How hornfels form?
Hornfels is a group designated for a series of contact metamorphism that have been baked and by the heat of magma chamber or from the intrusive igneous masses and are made into massive, hard, splintery, and in some cases exceedingly tough and durable. As of the contact metamorphism, pressure is not a factor in the formation of hornfels, it lacks the foliation as seen in many metamorphic rocks formed under high pressure and temperature. Pre-existing bedding and structure of the parent rock is generally destroyed in hornfels.
Types and colour of hornfels
The most common hornfels are the biotite hornfels which are dark brown to black with somewhat velvety luster owing to the abundance of small crystals of shinning black mica. The limestone hornfels are often white, yellow, pale green, brown and other colours. Although for the most part the constituent grains are too small to be determined by the unaided eye, there are often larger crystals of cordierite, garnet or andalusite scattered through the fine matrix, and these may become very prominent on the weathered faces of the rock.
Uses of hornfels are as an aggregate in the construction and road building.