Using a microprobe equipped with interference filters for making luminescence measurements, the intensity of cathodo-luminescence in zoned cassiterites of hydrothermal origin is found to be directly related to the concentration of the activators Ti and W, provided no quenching combination (Si, Fe), (Fe, W) or (Ta, Nb) is present. Distinct emission bands are observed at ~565 nm for Ti-activation and ~440 nm for W-activation. The Ti-activated emission is similar to the long-wavelength excited fluorescence of the Ca, Sr and Ba metastannates. Iron appears to enhance the Ti-activated emission. An orange cathodoluminescence is observed when Si is present with Ti in the absence of Fe.
Luminescence intensity exhibits strong thermal quenching, and the activation energy for thermal de-excitation is less than 0.015 eV. The temperature broadening and shift of the peak intensity for the Ti-activated emission are small, and decay time exhibits no appreciable temperature dependence between 21 and 275°C. Thus the Ti-activated emission appears to be a long fluoresence process. The wavelength of W-activated emission is similar to that observed for self-activated CaWO4. The presence of Fe with W suppresses the W-activated emission, while Fe occurring with Ti enhances the probability of luminescence transitions.
Pegmatitic cassiterites that contain Ta and Nb in addition to Fe and Ti, are seldom zoned and exhibit no appreciable cathodo-luminescence, but they are usually pleochoric. It appears that both pleochroism and luminescence-quenching in cassiterites are related to the presence of Ta and Nb.