visite en nacelle du gouffre de proumeyssac
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"The beautiful hollow triangle crystal formations are so rare that the recent "Cave Minerals of the World", by Hill and Forti only mentions them incidentally, and the ones found in Proumeyssac, due to their size and presentation, are worthy of attention." (Ph. Renault).

"A small, naturally hollowed out corridor, entirely out of sparkling white calcite, leads to a small room where the crystal formation, under a thin layer of water, has taken on (a) curious triangular shape," wrote speleologist E.-A. Martel, following his visit of the cave on 26 July 1907..."
"May we remind you that calcite is the mineral with the most combinations of crystal structures: more than a thousand! The calcite found in caves is limited to a few crystal shapes which are always the same. Among them, the triangular shape, which has typical rhomboidal symmetry, was allegedly described for the first time by the mineralogist Des Cloizeaux in his Manuel de Minéralogie (Handbook of Mineralogy) in 1874 in the gallery of Eaux-Bonnes (Basses-Pyrénées). They can be observed both at the microscopic level, in the floating calcite (the calcite crystallizes in a thin layer at the surface of the bodies of water) as well as the centimetric or decimetric level, with triangular stalagmites (there are many examples measuring approximately one metre high and ten centimetres in diameter, in the Grotte de l'Aguzou, Ariège, France), and the hollow triangles in the pools of water." (Ph. Renault).

It should be noted that these hollow triangles only form in pools of still water, which is more or less stagnant and therefore saturated with CO3Ca. On the walls or bottoms of these pools of water, "scalenohedral" (in mineralogy: name of scalene triangles = with unequal sides) crystalline tips develop, and they stop growing when the crystals reach the surface of the water. Their growth is obviously very slow, and is most often linked to the development of gour and its water supply.

The rarity of hollow triangles proves that they require a very specific environment, both atmospherically and morphologically. Their diversity, from one cave to another, helps define the factors determining the geological evolution of each site. In fact, all the triangles examined in the different caves are located in quiet and "confined" places. In short, regarding the formation of these hollow triangles, in Proumeyssac and elsewhere, but which are particularly remarkable in Proumeyssac, we must remember that it depends on the "confined" location of the pool, its water supply (still and shallow water), the chemical composition of the water (high levels of CO3Ca), and the surrounding atmosphere. Similarly, it should be noted that the large triangles of Proumeyssac are hollow, while those of the Grotte de l'Aguzou (Ariège) are very thick, especially inside the corolla, due to recent calcite. Which poses the problem of the difference between the large isolated triangles of Proumeyssac and the small centimetrical triangles that can be seen on the stalagmite floors of certain other caves. The thickness of the water layer, in which the crystals collect, could explain that difference.

But the fact remains that Proumeyssac is delighted to be able to show its visitors this beautiful, surprising and rare work of nature.