Thematic Articles

Luminescence applications in ore geology, mining, and beneficiation include remote prospecting, ground-based exploration, and radio metric sorting. Remote prospecting for ores with a drone or helicopter- borne luminescent sensing using laser excitation and time-delayed detection is becoming commonplace. Modern ground-based exploration increasingly utilises outlining of luminescing “fugitive calcite” veinlet halos, whose characteristics can rapidly and inexpensively give information on the overall size of a mineralised system and the principal structural controls on ore fluid migration pathways. Diamonds and scheelite have been found and recovered through X-ray luminescent radiometric sorting, while laser-induced luminescence has great potential, especially for sorting diamonds lacking X-ray luminescence, fluorite, spodumene, and rare earth element (REE)–bearing minerals. The luminescence concept is expanded here to include laser-induced breakdown spectroscopy; its fusion with X-ray techniques provides simultaneous information on both the mineralogical and chemical composition of a rock.

Luminescence imaging and spectroscopy have become essential in gem testing, as most gem minerals and materials exhibit specific luminescence when properly excited. With a range of techniques introduced in gem testing laboratories in the past quarter century, such as luminescence imaging and photoluminescence (PL) emission and excitation spectroscopies, there are many applications to gem materials for establishing their identity, separating natural from synthetic gems, and detecting potential treatments. Further, these techniques often give clues towards the identity of emitting defects. Luminescence-based testing has recently gained attention even outside the gemmological laboratory as many simple luminescence-based instruments are offered to the gem and jewellery trade to separate natural from synthetic diamonds or from their imitations.

Luminescence is a powerful tool to infer physical and chemical conditions during mineral growth. It is very subtly linked to temperature of formation, composition and structural state, and related changes during rock evolution that often cause striking contrasts in the light emitted. This information can show magma sources and the hydrothermal evolution of igneous rocks, sources and diagenesis in sedimentary systems, and the pressure–temperature evolution during metamorphism. However, luminescence is most powerful when it goes beyond imaging, coupling with spectroscopies and microgeochemical techniques. We present examples of luminescence spectroscopies in igneous, sedimentary, and metamorphic rocks to show how these methods elucidate geological processes. Luminescence imaging is an exciting scientific frontier in which novel methods provide ever deeper insights into petrogenesis.

Luminescence imaging and hyperspectral luminescence mapping are powerful analytical tools with widespread applications in geosciences and materials science. The luminescence of minerals is mainly a defect phenomenon caused by lattice defects and/or impurity elements. This in turn allows one to study trace-element composition and the structural state of a sample by means of its emission. One of the most spectacular and widely used applications of luminescence images is to visualise internal textures in minerals that are not revealed by other analytical techniques. Herein we present a selection of examples for the extraordinary sensitivity of luminescence imaging. We also show that precise information on samples is obtained if luminescence imaging is combined with spectroscopic analysis of the emission and/ or complementary analytical techniques.

Luminescence in minerals is created by ions, groups of ions, or electronic defects that can absorb energy and emit it as visible light. These units are commonly referred to as “centers” or “activators.” They can be impurities in the mineral or intrinsic constituents. In some cases, separate ions (so-called “sensitizers”) act to aid the luminescence process by preferentially absorbing energy and sending it to the emitting unit. In other cases, ions or electronic defects can slow the emission process by trapping excited electrons. Ions preventing emission from other luminescence centers are called “quenchers.” Some impurities can potentially create almost any luminescent color, while others are known for particular colored emission. Luminescence may exhibit strong zonation in crystals due to selective uptake of the activating ions.

Luminescence is the eye-catching phenomenon of light emission by a mineral after some input of energy (the excitation). Although commonly used in Earth sciences only to produce images, much more can be extracted from this phenomenon. Luminescence is extremely sensitive to low levels of emitters (activators), which helps to reveal the geochemistry or the creation of defects. We give an overview of the great variety of techniques (cathodoluminescence, photoluminescence, and more), discuss vocabulary issues (such as excitation versus stimulation, or the different types of persistent luminescence phenomena), and propose wording we feel best reflects today’s knowledge. We explain the basics of luminescence spectroscopy with emission, excitation, and time-resolved spectra to obtain useful data for Earth scientists.