Comparison of ‘blue’ and ‘infrared’ emission bands in thermoluminescence of alkali feldspars

Dating quaternary sediments by thermoluminescence (TL) or optically stimulated luminescence (OSL) calls for a detailed knowledge of the luminescence of feldspars. TL of the various alkali feldspars ((K, Na) Si₃AlO₈) display many common features, and some of these cause great difficulties for dating. After long storage following ionizing irradiation, the TL of most alkali feldspars is known to fade away by “anomalous fading”, which is incompatible with dating. This effect had been attributed to tunnel recombination. Following irradiation, a very intense tunnelling afterglow is observed at temperatures down to liquid nitrogen, in accordance with the observed rate of fading. This emission has a Gaussian spectrum entirely in the infrared (IR) with a maximum at 1.7 eV. It displays an important thermal quenching from 77 to 300 K. Its intensity is related with the ‘disorder’ of the crystal lattice. At higher temperatures, in TL proper, two emission bands can be separated. One is the well-studied complex visible emission, distributed over the spectral region from UV to orange, but mostly ‘blue’. The other is the ‘infrared’ band already observed at lower temperatures, which is attributed to Fe³⁺ ions. These two bands are clearly separated, with the spectral maxima, respectively, below and above 600 nm. They have also different kinetics, the glow peaks temperatures are different. But these two different bands are also coupled in many ways, they have parallel growth and fading. With ‘disordered’ feldspars, the ‘blue’ emission displays anomalous fading, which is stronger than that of the ‘infrared’. The infrared emission is more stable, which may be interesting for the purpose of dating.