Tunnelling radiative recombination in labradorite: its association with anomalous fading of thermoluminescence

Anomalous fading was first observed in feldspars. In the feldspar mineral labradorite the fading rate has been measured and shown to follow a temperature independent, logarithmic decay law. Among the various possible mechanisms, tunnelling recombination appears to account for these effects. It leads to such a logarithmic decay. Also, it is expected to produce a low temperature afterglow which has not yet been observed. A sample of the labradorite used by A.G. Wintle for the original fading studies has been investigated. It displays a low temperature afterglow. Furthermore, it reproduces the characteristic features of tunnelling radiative recombination. The emission spectrum is mainly in the red to i.r. at temperatures below room temperature. It shifts into the visible and u.v. as the temperature rises and the usual thermoluminescence is observed. It is proposed that this spectral shift explains why the afterglow was not detected earlier. Also, this afterglow accounts quantitatively for the observed anomalous fading. It is concluded that tunnelling radiative recombination occurs in labradorite and can account for the anomalous fading. (author)