INFLUENCE OF CATION DISTRIBUTION ON THE OPTICAL ABSORPTION SPECTRA OF FE3+-BEARING SPINEL S.S.-HERCYNITE CRYSTALS: EVIDENCE FOR ELECTRON TRANSITIONS IN VIFE2+-VIFE3+ CLUSTERS

Abstract

Flux-grown Fe3+-bearing spinel s.s.–hercynite solid-solution crystals, (Mg1-y Fe2+y )Al2O4 (0 < y≤ 1), have been investigated by means of electron microprobe technique and Mössbauer and electronic spectroscopy. Obtained results show that different electronic processes cause intense optical absorption bands in the near-infrared spectral region. In addition to an electronic d–d transition in single-ion IVFe2+, observed at 5200 cm−1, intense and broad bands at 9500 and 14 500 cm−1 are assigned to exchange-coupled pair (ECP) and intervalence charge-transfer (IVCT) transitions in VI Fe 2+ VI Fe3+clusters, respectively. The net linear extinction coefficients of these bands (α) were calibrated against Fe2+ and Fe3+ concentrations and site distributions previously defined by combined microchemical, Mössbauer, and XRD structural refinement data. The following expressions were obtained: $$$$ where α is measured in cm−1 and concentrations are expressed in mol l−1. The present results show that optical absorption spectroscopy may be used as a probe to obtain high spatial resolution (Ø ~ 10 μm) information on Fe2+ ordering as well as on Fe3+ concentrations in minerals belonging to the spinel group.