What are hornfels in geology ?

Hornfels - Metamorphism

Metamorphic facies diagram
An hornfel is a rock of contact metamorphism of granitoids, usually very hard.

Its mineralogical composition varies according to the nature of the host rocks :

- pelitic rocks give micaceous hornfels with andalusite and cordierite, evolving into feldspathic hornfels
- marl-limestone rocks and calcareous pelites give calcic hornfels, called tactites, of variable mineralogical composition, usually rich in garnet, epidote and hornblende.
- carbonate rocks give skarns and marbles
- the other rocks evolve according to their original mineralogical composition (for example, for quartzose sandstones the hornfel ones will be quartzites).

The hornfels can give rise to exploitable concentrations of metals (gold, copper, tungsten, etc...) or industrial minerals (andalusite from Glomel in Côtes-d'Armor, France for example).