3.2.3. Cathodoluminescence Imaging

Unfortunately the MBX microprobe is not configured to do cathodoluminescence imaging; however, this method provides valuable information about a sample and is described briefly here. Cathodoluminescence provides a means of mapping trace element zonation in minerals and is  commonly used to study carbonates, where variations in luminescence result from the interplay of Mn²⁺ and Fe²⁺ substituting for Ca²⁺. The presence of Mn²⁺ activates luminescence, whereas Fe²⁺ quenches (inhibits) it. Thus cathodoluminescent areas will contain Mn²⁺ (with no or some Fe²⁺) and non-luminescent areas will lack Mn²⁺ (or have sufficient Fe²⁺ present to quench the luminescence.)

Cathodoluminescence can be observed by photographing light emitted from a sample through a glass port (optical). Alternatively, the light may be detected using an SEM detector. The latter method is more sensitive, but because of the relatively long persistence of cathodoluminescence. Thus, as the beam is rastered over the sample, light will still be emitted from parts no longer being hit be electrons. This problem is overcome by rastering the bean more slowly over the sample (increasing the dwell time).

Figure 3.2.3. (upper left) Transmitted light image of calcite-lined cavity. Number indicate approximate positions of different generations of calcite. Calcite precipitating from water in reducing environments commonly contains Mn2+ and Fe2+. (lower left) Optical cathodoluminescence image of same area. Generation 1 calcite is orange. (upper right) SEM cathodoluminescence image of same area. The dwell time was 2300 msec. Note that all three generations of calcite can be distinguished and that detail is revealed in the non-luminescent generation 2 calcite. (lower right) Detail of SEM-CL image. From Lee, M.R., 2000, Imaging of calcite by optical and SEM cathodoluminescence: The Americas Microscopy and Analysis, p. 21-22.