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

Flux-grown Fe3+-bearing spinel s.s.-hercynite solid-solution crystals, (Mg1-yFey2+)Al2O4 (0 < y less than or equal to 1), have been investigated by means of electron microprobe technique and Mossbauer 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 Fe-IV(2+), 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 Fe-VI(2+) Fe-VI(3+) clusters, respectively. The net linear extinction coefficients of these bands (alpha) were calibrated against Fe2+ and Fe3+ concentrations and site distributions previously defined by combined microchemical, Mossbauer, and XRD structural refinement data. The following expressions were obtained: alpha(5200) = 59 +/- 4 [Fe-IV(2+)], alpha(9500) = 189 +/- 36 [Fe-VI(2+)][Fe-VI(3+)] and alpha(14500) = 141 +/- 25 [Fe-VI(2+)] [Fe-VI(3+)], where a 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 (phi similar to 10 mum) information on Fe2+ ordering as well as on Fe3+ concentrations in minerals belonging to the spinel group.