Hornfels

The group name for a set of contact metamorphic rocks that have been baked and hardened by the heat of a nearby intrusive igneous body, forming in the metamorphic aureole that surrounds a granitic intrusion in the upper or middle crust. Hornfels develops through thermal metamorphism without significant accompanying deformation, so its composition varies with that of the original rock: biotite hornfels forms from clay or shale, calc-silicate hornfels from impure limestone, and mafic hornfels from igneous rocks such as diabase or basalt, with characteristic minerals including biotite mica, quartz, feldspar, cordierite, garnet and andalusite. Most hornfels is fine grained with a texture geologists call pflaster or pavement structure, in which small mineral grains of roughly equal size fit closely together like the pieces of a mosaic, and any sheet minerals present are aligned at random rather than in parallel layers, giving hornfels a massive, hard, splintery character that tends to break into cubical fragments rather than thin plates. Biotite hornfels typically appears dark brown to black with a velvety luster, while carbonate hornfels can be white, yellow or pale green, occasionally with larger porphyroblast crystals standing out from the fine grained matrix.