A TECHNIQUE FOR AVOIDING IMAGING PROBLEMS ASSOCIATED WITH CARBONATE MINERALS ON SEM-BASED CATHODOLUMINESCENCE SYSTEMS

SEM-based cathodoluminescence systems (scanned CL) afford higher magnifications, better detection of weak luminescence, detection of a broader spectrum of luminescence, and more stable operating conditions than conventional, light-microscope-mounted CL systems. However, persistent luminescence of carbonate minerals causes problems with image quality. Such problems create significant obstacles to the use of scanned CL on most carbonate minerals; hence, methodology for overcoming image problems is beneficial. A previously suggested approach involving very long dwell times (3200 µsec) requires prohibitive image acquisition times and can lead to increased beam damage of the specimen.

We have found that persistence problems can be overcome by using a broadband, short-wavelength (UV-blue range) filter. The filter used provides 80 to 90% transmissivity in the range 385 to 495 nm. Minor transmissivity of 10 to 30% is present in the range of 760 to 800 nm. Thus, this filter largely blocks the persistent luminescence in carbonates, which seems to be primarily in the orange-red wavelengths. The filter allows transmission of UV-violet luminescence in the range of 350 to 425 nm, which seems to be present in most luminescent carbonates. The total range of the CL detector is 185 to 800 nm. This technique requires longer dwell times (typically 600 µsec) than scanned CL of silicate minerals, but image acquisition time is not prohibitively long. The SEM is typically operated at 20 kV with a relatively high sample current.

Scanned CL images obtained using this technique are comparable in detail to those produced using light-microscope-based CL systems. In almost all examples, features visible in the orange-red wavelengths show corresponding variations in luminescence in the shorter wavelengths. This technique has proven most useful with carbonate-dominated rocks such as limestones, dolomites, and marbles. Imaging of rocks with mixed silicate-carbonate lithology, such as calcite-cemented sandstones, also benefits from this approach. In this case, however, imaging of silicate minerals commonly suffers slightly from the loss of the longer wavelengths.