Heating and radiation effects on optical and mössbauer spectra of Fe-bearing cordierites

Optical spectroscopy indicated that the partial oxidation of Fe²⁺ to Fe³⁺ on heating natural cordierites above ～500° C was not sensitive to oxygen partial pressure. This result suggested that the partial oxidation takes place by loss of mobile charge compensators such as protons. Ionizing radiation also produced partial oxidation of the minor-site Fe²⁺ to Fe³⁺, but this effect could be reversed by annealing at ～300° C and so was due only to an electronic redistribution. Mössbauer spectroscopy confirmed these results, although there were complications due to apparent thermal decomposition in localized regions of the cordierites on heating in air. Ionizing radiation also produced strong absorption in the near-ultraviolet and blue regions of the optical spectrum. This absorption, which was strongest in the α polarization, had different annealing characteristics to those of the crystal-field absorption bands near 10,000 cm^?1. Dehydration by heating above ～900° C severely reduced the subsequent effect of ionizing radiation on the optical spectra. On heating over a temperature range spanning the dehydration temperature, the lack of any changes in the absorption energies of the β- and γ-polarized crystal field bands near 10,000 cm^?1, of any changes in the ratio of the intensities of these β- and γ-polarized crystal field bands, and of any singularity in the intensity variations of the bands with heating temperature, all suggest that these bands originate from substitutional Fe²⁺ rather than Fe²⁺ in channel sites. Mössbauer evidence also supports this. The formation of pleochroic haloes in cordierite is discussed briefly. No evidence of CO _- ³ centres in the irradiated samples was found.