COMPARISON OF BLUE AND INFRARED-EMISSION BANDS IN THERMOLUMINESCENCE OF ALKALI FELDSPARS

Dating quaternary sediments by thermoluminescence (TL) or optically st imulated luminescence (OSL) calls for a detailed knowledge of the lumi nescence of feldspars. TL of the various alkali feldspars ((K, Na) Si3 AlO8) display many common features, and some of these cause great diff iculties 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 att ributed to tunnel recombination. Following irradiation, a very intense tunnelling afterglow is observed at temperatures down to liquid nitro gen, 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. It s 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 t he spectral region from UV to orange, but mostly 'blue'. The other is the 'infrared' band already observed at lower temperatures, which is a ttributed to Fe3+ ions. These two bands are clearly separated, with th e spectral maxima, respectively, below and above 600 nn. They have als o different kinetics, the glow peaks temperatures are different. But t hese two different bands are also coupled in many ways, they have para llel growth and fading. With 'disordered' feldspars, the 'blue' emissi on displays anomalous fading, which is stronger than that of the 'infr ared'. The infrared emission is more stable, which may be interesting for the purpose of dating.