Hornfels

Hornfels is a dense, hard, fine-grained metamorphic rock produced by contact metamorphism when igneous intrusions bake surrounding rocks. It forms at relatively low pressure close to the heat source, resulting in non-foliated rocks with a compact, uniform texture. The mineralogy is strongly influenced by the protolith and metamorphic grade, and hornfels typically lacks the mineral alignment seen in foliated rocks.

Because hornfels forms from a wide range of protoliths—such as shale, mudstone, limestone, or dolostone—the mineral assemblage is highly variable. Common constituents include quartz and feldspar, with mica such as biotite or muscovite in many varieties. Higher-temperature aureoles can host minerals associated with higher metamorphic grades, such as andalusite, cordierite, or sillimanite. Calc-silicate hornfels derived from limestones or dolostones may contain wollastonite and diopside.

Texture and structure of hornfels are typically non-foliated, with a massive, dense appearance and a fracture that is often conchoidal or splintery. The rock is usually resistant to weathering and can be locally quite hard.

Occurrence and uses: Hornfels is commonly found in contact aureoles surrounding granitic or other intrusive bodies in continental crust and is widespread in mountain belts worldwide. It has been used as dimension stone and for decorative purposes owing to its hardness and compact texture. As a geologic proxy, hornfels helps reveal the temperature and conditions of contact metamorphism near igneous intrusions.