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Hornfels

Hornfels is a fine grained metamorphic rock and It is the group for a series of contact metamorphic rocks that have been baked under high temperatures by the heat of igneous intrusions and as a result, have become massive, splintery, extremely hard, and in some cases exceedingly tough and durable. The generally of hornfels are fine-grainded and dark colour. Biotite hornfels is most common that are dark-brown to black with a velvety luster.There are also lime hornfels that are commonly white, yellow, brown, pale-green and other colors. The green and dark-green color tint of the hornfels is established by the alteration of igneous rocks.

The shape of the Hornfels can be multifunctional. Most of the time, none of the minerals show a crystalline form, but small grains are very close to each other, such as parts of a mosaic; they are usually almost the same size. Similar to hard coating images, pflaster or pavement structure is called. Each mineral may also contain debris of others; In addition, small crystals of quartz, for example graphite, biotite, iron oxides, sillimanite or feldspar, may appear in great numbers. Generally all of the grains are rendered semi-opaque. The smallest crystals may also indicate strains of crystalline outlines; certainly, they are in new formations and are in situ. This has allowed us to agree that the mineral rock is recrystallized at an extreme temperature and in the powerful kingdom so that the mineral molecules have little freedom to accumulate beautifully individualized crystals. The regeneration of the rock has been enough to influence most of the original systems and to update the previous minerals with more or less than ever. However, crystallization has been hampered by the strong state of mass and the new minerals are amorphous and unable to reject the impurities, but have grown around them.

Texture – Granular, platy or elongate crystals randomly oriented so no foliation evident.

Grain size: Very fine grained; grains need to be observed under a microscope; can contain roundedporphyroblasts.

Hardness: hard (commonly displays conchoidal fracture).

Colour: variable, generally grey to black, but can form in a variety of colours dependent on parent rock composition.

Mineralogy: Extremely variable, dependent on the original composition of the parent rock; generally contains minerals only formed under high temperature conditions, e.g. andalusite (Al 2SiO5), cordierite ((Mg, Fe) 2Al 4Si 5O 18).

Other features: Generally smooth to touch.

Parent Rocks and Protoliths: Hornfels is not a rock that is “deposited”. Instead it is a rock type that forms when an existing rock is metamorphosed. The original rock that was metamorphosed is usually referred to as the “parent rock” or “protolith”. A variety of sedimentary, igneous, and metamorphic rocks can be the protolith of hornfels. Common protoliths of hornfels include sedimentary rocks such as shale, siltstone, sandstone, limestone and dolomite; igneous rocks such as basalt, gabbro, rhyolite, granite, andesite and diabase; or, metamorphic rocks such as schist and gneiss.

Name origin: German, meaning “hornstone”

Classification of Hornfels

The Hornfels classification of mineral composition that can be seperate into one of three general group

Pelitic Hornfels is derived from shale, slate, and schist

Carbonate Hornfels is derived from limestone, dolomite or marble

Mafic Hornfels is derived from mafic igneous rocks

Chemical Composition of Hornfels

Pelitic

Biotite hornfels yield of clay, sedimentary slates and shales, the small scales of transparent under the microscope and have a dark reddish-brown color and strong dichroism. There is also quartz, and often a considerable amount of feldspar, while graphite, tourmaline and iron oxides frequently occur in lesser quantity. In these biotite hornfels the minerals, which consist of aluminiun silicates, are commonly found; they are usually andalusite and sillimanite

Carbonate

The Calc-Silicate Hornfels is a second great group of hornfels. That arise from the thermal alteration of impure limestone.The purer beds recrystallize as marbles, but where there has been originally an admixture of sand or clay lime-bearing silicates are formed, such as diopside, epidote, garnet, sphene, vesuvianite and scapolite; with these phlogopite, various feldspars, pyrites, quartz and actinolite often occur. These rocks are fine-grained, and though often banded, are tough and much harder than the original limestones.

Mafic

Third biggest group in hornfels is produced from diabases, basalts, andesites and other igneous rocks. The consist minerals are  feldspar with hornblende (generally of brown color) and pale pyroxene. Sphene, biotite and iron oxides are the other common constituents, but these rocks show much variety of composition and structure. Where the original mass was decomposed and contained calcite, zeolites, chlorite and other secondary minerals either in veins or in cavities, there are usually rounded areas or irregular streaks containing a suite of new minerals, which may resemble those of the calcium-silicate hornfelses above described.

Formation of the Hornfels

The Hornfels formed is a 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.

Where is Hornfels Located

Hornfels occurs worldwide. In Europe, the largest reserves are in the United Kingdom. In North America, hornfels occurs in primarily in Canada. South American countries with large reserves include Bolivia, Brazil, Ecuador, and Colombia. Asian reserves are found in China, Russia, India, North Korea, South Korea, and Thailand. In Africa, hornfels is found in Tanzania, Cameroon, East Africa, and Western Africa. The rock is found in Australia and New Zealand, as well.

Characteristics and Properties of Rock

Hornfels often retains the stratification, large-scale geometry, and also some textural characteristics of the protolith. The changes of contact metamorphism that convert rocks to hornfels can include recrystallization, cementation, silicification, partial melting, and more.

The result is often a dense, hard, fine-grained rock that is generally homogenous and exhibits a semi-conchoidal fracture. Hornfels can be almost any color, but black, gray, brown, reddish and greenish rocks are common.

  • It is a type of metamorphic rock that gets its name from its resemblance to animal horn.
  • It forms when magma heats other rock, which may be igneous, metamorphic, or sedimentary.
  • The most common colors of hornfels are black and dark brown. It may be banded or occur in other colors. The colors depend on the composition of the original rock.
  • Key properties of the rock include velvety texture and appearance, conchoidal fracture, and fine grain. It may be very hard and tough.
  • It is a contact metamorphic rock, formed when magma bakes its source material.

Uses of Rock

Uses of hornfels are as an aggregate in the construction and road building.

The primary use of hornfels is in architecture. The hard, interesting-looking stone may be used to make interior flooring and decorations as well as exterior facing, paving, curbing, and decorations.

The rock is used in the construction industry to make road aggregate. Historically, hornels has been used to construct monuments, cemetery markers, whetstones, artworks, and artifacts.

One noteworthy use of hornfels is to construct lithophones or stone bells. In South Africa, the rock may be called “ring stones.” The “Musical Stones of Skiddaw” refers to a series of lithophones made using hornfels mined from Skiddaw mountain, near the town of Keswick in England. In 1840, stonemason and musician Joseph Richardson built an eight-octave lithophone, which he played on tour. The lithophone is played like a xylophone.

Facts About Rock

  • The structure of the hornfels is characterized by the small-grained mosaic make-up.
  • Thye are used in a number of applications like in the field of construction and landscaping. It is used as a decorative rock in gardens. In the olden times, it is used as a tool like scrapers and knives.
  • It is used as a road base and in concrete and is most often dark blue or almost a black color.
  • The interior use of hornfels is found in homes and businesses in the decorative aggregates, flooring, countertops, and bathrooms.
  • The exterior use of hornfels is viewed in building construction, paving stones, and a variety of gardening decorations.
  • In prehistoric times, hornfels was used to make simple tools such as knives, scrapers, and arrowheads.
  • Thye are defined by the physical properties such as hardness, strength, grain size, fracture, porosity, and streak. It is these physical properties that determine usage.
  • Because pressure is not a main factor in the formation of hornfels, and the texture is granular, platy or elongated crystals, there is a lack of foliation as often seen in many metamorphic rocks formed under high pressure.
  • During the formation of hornfels, the pre-existing rock is destroyed.
  • They are typically found only by microscopic observation and not witnessed by eye alone. However, under a microscope the structure becomes very distinctive revealing the small-grained mosaic design.
  • There is a second group of hornfels are called the calc-silicate hornfels which originate from the thermal alteration of impure limestone. These rocks are fine-grained, and even though they are often banded, they are tough and much harder than the original limestone.
  • It have the ability to resonate when struck. The stones in South Africa are called “ring-stones” due to their ability to ring like a bell after being struck with an object.

References

Bonewitz, R. (2012). Rocks and minerals. 2nd ed. London: DK Publishing.

Helmenstine, Anne Marie, Ph.D. (2018, October 19). What Hornfels Is and How It Forms. Retrieved from https://www.thoughtco.com/hornfels-definition-and-formation-4165525

http://www.softschools.com/facts/rocks/hornfels_facts/2985/