In the nineteenth century, before the development of modern technology as we know it now, vacuum tubes were widely used to study the fundamental properties of materials and objects. During this period, William Crookes, the inventor of the vacuum tube, was studying various objects with these tools. At the time, scientists were unsure of the exact processes that were occurring, but in these vacuum tubes electrons were essentially being fired at objects at the end of the vacuum tube. These electrons excited the materials which caused them to produce light when they returned back to their equilibrium state, a process known as cathodoluminescence (CL). This process is still being used in geology as well as other scientific fields, albeit with much more complex and efficient technology. To this day, CL is an extremely useful tool in the geosciences and is experiencing a resurgence in popularity with the ability to integrate it with a scanning electron microscope .
CL is a very useful method of data acquisition in geosciences because it reveals information not readily provided by other techniques. The excitation of electrons in the sample that produces the light seen through a CL system occurs in specific chemical impurities or intrinsic defects within a geological structure. This allows the scientist to visualize sites that are damaged by radiation or deformation, or sites that have a different chemical composition or crystal structure for example. Some studies of the texture of a particular geological sample can be linked to particular geological events using CL . CL is thus not only important for understanding the composition of various materials in a site, but also the changes that have occurred over time.
Image: Zircon is a robust mineral which can persist for long periods of time in the earth’s crust. During growth, changing conditions lead to zonation in the crystal which can be visualized with CL. Zircons are often rich in heavy metals and as such are very useful for radiometric dating with advanced mass spectrometry tools. CL is often used as a pre-screening tool for such studies owing to its speed and relatively low cost.
Sample courtesy of dr. Chen Zhenyu (Institute of Mineral Resources Beijing)
CL reveals an explosion of color when used on geological samples, which provides clues as to the composition of a sample. When Crookes was using his vacuum tubes in the 1800s, he found that zircons emit a whole range of hues depending on their refractive indices and thus their composition. In 1885, Becquerel found that different materials emit different colors: calcite luminesces orange, quartz yellow, and aragonite green. Further studies have found that with CL rubies glow red, emeralds are crimson red, sapphires are blueish gray, and cassiterite is pale yellow . In the study of quartz, blue luminescence can point to a high titanium content  (to learn more about quartz, see DELMIC’s white paper: Cathodoluminescence imaging on sedimentary rocks: Quartz sandstones).
It is in combination with a scanning electron microscope (SEM) that the benefits of CL can most effectively be reaped in geosciences. In such a system, the functional information obtained from the optical response generated by the electron beam is combined with the extremely high spatial resolution associated with an electron microscope.
Zircons in particular are frequently studied using a SEM CL system. This mineral is extremely resistant to weathering processes and can be found in a wide variety of rocks. Changes in the chemical composition of rocks over time lead to zonations in the zircon, in which different zones of zircon have varying chemical compositions. Comparable to rings in trees, the study of zircons can open up a breadth of information on historical changes in the rock. In fact, the oldest native crustal rock from earth ever to be dated was done so by observing zircons through a combined SEM CL system and quantitative isotope analysis in a mass spectrometer  (to learn more about the application of CL for the study of zircons, see DELMIC's application note).
Even before the invention of the electron, cathodoluminescence was being used as a tool to understand the fundamental nature of our planet by looking closely at geological samples. The power of cathodoluminescence in geosciences, especially in petrology, is, however, still heavily underestimated . Nevertheless, with the advent of the integrated SEM CL system, cathodoluminescence will increasingly be recognized as an indispensable tool for understanding the chronology and chemical composition of rocks and minerals.
 McLemore, Virginia T, and James M. Barker. "Some geological applications of cathodoluminescence."
 Pagel, Maurice, et al., eds. Cathodoluminescence in geosciences. Springer Science & Business Media, 2013.
 S. A. Wilde et al., Nature 409 (2001) 175-178.