On the yellow color of gamma-irradiated brazilianite from Minas Gerais (Brazil)

The origin of gamma irradiation-induced strong yellow color in brazilianite from Brazil is investigated. The irradiation-induced optical absorption band responsible for the color shows an onset in the blue spectral region at about 2.5 eV and maximum centered in the UV at ~ 4.24 eV. From the ratio between the squared value of the line width (W2) and the peak energy (M), a value of about 0.09 eV is estimated, which is consistent with an absorption band caused by a Schirmer´s-type O− bound small polaron. By electron paramagnetic resonance (EPR), we are able to confirm its microscopic structure. This O− hole center is in fact the Al3+–O−X2+–P5+ hole center already identified by EPR earlier in the literature (where X2+ stands for a nearby divalent cation with negligible abundance of magnetic isotopes). The EPR spectrum of the Al3+–O−X2+–P5+ hole center, along with the O− bound small polaron absorption band responsible for the yellow color, appears simultaneously at high concentrations after gamma irradiation, and vanishes together for thermal annealing above 300 °C, returning after re-irradiation in a reversible way. Their appearance is concomitant with the H0 centers and Ti3+ electron centers, and possible charge-compensating centers.