Cathodoluminescence imaging is an ideal tool for studying geological samples and to get additional contrast and spectroscopic information down to the resolution of a scanning electron microscope. When a sample is bombarded by fast electrons, the material becomes excited and as it returns to a ground state, it luminesces in various colors. The different colors provide clues as to the composition of a sample. Not only can one gather data at the nanoscale, but the colors produced by CL reveal information that no other optical microscope can obtain.
The cathodoluminescence emission from a rock gives insights into crystal growth, zonation, cementation, replacement, deformation, provenance, trace elements, and defect structures. This can be used to fingerprint rocks and reveal interesting spatial textures on a submicron scale. CL is often combined with other analytical tools such as SIMS, BSE, EDS, WDS and μCT for a more complete understanding of all relevant rock properties.
CL is a particularly useful method of data acquisition in geosciences because it reveals information not readily provided by visible or ultraviolet light. It allows for the collection not only of spectroscopic, but geochemical data. The excitation of electrons in the sample that produces the light seen through a CL system occurs in specific chemical impurities within a geological structure. This allows the scientist to visualize sites that are damaged by radiation, chemical composition, electron charge displacements and structural defects. Some studies of the texture of a particular geological sample can also only be done 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.
CL presents a valuable technique for fundamental sedimentary, metamorphic, and igneous rock studies but also for more applied studies on ores, precious gems, reservoir rocks, and construction materials.
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.
CL imaging can be used to quantitatively map the quartz composition of a sandstone which allows rigorous segmentation of granular and cemented material for example. By combining such results with in-depth analysis of the observed textures such as (healed) fractures and grain contacts, the geological history as well as the porosity of the rock can be studied in great detail.
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