Thermoluminescence and optical absorption studies of natural grossular minerals

The optical absorption spectra of two samples of grossular have been measured at room temperature. An intense charge transfer band (UVCT) of iron extends to the visible and near infrared region. Some peaks associated to Fe3+Fe3+ ions in tetrahedral and octahedral positions have been identified and their energy levels were computed. Mn2+Mn2+ and Fe2+Fe2+ ions are responsible with some bands and probably these ions occupy dodecahedral positions. No change in the intensity of optical absorption spectra were found after gamma dose, but only the 505 nm band decreases with irradiation. The OH spectra, consisting of OH overtones at 2750 nm and asymmetric OH bands in the near infrared region were observed in the two samples. The heat treatment produces Fe2+→Fe3+Fe2+→Fe3+ and Mn2+→Mn3+Mn2+→Mn3+ by oxidation. This last was observed in sample II only.The thermally stimulated luminescence of both grossular samples has been investigated. Due to differences in iron and manganese concentration, not only a large difference has been observed in their optical absorption behavior, but also a striking difference in their thermoluminescent behavior. Actually, it is not clear whether other impurities such as Ti, Na and K that are present in quite different concentration in grossular I and II are also contributing to the thermoluminescenct properties of both samples.