Les cristaux autour du Mont-Blanc ont cet aspect fumé caractéristique à cause de la radioactivité du granit environnant. © Thierry Berguerand

A look at 15 million years of geology: a portrait of a crystal maker

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Initiated in the crystal hunt of his adolescenceJean-Franck Charlet is a guide to high mountain, but also crystal. This term does not designate a profession, but a passion: to go and collect the crystals exposed on the slopes of the mountains. “They are in what are called ‘furnaces’, cracks open in the rock lined with crystals, which have been released by the erosion of the mountains”explains J.-F. Charlet.

These crystals are the result of 15 million years

As Jean-Franck Charlet explains, “These crystals formed at a depth of about 10 kilometers, 15 million years ago, during the collision between the African plate and the European plate. The collision removed granite from the magma earthly. During the collision, everything was compressed so hard that cracks formed in the rock. Then water accumulates in these cracks, or rather in aqueous solution which contains water, but also several minerals which dissolve in it. »

All these processes take place at a very high level pressure (400 times more atmospheric pressure) and high temperature, about 500 degrees Celsius. The elements contained in the water are dispersed and dissolved in it atom per atom until the solution is saturated: “Gradually, the rock rises, about a millimeter per year, because what was 10 km land Today, 15 million years ago, it is today at the top of Mont Blanc: it is the natural process of mountain formation. The temperature and pressure decrease, then the water supersaturates elements and then begins to precipitate. Added to the various compressions and decompressions that the rock undergoes during its ascent, the aqueous solution is transformed into crystals that then cover the sides of the cavity ”. Of course, this mechanism of crystal formation it is the basis for the formation of all minerals, and these depend on the composition of the source rock where they were formed.

There are more than 3,000 different minerals depending on the base molecule

Then there are the conditions that accompany it rise of the crystal who will do it as he is on the mountain today. “There are two possible states for solid : the crystallized state, where atoms form a regular lattice, and the state amorphous where everything is broken. In the crystallized state the crystals are found transparent. The type of crystal is then defined by the molecule It is she who will impose the structure of the atomic lattice: there are, therefore, more than 3,000 different minerals! So for the quartz which is an oxide of siliconthe base molecule is a tetrahedron, and a six-sided hexagonal crystal is formed from it. “continues Jean-Franck Charlet.

Jean-Franck Charlet hunts crystals all over the world, but with a particular appetite Mont Blanc massif, in the Alps. Some groups of crystals can reach 50 kilograms part, which must then be lowered from the high rock walls with ropes and then carried in the backpack. back Through glaciers then steep paths. “Only crystals found in cracks opened by natural and continuous erosion of the walls are allowed for collection”explain.

But the most sought after in the region is still the pink fluorite which, as J.-F. Charlet is sometimes nicknamed “gold from crystal makers”. It is a mineral much sought after by collectors and glassmakers, because it is very rare in the world and the most beautiful specimens come from the Mont Blanc massif or central Switzerland which has the same geology. “In mineralogy, this is called fever rose gold, because of the fluorite rose found in the Mont Blanc massif and in Switzerland. It is a mineral much sought after by collectors and glassmakers, as they are much more aesthetic than the smoky and opaque crystals characteristic of the region. »

Because the crystals found mainly around Mont Blanc have a somewhat special look: they have a color smoke, caused by radioactivity from granite around. “The internal radioactivity of the granite bombs the glass and deforms it crystal lattice. Then, the rays of light that could pass are diverted, creating this smoky aspect characteristic of the Mont Blanc massif. »

Global warming, especially in the mountains, is causing the glaciers to gradually melt

Whileatmosphere it heats up more and more and faster, Icebergs are melting. A process that gets worse when you go up in altitude. “At around 2,500 meters, we lose almost 10 meters in thickness a year. explains JF. Charlotte. For crystal finders, this can be a big help, as they bring to light acres of rocks that were previously hidden under the ice, potentially digging up new glass kilns. »

“It simply came to our notice thencollapse in the mountains. Climate change is intensifying there, the appearance of glaciers is changing everywhere. “, worries J.-F. Charlet. In particular, in the Chamonix region where he is a high mountain guide, “The rocks that make up the great walls are held together by an old ice that serves as cement. When this ice melts, whole sections of walls can collapse, as happened recently in Les Drus, one of the most beautiful. rock needles above Chamonix. »

Catastrophic scenarios could occur in the next 20 or 30 years, and even reach the valley depending on the evolution of temperatures. “Therefore, we are constantly monitoring the mountain: some glaciers whose base has so far been stuck to the rock could movement as soon as the ice in contact with the rock would begin to melt, potentially creating large avalanches. Thus, scientists measure the temperature under certain glaciers, where we observe a continuous increase temperatures. We also look at the altitude from which the rocks remain subject to ice all year round: what is called permafrost – On permafrost, in English. 30 years ago it was 3,400 meters, now we are more than about 3,700, and it only increases ”. alarm Jean-Franck Charlet.

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