A comparison of optical microscope- and scanning electron microscope-based cathodoluminescence (CL) imaging and spectroscopy applied to geosciences

Cathodoluminescence (CL) imaging and spectroscopy are outstanding methods in several fields of geosciences. Cathodoluminescence can be examined using a wide variety of electron-beam equipment. Of special interest to geologists are optical microscopes (OMs) equipped with an electron gun, scanning electron microscopes (SEMs) and electron microprobes. Despite the similar kind of excitation, the results obtained may show marked differences. These are related to the use of focused or defocused as well as a scanned or stationary electron beam and the kind of signal acquisition. Images obtained by OM-CL (hot or cold acceleration) and SEM-CL differ due to different spatial resolution, true colour, grey-scale, or monochromatic detection, contrast inversion, phosphorescence effects, etc. Instrumentation used for spectroscopic studies may differ in sequential or parallel signal acquisition, wavelength range, spectral resolution, and the kind of analytical spot limitation. This is particularly important when investigating transient CL, rare earth element (REE) emissions, or luminescence in the near UV and TR regions as well as samples with small grain sizes and contrasting CL behaviour of adjacent mineral phases. In the present study, the influence of analytical parameters is demonstrated for certain mineral examples including zircon, fluorite, apatite, feldspar, quartz, corundum, kaolinite, and dickite.