Möessbauer spectroscopy and crystal chemistry of natural Fe–Ti garnets

Anomalies in site occupancy data for iron cations from recent Mössbauer studies of titanian garnets have led to a reassessment of fitting and assignment of doublets in these complex spectra. Component peaks of doublets have been resolved in spectra of a suite of melanites and schorlomites with a significant decrease in the statistical parameters, χ² and misfit, as well as a decrease in the uncertainty of calculated positions, widths, and intensities. Doublets have been assigned to Fe³⁺ in octahedral [Y] and tetrahedral (Z) sites, Fe²⁺ in 8-fold triangular dodecahedral {X} and octahedral [Y] sites, and Fe²⁺ {X} → Fe³⁺(Z) electron delocalization. The latter assignment, which obviates tetrahedral Fe²⁺ replacing Si as suggested in recent studies, is supported by theoretical considerations (ionic radii, crystal field site preference energies, short X–Z separation of 3.015A, edge-shared XO₈ and ZO₄ polyhedra), and correlation with previous optical studies. The absorption band at 5280 cm⁻¹, whose temper-ature-dependence argues against assignment as a tetrahedral Fe²⁺ crystal field transition, agrees with assignment as Fe²⁺ {X} → Fe³⁺(Z) charge transfer. Discrepancies between calculated Fe²⁺/∑Fe ratios from Mössbauer and wet-chemical analyses indicate the presence of substantial Ti³⁺ in some natural Fe–Ti garnets. Site occupancy data show the relative enrichments in the tetrahedral (Z) site to be Fe³⁺ > (Al³⁺,Ti⁴⁺) in samples of non-metamorphic origin.