Photo: Green speleothems in the Aven du Mont Marcou (Hérault, France) © Stéphane Pire, Gaëtan Rochez (UNamur)

Speleothems, for instance stalactites and stalagmites, are commonly composed of calcite or aragonite (CaCO₃). This mineral compound comes directly from the rock in which the cave was formed and naturally has a white to brownish colour. However, speleothems can sometimes exhibit unique and unusual colours. From yellow to black, blue, red, green, and even purple, there is something for everyone! 

Such a diversity of colours reflects the many possible causes: mineralogical, chemical, biological, or even physical. A speleothem, like any natural formation, is never perfectly pure. Their deposition process, through the precipitation of calcium carbonate dissolved in water, is necessarily accompanied by the deposition of numerous impurities carried along with the water circulating underground. Even if these impurities are sometimes too low in concentration or simply uncoloured, they can still have a visible impact on the colour. 

OK, but what is the point?

The formation of speleothems is very often linked to impurities dissolved in groundwater. Therefore, studying coloured speleothems provides valuable information about potential contamination of surface water with heavy metals or other harmful organic compounds, which in some cases may be consumed by residents. It is therefore a simple and direct way to identify areas with potentially contaminated water and to determine whether this contamination poses an environmental or health risk.

This is the objective of Martin Vlieghe's thesis: to apply a range of cutting-edge analytical techniques to samples of these speleothems to determine these causes and propose an explanation for the origin of the colouring elements. 

Here are a few examples.

An initial project explored the green speleothems of the Aven du Marcou (see photo above). Located in the Hérault department of France, this chasm is well known in the area for its series of impressive shafts, the largest of which is over 100 meters deep. It also has a tiny chamber hidden at the top of a steep wall, which houses an impressive concentration of deep green speleothems. After all the effort of descending and climbing ropes to progress through this very vertical cave, what a wonderful reward to discover this true underground gem! Once the initial wonder has passed, it's time to get to work!  We observe, describe, interpret, and collect a few green fragments from the ground, while respecting the integrity of the site as much as possible. Back in Belgium, it's time to move on to the analyses.

The discovery is all the more surprising given that there are no nickel deposits in the vicinity of the cave! Further study of the composition of the nepouite reveals that they contain a high concentration of zinc, which is also very unusual and suggests that they are in fact quite different from those commonly mined in volcanic deposits. Finally, this mystery was solved by a thorough examination of the rock outcrops in the immediate vicinity of the cave. Just above the cave are siliceous deposits particularly rich in pyrite, an iron sulphide commonly found in this type of settingst. Analysis of these sulphides reveals high concentrations of nickel, which is also found in the natural water source closest to the cave. 

The result of this "investigation" and final explanation: nepouite was able to settle underground through the dissolution of various chemical elements contained in the pyrite of the overlying rocks, which were transported into the cave by surface water and were able to crystallize on site. 

The Malaval cave is very different from the Aven du Marcou. Located in Lozère (France), it extends largely along a high underground river that winds beneath the Cévennes massif. At the bend of a meander, one can find magnificent blue speleothems. 

As in the Aven du Marcou, the coloured speleothems are found only in two specific locations in the cave and nowhere else, suggesting that the origin of the chromophore elements is probably very localized.

Photos - Left: Blue stalagmite in Malaval Cave. Right: Cluster of blue aragonites in Malaval Cave © Gaëtan Rochez (UNamur)

Once again, a few fragments were collected, including a large bluish stalactite found broken on the cave floor. A series of microscopic observations and mineralogical and geochemical analyses were carried out. The first striking finding was that several blue fragments contained no minerals other than aragonite, suggesting that, unlike the green ones from Marcou, it was the aragonite itself that was coloured by the presence of metallic elements. After examining the analyses, three of these elements stood out: copper, commonly cited as the cause of blue colouring in aragonite, as well as zinc and lead. 

While copper appears to be the main cause of the blue colouration, zinc and lead also play a role here. 

Lead also has a marked colouring power, producing green to blue hues, but statistical analysis of coloured and uncoloured areas shows that these colours only appear in the absence of zinc, which seems to inhibit lead-induced colouring. This study clearly demonstrates that, even if a problem seems easy to explain at first glance, it can sometimes hide unexpected subtleties that need to be explored in greater depth in order to uncover all its secrets. 

The Cigalère Cave is one of a kind. Not only does it contain impressive quantities of gypsum, a calcium sulphate found in certain caves, but this gypsum also displays a wide variety of colours rarely seen in nature. Because of this rarity, the cave is particularly well protected, to the point that we were not allowed to collect any fragments from inside it. 

This study was therefore the ideal opportunity to test the Geology Department's new acquisition: a portable X-ray fluorescence spectrometer (pXRF), which allows rapid, in situ, and above all completely non-destructive analysis of coloured speleothems.

Photos - pXRF analysis of a blue stalactite core (left) and a yellow flowstone (right) in the Cigalère Cave © Stéphane Pire (UNamur)

Black is also found in the Chapelle de Donnea, but contrary to what one might think, no iron has been detected. Here, it is manganese in the form of oxides that is responsible for the colouration. This observation is interesting because it clearly demonstrates that black colouration in gypsum, two phenomena that appear similar at first glance, can have very different causes, hence the importance of being able to carry out analyses directly in the field. 

A little further downstream, blue dominates along the main gallery, and analyses have shown strong similarities with the blue speleothems of Malaval, with a marked influence of copper and potentially zinc. 

All this highlights that, despite certain limitations of the device, this type of non-destructive analysis method is a very valuable tool for studying rare, fragile, precious, or protected objects, of which the Cigalère cave is an excellent example! 

Martin Vlieghe's doctoral thesis on "The origin(s) of colored speleothems in caves," supervised by Professor Johan Yans and in collaboration with Gaëtan Rochez, began in February 2022. All three researchers are members of the Faculty of Sciences, Department of Geology at UNamur and the ILEE Research Institute. 

ILEE (Institute of Life, Earth and Environment) is directly involved in issues related to the study and preservation of the environment, to which this subject is directly linked. 

The various analyses were carried out with the support of UNamur's technological platforms:

• Physicochemical characterization (PC²)
• Lasers, optics, and spectroscopy (LOS)
• Electron microscopy
• Synthesis, Irradiation and Analysis of Materials (SIAM)

Some analyses were carried out in partnership with KUL, MRScNB and UMontpellier, and access to the caves was provided by the Association Mont Marcou, the Malaval Association and the Association de Recherche souterraine du Haut Lez.

This thesis was originally funded by the ILEE institute and institutional funds from UNamur, and by an Aspirant F.R.S. - FNRS grant (FC 50205) since October 2023.

It is also closely linked to the new research partnership supported by the RELIEF network (Réseau d’Échanges et de Liaisons entre Institutions d’Enseignement supérieur Francophones), the ILEE research institute at UNamur, and EDYTEM (Environnements, Dynamiques et Territoires de Montagne, Université Savoie Mont Blanc).  Mobility programs between these entities will strengthen a common research area: the study of the critical zone, the most superficial zone of the Earth, where rocks, water, air, and living organisms interact. The perspective is to develop other transdisciplinary research areas and potential teaching projects in the field of environmental sciences and sustainable development.

Studying geology means developing a solid foundation in physics, chemistry, and biology in order to understand the Earth, from its internal dynamics to surface processes and their interactions with our environment and human activities. 

Thanks to their interdisciplinary training, geologists are ideally positioned to perform a variety of roles that require a rigorous scientific approach to solving complex problems (research and development, project management, consulting, and education).

What are the advantages of studying at UNamur? 

• Practical training and numerous field activities
• Strong scientific foundations
• Immersion in geology from block 1
• The possibility of ERASMUS from block 3 onwards
• Close contact with teachers

Dans le monde naturel, de nombreuses espèces animales présentent de la fluorescence, c’est-à-dire qu’ils émettent de la lumière visible sous un éclairage ultraviolet. De façon générale, les propriétés physiques, chimiques ou biologiques de fluorescence de ces espèces sont très mal comprises. Par exemple, la fluorescence des ailes transparentes des plus de 3000 espèces de cigales n’avait jamais été rapportée dans la littérature scientifique jusqu’à cette année.

Dans une nouvelle étude publiée en février 2023 dans le Journal of Luminescence, une équipe internationale de recherche menée par Sébastien Mouchet, chercheur au Département de physique et membre des Instituts NISM (Institut namurois de la Matière Structurée) et ILEE (Institute of Life, Earth and Environment) de l’UNamur, a mis en évidence les propriétés inconnues de fluorescence d’ailes transparentes de certains insectes, dont les cigales que l’on retrouve dans le sud de la France, et du sphinx gazé, un lépidoptère qui vit, entre autres, en Belgique.

Cette étude indique que l’émission de lumière par fluorescence serait plus commune dans les ailes transparentes d’insectes que ce que l’on admettait auparavant. Tout porte à croire que l’origine de cette fluorescence est la présence de résiline au sein des ailes. Cette protéine est connue pour contribuer à la flexibilité des ailes.

Contrairement à ce que l’on pensait, l’émission par fluorescence observée chez ces insectes pourrait être une conséquence fortuite de la présence de résiline dans les ailes à des fins mécaniques et ne pas jouer de rôle particulier dans la communication visuelle de l’insecte, qu’il s’agisse de parade nuptiale ou de défense territoriale.

L’étude fondamentale de la fluorescence d’organismes vivants n’est pas uniquement cruciale du point de vue zoologique. La mise en évidence de la protéine fluorescente verte (connue comme GFP, de l’anglais Green Fluorescent Protein) au sein des tissus d’une espèce de méduse a révolutionné technologiquement le monde de la génétique et de la microscopie à fluorescence. Cette découverte a d’ailleurs été couronnée par le prix Nobel de Chimie en 2008. 

A l’UNamur, c’est grâce aux équipements de la plateforme technologique LOS (Laser, Optique et Spectroscopies) que ces études ont été réalisées.

La plateforme offre une expertise unique dans la caractérisation des propriétés optique et électroniques de la matière au moyen de mesures optiques linéaires et non-linéaires, réalisées notamment par des lasers dans un cadre de recherches fondamentales ou appliquées. La plateforme permet l'analyse de systèmes structurés à l’échelle nanométrique à une, deux ou trois dimensions, de films moléculaires aux interfaces ou de traces de gaz. En plus des compétences expérimentales, la plateforme développe des modèles analytiques et numériques permettant d’interpréter les réponses spectroscopiques mesurées.

• Sébastien Mouchet, Louis Dellieu et Olivier Deparis (Université de Namur, Belgique)
• Charlotte Verstraete, Dimitrije Mara et Thierry Verbiest (KU Leuven, Belgique)
• Bojana Bokic et Branko Kolaric (Université de Belgrade, Serbie)
• Albert Orr (Griffith University, Australie)
• Rik Van Deun (Université de Gand, Belgique)
• Pete Vukusic (University of Exeter, Royaume Uni)

Cette propriété est souvent utilisée dans notre vie quotidienne. Quelques exemples :

• Les vêtements à haute visibilité ou dans un simple but esthétique ;
• Les marqueurs surligneurs ;
• Des peintures « anticollision » dont on peint, par exemple, certaines parties des avions ;

La fluorescence est également utilisée dans le cas de la lumière dite « noire », à savoir celle émanant d’une source lumineuse composée essentiellement de proches ultra-violets, qui fait ressortir les blancs et les objets fluorescents afin de créer une ambiance spéciale, de vérifier qu’un billet de banque n’est pas contrefait ou de déceler certaines substances invisibles à l’œil nu.

En recherche de fuites, la fluorescence est très employée en mélangeant à l'eau des traceurs fluorescent. Cela permet de déceler tout type d'infiltration ou de passage d'eau. La spin-off TRAQUA de l’UNamur, experte en monitoring hydrologique et hydrogéologique, qui a développé le fluorimètre/turbidimètre STREAM®, en est un bel exemple.

Sébastien Mouchet et Oliver Deparis sont les auteurs d'un ouvrage intitulé Natural Photonics and Bioinspiration.  Publié en novembre 2021, c'est un livre sur le thème de l’optique physique et de la biologie environnementale. Sur les traces des recherches du Prof Jean-Pol Vigneron, ce livre, avant-gardiste selon l’éditeur, ouvre la porte aux applications bio-inspirées dans le monde de l’optique, de l’énergie et de l’environnement.