concentration of ferrous iron in the M(2) site. From this correlation, calibrations are established for the intensities of the near infrared bands so that quantitative site distributions can be determined for single orthopyroxene crystals from optical spectra. Thermally induced cation disorder allows assignments to be made for spin-allowed and spin-forbidden ferrous iron bands originating from both the M(1) and M(2) sites. The electronic absorption and Mössbauer spectra of calcic amphiboles, Ca₂(Mg,Fe)₅Si₈O₂₂(OH)₂, are reinterpreted to include previously neglected contributions from ferrous iron in the calcium-rich M(4) site. Bands due to ferrous iron in M(l), M(2) and M(3) sites are examined in the electronic spectra and the intensity of the Fe²⁺/Fe³⁺ intervalence charge-transfer band is found to linearly correlate with the ferric iron content. Next­ nearest neighbor variations of ferric iron and aluminum are found to affect the ferrous iron peak parameters in the Mössbauer spectra of calcic amphiboles which impairs the capability of determining accurate site distributions. Ferrous iron is found to be present in the channel cavities and the octahedral site in cordierite, (Mg,Fe)₂Al₄Si₅O₁₈, osumilite, K(Mg,Fe)₂Al₃(Si₁₀Al₂)O₃₀, and beryl, Be₃Al₂Si₆O₁₈. In cordierite, two types of water are suggested to be present in the channel cavities that differ in crystallographic orientation and relationship to other channel constituents, whereas osumilite is found to be virtually anhydrous. In cordierite, cation migration within the channels is suggested to occur after dehydration which could explain the observed change in the lattice geometry. The blue color in these minerals is suggested to be due to intervalence charge transfer between ferrous iron and channel ferric iron. Electronic spectra suggest that structural state variations occur in osumilite, whereas significant variations in cordierite are not apparent. Ferric iron in tetrahedral coordination in osumilite is indicated from Mössbauer spectra. The effect of site size and distortion on the spectroscopic properties of ferrous iron in terms of band position, intensity and polarization anisotropy is examined. As a non-centrosymmetric site becomes larger, absorption bands migrate to longer wavelengths (lower energy), become more intense, and exhibit greater polarization anisotropy among each other. For these sites, intensification is correlated with a decrease in the quadrupole splitting determined from Mössbauer spectra. The spectroscopy of ferrous iron in large sites is distinctly different from that observed from ferrous iron in smaller sites.