is an example of a CL signal recorded under the electron beam on cassiterite mineral (SnO2, tetragonal): the optical image is produced under electron beam in the SEM. This mineral is known to be a good CL emitter and CL is frequently used for characterizing crystal growth (luminescent zones) due to trace element distribution (Farmer et al., 1991 and Hall and Ribbe, 1971). Note that CL emission intensity is dependent on several parameters, such as the sample and the electron beam setup. The first parameter controls luminescence as a function of its composition and trace elements. This has been highlighted by Farmer et al. (1991) on a cassiterite sample, where they have studied the effects of substitution of Sn by Ti, W and Fe on the luminescence of this mineral. They observed that Ti and W behave as luminescence activators, in contrast to Fe which quenches luminescence. According to these authors, the observed colours and intensity of the luminescence result from the interplay between these effects.