The anomalous fading of infrared-stimulated luminescence from feldspars

Measurements of anomalous fading of infrared-stimulated luminescence (IRSL) are presented for a suite of identified feldspars stored for periods of up to 15 months at temperatures of ∼ 10oC and 100oC. Examples of feldspars representative of each major compositional regime of the ternary system are seen to fade, and similarly some are stable. Measured luminescence characteristics (emission spectra, relative luminescence intensities, glow curve shapes) and storage durations at these temperatures do not permit reliable separation of stable signals. In a consequent study to determine if preferential removal of the anomalously fading component by bleaching was possible, the optical bleaching response spectra of selected samples exhibiting representative fading and non-fading behaviour were measured. No preferential bleaching of the anomalous fading component was found, precluding an ''optical wash'' approach for feldspar. These same measurements suggested that IRSL signal originated from one defect type only in all feldspars studied. The ground-to-excited state transition energy was found to be 1.44 ± 0.01 eV (corresponding to ∼ 861 ± 6 nm) regardless of mineral composition or signal stability. However, the observation of stable IRSL [and optically stimulated luminescence (OSL)] from a variety of feldspar species indicates that anomalous fading is not an inherent property of this defect. It was deduced that the anomalous fading mechanism, at least at and above ambient temperatures, is dominated by a quantum-mechanical tunnelling mechanism. As such anomalous fading cannot be circumvented by any current laboratory procedures, monitoring is essential; discovery of significant anomalous fading is a sample rejection criterion. (author)