Hornfels is the group designation for a series of contact metamorphic rocks that have been baked and indurated by the heat of igneous intrusions and have been rendered massive, hard, splintery, and in some cases exceedingly tough and durable. Most hornfelses are fine-grained, although protoliths (such as sandstone, shale and slate, limestone and diabase) may have been more or less fissile owing to the presence of bedding or cleavage planes, hence these structures may be effaced or even obliterated. Relict banding due to bedding, fracture surfaces, etc., are often planes of weakness across which hornfels separate into cubical fragments rather than into thin plates. Biotite hornfelses are the most common hornfels. They are dark-brown to black with a somewhat velvety luster owing to the abundance of small crystals of mafic mica. The lime hornfelses are often white, yellow, pale-green, brown and other colors. Green and dark-green are the prevalent tints of the hornfelses produced by the alteration of igneous rocks. 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-grained matrix, and these may become very prominent on weathered surfaces. The structure of the hornfels is very characteristic. Very rarely do any of the minerals show crystalline form, but the small grains constitute an equant mosaic structure. Each mineral may also enclose particles of associated minerals. Within quartz, for example, small crystals of graphite, biotite, iron oxides, sillimanite or feldspar may appear, rendering grains semi-opaque. The minutest crystals may show traces of crystalline outlines, undoubtedly as secondary overgrowths. It is therefore believed that the entire rock has been recrystallized at a high temperature and in the solid state so that there was little diffusive freedom for minerals to form well-developed crystals. The regeneration of the rock has been sufficient to eradicate most of the original structures and to completely replace minerals. Often the crystallization of new minerals is hampered by the solid condition of the mass rendering new mineral grains as microscopic, with poorly defined grain boundaries, and saturated with inclusions.
