What is an alpine cleft in geology ?

Alpine cleft (or alpine fissure) : definition

Alpine fissures are open fractures in which remarkable crystallizations have formed, in particular of quartz, albite, epidote, chlorite, hematite, axinite, and adularia. Their name comes from the fact that they are found in large numbers in the Alps, and more particularly in the Mont-Blanc Massif, while they are very rare in other regions. Outside the Alps, they are only known in the subpolar Urals and in the Himalayan massif. These crystal slots are not very thick (with a thickness of a few centimeters to 1.20 m) and of weak extension (decimetric to decametric). They are found in granites as well as in metamorphic rocks.

These alpine clefts are of metamorphic origin and were born during the formation of the Alps, between -19 and -40 million years ago. At that time, the compressed rocks underwent a metamorphism of green schist facies which caused the opening of cracks and the circulation of aqueous metamorphic fluids rich in silica, aluminum, potassium, etc... These fluids were able to deposit their mineral charge and develop magnificent crystallizations in the open spaces formed by these alpine fissures (or clefts).