Hornfels is a non-foliated and very fine-grained metamorphic rock with a variable composition. It is a very hard rock with a mottled brownish color. It is usually found adjacent to igneous intrusion sites at shallow depths.
The word Hornfels is often used as a group name for a set of contact metamorphic rocks that have formed due to the heat of intrusive igneous masses. This excessive heat has made these rocks hard, splintery, super tough, and durable.
Hornfels is divided into three categories based on its composition.
Pelitic hornfels are also known as biotite hornfels. Clays, sedimentary slates, and shales produce biotite hornfels. These hornfels contain biotite mica in large concentrations, giving the rock a dark-reddish brown color and strong dichroism. These rocks also contain quartz and feldspar in moderate numbers. Graphite, tourmaline, and iron oxides are also usually present but in low quantities.
Carbonate hornfels or calc-silicate hornfels are formed after a thermal alteration of impure limestone. These rocks are fine-grained, banded, and much harder and tougher than the original limestone. These stones vary greatly in their mineralogical compositions and frequently alternate in thins seams with biotite hornfels and quartzites.
Mafic hornfels are produced from diabases, basalts, andesites, and some other igneous rocks. These rocks consist of feldspar with hornblende (brown) and pale pyroxene. These rocks vary in composition and structure, but most have sphene, biotite, and iron oxides in different amounts.
Hornfels Metamorphic Facies
The hornfels facies occupy the lowest spot in the facies chart. They have the lowest temperature and lowest pressure. The facies temperature range from low to high. Petrologists and geologists study facies to learn more about rocks and compare them with other ones.
Hornfels is formed due to heat conducted by a nearby magma chamber, dike, sill, or lava flow. The required temperature for hornfels formation is about 1300-1450 F (700-800 C). Directed pressure doesn’t play a big role in hornfels formation; it is often made up of minerals grains of the same size and spread in multiple directions. The grain’s shape and orientation might also be inherited from its parent rock.
One interesting thing about hornfels is that they are not deposited; they are formed when an existing rock is metamorphosed. The metamorphosed rock is often known as the parent rock or protolith.
Common sedimentary protolith or parent rock for hornfels are shale, siltstone, limestone, sandstone, and dolomite. Granite, andesite, diabase, basalt, gabbro, and rhyolite are common igneous rock protoliths. Some metamorphic rocks like schist or gneiss can also become parent rocks for hornfels.
Where is Hornfels Found?
Hornfels is found near their parent rock. Asian reserves are located in China, India, Russia, North Korea, South Korea, and Thailand. Hornfels reserved are also located in Tanzania, Cameroon, East Africa, and South Africa. The largest hornfels reserves in Europe are located in England. Hornfels is also found in Canada and South American countries.
Hornfel is used as an aggregate in the construction and road industries. It gives great results as a filler because it is extremely hard.
Minerals found in Hornfels
Hornfels are economically valuable because of the minerals found in them. These minerals include garnet, graphite, hornblende, kyanite, pyrite, scapolite, sillimanite, sphene, tourmaline, actinolite, andalusite, augite, biotite, calcite, chlorite, cordierite, diopside, epidote, feldspars, and vesuvianite.