Dynamic biaxial absorption spectra of Ti3+ and Fe2+ in a natural rose quartz crystal

The absorption spectrum of Ti3+ in quartz is typical for that element in an octahedral site, except for the high oscillator strength of the band which indicates intensive charge transfer, most likely between a substitutional Ti4+ and an interstitial Ti3+. The biaxial anisotropy of the band is similar to that of octahedral Fe2+ in the same crystal, suggesting the same site for the Ti3+, an interstitial site at the intersection of the threefold axis with two twofold axes. The dynamic nature of the Ti and Fe ions is indicated when the crystal is subjected to X-irradiation and then to bleaching treatment with UV-light or heat. Ti(IV) ions are acceptors of electrons donated by Fe2+ during X-irradiation. This is indicated by increase of the 2.4 eV band intensity with decrease in the Fe2+ band intensities and appearance of Fe3+ charge transfer bands in the UV-region. Heat and UV-light treatment decrease the Fe3+ CT bands and increase the Fe2+ crystal-field bands along with decrease in the 2.4 eV band related to titanium.