Hornfels is a variety of metamorphic rock; which means that the rock's original texture, mineralogy, or both have been changed. The most obvious changes occur in the minerals that make up the rock. Although the rocks never quite melt, some or all of the minerals recrystallize into different shapes and new minerals form by a chemical process that "cannibalizes" the minerals in the original, or parent rock.
Geologists recognize two main groups of metamorphic rocks, separating them on the basis of the conditions in which metamorphic change takes place. One group is formed by alteration of rocks in a broad region of high heat and elevated pressure. This group of rocks, formed by regional or "dynamothermal" metamorphism, is typically characterized by the presence of foliated minerals. In foliated rocks, recrystallized and newly formed mineral grains align themselves in the same direction, giving the resulting rocks a banded or layered appearance. Foliated rocks include schist, phyllite, and gneiss.
Hornfels is a product of a different metamorphic environment, one that is known as contact metamorphism. The contact metamorphic environment lacks the high pressures found in a regional metamorphic environment. Instead, mineral changes take place entirely as a result of exposure to high temperatures, almost always produced by a nearby igneous body. Such rocks may be found in thin "baked contacts" adjacent to dikes and sills or in larger bodies surrounding large igneous intrusions such as a granite batholith.
The pre-metamorphic parent rock of a hornfels sample can be almost any sedimentary or igneous rock, and its original form often cannot be determined by examining the hornfels itself. Metamorphism of sedimentary rocks is perhaps most common, especially of mixed sandstone-shale and limestone-shale deposits. More pure layers of sandstone and limestone are altered to quartzite and marble, respectively, by contact metamorphism rather than to hornfels.
Most samples of hornfels are black to gray or brown with a very fine-grained texture, making identification of minerals difficult or impossible without microscopic study. Hornfels mineralogy spans a broad range that is mainly dependent on what minerals were present in the parent rock and in what proportions, though many are dominated by a mixture of micas (especially biotite), hornblende, quartz and feldspar. Mineral grains are usually all of approximately the same size, although larger grains called porphyroblasts may be present. The grains are randomly oriented, as opposed to the alignment that creates the foliation characteristic of regional metamorphism.
Since the rock type lacks either a characteristic texture or mineral assemblage, hornfels is generally identifiable only by its proximity to a high-temperature igneous body. Other potential diagnostic characteristics of hornfels are a tendency to break into blocks without regard to any apparent layering, and an unusual resistance to mechanical and chemical weathering.