What is an alpine cleft in geology ?

BORNET Rémi (2025)

Alpine clefts : Crystalline treasures of the mountains

Alpine clefts (or fissures) are fascinating geological fractures distinguished by their exceptional crystallizations. Formed under unique conditions, they represent a striking element of Alpine mineralogy. Their name evokes their characteristic presence in the Alps, and more particularly in the Mont Blanc Massif. On the other hand, they remain rare in other regions, being only identified in the subpolar Urals and the Himalayan massif beyond the Alps.


Origin and formation

The Alpine clefts originate from geological processes dating back to the time of the formation of the Alps, approximately 19 to 40 million years ago. During this period, colliding tectonic plates compressed the rocks, leading to greenschist facies metamorphism. This metamorphism led to the opening of fissures and the circulation of aqueous fluids rich in various chemical elements, such as silica, aluminium and potassium.

These metamorphic fluids, under the effect of pressure and temperature, gradually deposited their mineral load in these open spaces. This process allowed the formation of exceptional and perfectly developed crystals for which the Alpine fissures are famous.


Engraving from 1872, Mosengel / Roth - Alpine Museum Collection, Chamonix-Mont-Blanc

Characteristics and mineralogy of alpine clefts

These clefts, also called "crystal cracks", are of modest size. Their thickness varies from a few centimeters to about 1.20 meters, and their extension is limited, oscillating between a few tens of centimeters and several meters. These structures can form in both granites and metamorphic rocks, revealing a remarkable variety.

Among the emblematic minerals of alpine clefts, we find in particular :

Quartz : often in the form of clear or smoky crystals.

Albite and adularia : classic feldspars, generally white and translucent prismatic.

Epidote and chlorite : two minerals typical of metamorphic conditions of green color.

Hematite, ilmenite, anatase and rutile : classic opaque minerals;

Red or pink
fluorite : a rare and prized variety that brings a bright touch to alpine cracks. These fluorite crystals, often transparent or translucent, are distinguished by their unique color resulting from the presence of impurities and the specific conditions of formation. They are particularly sought after by collectors because of their aesthetics and rarity.

Others :
axinite, apatite, etc...

In addition, these cracks can house fluid
inclusions in some crystals, providing valuable clues on the conditions of formation and on the chemistry of hydrothermal fluids. The interactions between fluids and host rocks are closely linked to the diversity of minerals encountered.
Pink fluorite and smoky quartz from Argentière, Haute-Savoie, France © Rémi Bornet
Smoky quartz from Mont-Blanc, Haute-Savoie, France © Rémi Bornet
Anatase on quartz from Pointe du Rozet, Savoie, France © Rémi Bornet
Axinite from Rocher de l'Armentier, Isère, France © Rémi Bornet

Conclusion

Alpine clefts are not only objects of scientific study, but also natural wonders that illustrate the beauty and complexity of geological processes. Their crystallizations, unique in their aesthetics and rarity, continue to amaze mineralogists, collectors and mountain enthusiasts around the world.