The spectroscopy of luminescent geological materials

Luminescence is the non-incandescent emission of light from materials excited by an electron beam. Electron irradiation raises sample electrons to an excited state, which then emit a photon as they return to a lower energy. Luminescence phenemona may be studied in several ways, including spectral and spatial methods. Whilst cathodolominescence (CL) has become an established method of analysis for Earth materials, other forms of luminescence in minerals should not be overlooked. Photoluminescence (PL) studies, for example, allow for emission and excitation spectroscopy to be examined in weak- to strongly luminescent minerals, such as wilmenite. In the case of quartz PL, emission spectroscopy investigations have shown that excitation at different wavelengths produces highly variable emission spectra that relate to one or more transitions for excitation. As the building blocks to rocks, minerals and the atoms or ions within preserve critical information concerning the conditions attending growth or subsequent evolution - thus, investigation of these can inform on the origin or surface/near surface interactions relating to environmental change. As a non-destructive technique used in the study of rare materials (e.g., Lunar meteorites) luminescence imaging and spectroscopy have the potential to help characterise, as well as, elucidate domains or reveal fine-scale features not resolvable by optical methods. Advances in instrumentation now permit the collection of multi-dimensional data sets (e.g., hyperspectral) that can be interrogated off-line. The simultaneous capture and interpretation of compositional and luminescence signals has the potential to greatly improve our understanding of causes of luminescence, be these trace activator or defect.