Hornfels

Hornfels is the group name for a set of contact metamorphic rocks that have been baked and hardened by the heat of intrusive igneous masses and have been rendered massive, hard, splintery, and in some cases exceedingly tough and durable. These properties are due to fine grained non-aligned crystals with platy or prismatic habits. The term is derived from the German word Hornfels, meaning "hornstone", because of its exceptional toughness and texture both reminiscent of animal horns. These rocks were referred to by miners in northern England as whetstones.[1][2]

Most hornfels are fine-grained, and while the original rocks (such as sandstone, shale, slate and limestone) may have been more or less fissile owing to the presence of bedding or cleavage planes, this structure is effaced or rendered inoperative in the hornfels. Though they may show banding, due to bedding, etc., they break across this as readily as along it; in fact, they tend to separate into cubical fragments rather than into thin plates.

The most common hornfels (the biotite hornfels) are dark-brown to black with a somewhat velvety luster owing to the abundance of small crystals of shining black mica. The lime hornfels are often white, yellow, pale-green, brown and other colors. Green and dark-green are the prevalent tints of the hornfels 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 matrix, and these may become very prominent on the weathered faces of the rock.

Hornfels
A sample of banded hornfels, formed by contact metamorphism of sandstones and shales by a granite intrusion

Structure

The structure of the hornfels is very characteristic. Very rarely do any of the minerals show crystalline form, but the small grains fit closely together like the fragments of a mosaic; they are usually of nearly equal dimensions. This has been called pflaster or pavement structure from the resemblance to rough pavement work. Each mineral may also enclose particles of the others; in the quartz, for example, small crystals of graphite, biotite, iron oxides, sillimanite or feldspar may appear in great numbers. Often the whole of the grains are rendered semi-opaque in this way. The minutest crystals may show traces of crystalline outlines; undoubtedly they are of new formation and have originated in situ. This leads us to believe that the whole rock has been recrystallized at a high temperature and in the solid state so that there was little freedom for the mineral molecules to build up well-individualized crystals. The regeneration of the rock has been sufficient to efface most of the original structures and to replace the former minerals more-or-less completely by new ones. But crystallization has been hampered by the solid condition of the mass and the new minerals are formless and have been unable to reject impurities, but have grown around them.

Compositions of hornfels

Corneenne dielette manche
Hornfels sample (Normandy, France)

Pelitic

Clays, sedimentary slates and shales yield biotite hornfels in which the most conspicuous mineral is biotite mica, the small scales of which are 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, but kyanite appears also in hornfels, especially in those that have a schistose character. The andalusite may be pink and is then often pleochroic in thin sections, or it may be white with the cross-shaped dark enclosures of the matrix that are characteristic of chiastolite. Sillimanite usually forms exceedingly minute needles embedded in quartz.

In the rocks of this group cordierite also occurs, not rarely, and may have the outlines of imperfect hexagonal prisms that are divided up into six sectors when seen in polarized light. In biotite hornfels, a faint striping may indicate the original bedding of the unaltered rock and corresponds to small changes in the nature of the sediment deposited. More commonly there is a distinct spotting, visible on the surfaces of the hand specimens. The spots are round or elliptical, and may be paler or darker than the rest of the rock. In some cases they are rich in graphite or carbonaceous matter; in others they are full of brown mica; some spots consist of rather coarser grains of quartz than occur in the matrix. The frequency with which this feature reappears in the less altered slates and hornfels is rather remarkable, especially as it seems certain that the spots are not always of the same nature or origin. Tourmaline hornfels are found sometimes near the margins of tourmaline granites; they are black with small needles of schorl that under the microscope are dark brown and richly pleochroic. As the tourmaline contains boron, there must have been some permeation of vapors from the granite into the sediments. Rocks of this group are often seen in the Cornish tin-mining districts, especially near the ludes.

Carbonate

A second great group of hornfels are the calc–silicate 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. They are excessively variable in their mineralogical composition, and very often alternate in thin seams with biotite hornfels and indurated quartzites. When perfused with boric and fluoric vapors from the granite they may contain much axinite, fluorite and datolite, but the altiminous silicates are absent from these rocks.

Mafic

From diabases, basalts, andesites and other igneous rocks a third type of hornfels is produced. They consist essentially of 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. The original porphyritic, fluidal, vesicular or fragmental structures of the igneous rock are clearly visible in the less advanced stages of hornfelsing, but become less evident as the alteration progresses.

In some districts hornfelsed rocks occur that have acquired a schistose structure through shearing, and these form transitions to schists and gneisses that contain the same minerals as the hornfels, but have a schistose instead of a hornfels structure. Among these may be mentioned cordierite and sillimanite gneisses, andalusite and kyanite mica-schists, and those schistose calcite-silicate rocks that are known as cipolins. That these are sediments that have undergone thermal alteration is generally admitted, but the exact conditions under which they were formed are not always clear. The essential features of hornfelsing are ascribed to the action of heat, pressure and permeating vapors, regenerating a rock mass without the production of fusion (at least on a large scale). It has been argued, however, that often there is extensive chemical change owing to the introduction of matter from the granite into the rocks surrounding it. The formation of new feldspar in the hornfelses is pointed out as evidence of this. While this felspathization may have occurred in a few localities, it seems conspicuously absent from others. Most authorities at the present time regard the changes as being purely of a physical and not of a chemical nature.

Acoustic properties

Hornfels have the ability to resonate when struck. Michael Tellinger had described these stones in South Africa also known as "ring-stones" due to their ability to ring like a bell.[3] The Musical Stones of Skiddaw are an example of a lithophone made from hornfels.[4]

See also

References

  1. ^ "Holwick Scar & Low Force : Pamphlet" (PDF). Explorenorthpennines.org.uk. Retrieved 2015-03-17.
  2. ^ Lawrence, D. J. D et al 2004 Durham Geodiversity Audit, Durham: Durham County Council p20
  3. ^ "2014 Ancient Hidden Technology of the Annunaki (Fallen Angels)". see from 43 min. Archived from the original on 2016-08-09. Retrieved 2016-06-02.
  4. ^ "The Musical Stones of Skiddaw - Allerdale Borough Council". Allerdale.gov.uk. Retrieved 2015-03-17.

 This article incorporates text from a publication now in the public domainFlett, John S. (1911). "Hornfels" . In Chisholm, Hugh (ed.). Encyclopædia Britannica. 13 (11th ed.). Cambridge University Press. pp. 710–711.

External links

Media related to Hornfels at Wikimedia Commons

Bode Gorge

The Bode Gorge (German: Bodetal) is a 10 kilometres (6.2 mi) long ravine that forms part of the Bode valley between Treseburg and Thale in the Harz Mountains of central Germany. The German term, Bodetal (literally "Bode Valley"), is also used in a wider sense to refer to the valleys of the Warme and Kalte Bode rivers that feed the River Bode.

At the Bode Gorge, the River Bode, which rises on the highest mountain in the Harz, the Brocken, has cut deeply into the hard Ramberg granite rock. The ravine is about 140 m deep at Treseburg and some 280 m deep at Thale where it breaks out into the Harz Foreland. The Bode Gorge was designated a nature reserve as early as 5 March 1937; its boundaries being subsequently expanded. With an area of, currently 473.78 hectares (1,170.7 acres), it is one of the largest nature reserves in Saxony-Anhalt.

Boudinage

Boudinage is a geological term for structures formed by extension, where a rigid tabular body such as hornfels, is stretched and deformed amidst less competent surroundings. The competent bed begins to break up, forming sausage-shaped boudins. Boudinage is common and can occur at any scale, from microscopic to lithospheric, and can be found in all terranes. In lithospheric-scale tectonics, boudinage of strong layers can signify large-scale creep transfer of rock matter. The study of boudinage can, also, help provide insight to the forces involved in tectonic deformation of rocks and their strength.Boudinage can develop in two ways: planar fracturing into rectangular fragments or by necking or tapering into elongate depressions and swells. Boudins are typical features of sheared veins and shear zones where, due to stretching along the shear foliation and shortening perpendicular to this, rigid bodies break up. This causes the resulting boudin to take a characteristic sausage or barrel shape. They can also form rectangular structures. Ductile deformation conditions also encourage boudinage rather than imbricate fracturing. Boudins can become separated by fractures or vein material, this zone of separation is known as boudin necks.In three dimensions, the boudinage may take the form of ribbon-like boudins or chocolate-tablet boudins, depending on the axis and isotropy of extension. They range in size from about 20 m thick to about 1 cm.

Calcflinta

Calcflinta, or calc-flinta, is a fine grained calc–silicate rock found amongst the metamorphic rocks of the eastern Highlands of Scotland. It is a hornfels developed from calcareous mudstone. Calcflinta is also found, for example, around the northwest margin of the Dartmoor granite in England, and on King Island in Tasmania.

Calc–silicate rock

A calc–silicate rock is a rock produced by metasomatic alteration of existing rocks in which calcium silicate minerals such as diopside and wollastonite are produced. Calc–silicate skarn or hornfels occur within impure limestone or dolomite strata adjacent to an intruding igneous rock.

Carletonite

Carletonite is a rare silicate mineral with formula KNa4Ca4(CO3)4Si8O18(F,OH)·(H2O).

It is a phyllosilicate and a member of the apophyllite group. Its tetragonal crystals are a translucent blue, white, colorless or pink with a vitreous to dull lustre. It has a density of 2.45 and a hardness of 4-4.5.

It was discovered by G.Y Chao and named for the school he attended, Carleton University of Ottawa. It was first described in 1969 for an occurrence at Mont Saint-Hilaire, Quebec. The type locality at Mont Saint–Hilaire is the only reported occurrence.It occurs in hornfels and siliceous marble xenoliths within and adjacent to a nepheline syenite intrusion. It occurs in association with quartz, narsarsukite, calcite, fluorite, ancylite, molybdenite, leucosphenite, lorenzenite, galena, albite, pectolite, apophyllite, leifite, microcline and arfvedsonite.

Haycock Mountain

Haycock Mountain is a locally prominent hill with the highest summit in Bucks County. It rises above Nockamixon State Park, in the Delaware River drainage of southeastern Pennsylvania. Early settlers named it simply for its "resemblance to a cock of hay."Haycock is covered with numerous triassic diabase boulders, and is a bouldering destination with many established routes ranging from V0 to V10+.To the north northwest of the main peak is a secondary peak of approximately 820 feet (250 m)sometimes known as 'Little Haycock', and the main peak overlooks Lake Nockamixon to the southeast.

Contained within the Tohickon Creek watershed, Haycock Mountain is drained by Dimple Creek to the west and Haycock Creek to the east.

Since it lies within State Game Land Number 157, Haycock is used seasonally for hunting.

Hornfels in Victorian archaeological sites

Hornfels is an unusual and relatively rare stone used in making flaked stone tools, and which is found in Aboriginal archaeological sites in Victoria, Australia. A sample of places where it has been found can be seen in the geographic section below.The term has been used for ...a group of compact, fine-grained, metamorphic rocks that form as a result of contact between sedimentary country rocks and a magma intrusion. The mineral composition is variable, but commonly contains mica and pyroxene while porphyroblasts of pyroxene, cordierite or andulusite also develop. Sedimentary structures are rarely evident due to a high degree of recrystallisation.

Jabal al-Lawz

Jabal al-Lawz (Arabic: جَبَل ٱللَّوْز‎), also known as Gebel el-Lawz, is a mountain located in northwest Saudi Arabia, near the Jordanian border, above the Gulf of Aqaba at 2,580 metres (8,460 feet) above sea level. The name means 'mountain of almonds'. The peak of Jabal al-Lawz, consists of a light-colored, calc-alkaline granite that is intruded by rhyolite and andesite dikes which generally trend eastward.In discussions about the location of biblical Mount Sinai, Jabal Maqlā ('Burnt Mountain') is often confused with and misidentified as Jabal al-Lawz by various authors such as Bob Cornuke, Ron Wyatt, and Lennart Moller as shown by local and regional maps and noted by other investigators. In contrast to the real Jabal al-Lawz, the summit of Jabal Maqlā consists mainly of dark-colored hornfels derived from metamorphosed volcanic rocks that originally were silicic and mafic lava flows, tuff breccias, and fragmental greenstones. The middle and lower slopes of Jabal Maqlā consist of light-colored granite, which has intruded into the overlying hornfels. This is the same granite that comprises Jabal al-Lawz. Jabal Maqla is about 7 kilometers to the south, and a few hundred meters lower.

Claims made by some writers, including Bob Cornuke, Ron Wyatt, and Lennart Moller, that Jabal Maqlā, misidentified as Jabal al-Lawz, is the real biblical Mount Sinai have been rejected by such scholars as James Karl Hoffmeier (Professor of Old Testament and Ancient Near Eastern History and Archaeology), who details what he calls Cornuke's "monumental blunders". Creationist Gordon Franz has also argued against this identification.Remains both of pillars and cairns at the site have been described as "similar to rock cairns of uncertain use and often uncertain date found at other sites throughout northern and western Arabia."

Massachusetts Hornfels-Braintree Slate Quarry

The Massachusetts Hornfels-Braintree Slate Quarry is a prehistoric archaeological site in Milton and Quincy, Massachusetts. It consists of a series of pits and trenches used from 7,000 B.P. until the early 17th century as a source of slate and hornfels used for chipped and ground tools. Pieces made from material quarried at the site are found over much of eastern Massachusetts. The site was added to the National Register of Historic Places in 1980.

Metamorphic facies

A metamorphic facies is a set of mineral assemblages in metamorphic rocks formed under similar pressures and temperatures. The assemblage is typical of what is formed in conditions corresponding to an area on the two dimensional graph of temperature vs. pressure (See diagram in Figure 1). Rocks which contain certain minerals can therefore be linked to certain tectonic settings, times and places in the geological history of the area. The boundaries between facies (and corresponding areas on the temperature v. pressure graph) are wide because they are gradational and approximate. The area on the graph corresponding to rock formation at the lowest values of temperature and pressure is the range of formation of sedimentary rocks, as opposed to metamorphic rocks, in a process called diagenesis.

Metamorphic rock

Metamorphic rocks arise from the transformation of existing rock types, in a process called metamorphism, which means "change in form". The original rock (protolith) is subjected to heat (temperatures greater than 150 to 200 °C) and pressure (100 megapascals (1,000 bar) or more), causing profound physical or chemical change. The protolith may be a sedimentary, igneous, or existing metamorphic rock.

Metamorphic rocks make up a large part of the Earth's crust and form 12% of the Earth's land surface. They are classified by texture and by chemical and mineral assemblage (metamorphic facies). They may be formed simply by being deep beneath the Earth's surface, subjected to high temperatures and the great pressure of the rock layers above it. They can form from tectonic processes such as continental collisions, which cause horizontal pressure, friction and distortion. They are also formed when rock is heated by the intrusion of hot molten rock called magma from the Earth's interior. The study of metamorphic rocks (now exposed at the Earth's surface following erosion and uplift) provides information about the temperatures and pressures that occur at great depths within the Earth's crust.

Some examples of metamorphic rocks are gneiss, slate, marble, schist, and quartzite.

Metamorphism

Metamorphism is the change of minerals or geologic texture (distinct arrangement of minerals) in pre-existing rocks (protoliths), without the protolith melting into liquid magma (a solid-state change). The change occurs primarily due to heat, pressure, and the introduction of chemically active fluids. The chemical components and crystal structures of the minerals making up the rock may change even though the rock remains a solid. Changes at or just beneath Earth's surface due to weathering or diagenesis are not classified as metamorphism. Metamorphism typically occurs between diagenesis (maximum 200°C), and melting (~850°C).The geologists who study metamorphism are known as "metamorphic petrologists." To determine the processes underlying metamorphism, they rely heavily on statistical mechanics and experimental petrology.

Three types of metamorphism exist: contact, dynamic, and regional. Metamorphism produced with increasing pressure and temperature conditions is known as prograde metamorphism. Conversely, decreasing temperatures and pressure characterize retrograde metamorphism.

Metasomatism

Metasomatism (from the Greek μετά (change) and σῶμα (body)) is the chemical alteration of a rock by hydrothermal and other fluids. [1] It is the replacement of one rock by another of different mineralogical and chemical composition. The minerals which compose the rocks are dissolved and new mineral formations are deposited in their place. Dissolution and deposition occur simultaneously and the rock remains solid.

Synonyms to the word metasomatism are metasomatose and metasomatic process. The word metasomatose can also be used as a name for specific varieties of metasomatism (for example Mg-metasomatose and Na-metasomatose).

Metasomatism can occur via the action of hydrothermal fluids from an igneous or metamorphic source. In the igneous environment, metasomatism creates skarns, greisen, and may affect hornfels in the contact metamorphic aureole adjacent to an intrusive rock mass. In the metamorphic environment, metasomatism is created by mass transfer from a volume of metamorphic rock at higher stress and temperature into a zone with lower stress and temperature, with metamorphic hydrothermal solutions acting as a solvent. This can be envisaged as the metamorphic rocks within the deep crust losing fluids and dissolved mineral components as hydrous minerals break down, with this fluid percolating up into the shallow levels of the crust to chemically change and alter these rocks.

This mechanism implies that metasomatism is open system behaviour, which is different from classical metamorphism which is the in-situ mineralogical change of a rock without appreciable change in the chemistry of the rock. Because metamorphism usually requires water in order to facilitate metamorphic reactions, metamorphism nearly always occurs with metasomatism.

Further, because metasomatism is a mass transfer process, it is not restricted to the rocks which are changed by addition of chemical elements and minerals or hydrous compounds. In all cases, to produce a metasomatic rock some other rock is also metasomatised, if only by dehydration reactions with minimal chemical change. This is best illustrated by gold ore deposits which are the product of focused concentration of fluids derived from many cubic kilometres of dehydrated crust into thin, often highly metasomatised and altered shear zones and lodes. The source region is often largely chemically unaffected compared to the highly hydrated, altered shear zones, but both must have undergone complementary metasomatism.

Metasomatism is more complicated in the Earth's mantle, because the composition of peridotite at high temperatures can be changed by infiltration of carbonate and silicate melts and by carbon dioxide-rich and water-rich fluids, as discussed by Luth (2003). Metasomatism is thought to be particularly important in changing the composition of mantle peridotite below island arcs as water is driven out of ocean lithosphere during subduction. Metasomatism has also been considered critical for enriching source regions of some silica-undersaturated magmas. Carbonatite melts are often considered to have been responsible for enrichment of mantle peridotite in incompatible elements.

Mont Rougemont

Mont Rougemont (Abenaki: Wigwômedenek) is part of the Monteregian Hills in southern Quebec. It is composed of igneous rock and hornfels. The summit stands 366 m (1,201 ft) above sea level. The mountain is mostly covered with sugar maple-dominated forest. Apple orchards and vineyards are cultivated on many of the lower slopes, and much of the fruit is used to make cider.

Musical Stones of Skiddaw

The Musical Stones of Skiddaw are a number of lithophones built across two centuries around the town of Keswick, northern England, using hornfels, a stone from the nearby Skiddaw mountain, which is said to have a superior tone and longer ring than the more commonly used slate.The first documented lithophone from Keswick was built in 1785 by Peter Crosthwaite, an eccentric inventor who became interested in the musical properties of the local stone. However, this kind of instrument became widely known only decades later, when in 1840 Joseph Richardson, a local stonemason and self-taught musician, built a larger, eight-octave lithophone with which he and his sons toured the UK and Europe giving numerous concerts, including one in London for Queen Victoria.Richardson's lithophone initially featured 61 tuned and shaped hornfels rocks. It was later enhanced with steel bars, Swiss bells and various other percussions, and survives to this day, being on display at the Keswick Museum and Art Gallery.

Narsarsukite

Narsarsukite is a rare silicate mineral with chemical formula Na2(Ti,Fe3+)Si4(O,F)11 or Na4(Ti,Fe)4[Si8O20](O,OH,F)4.It was first described in 1900 for an occurrence in the Narsarsuk pegmatite in the Ilimaussaq intrusive complex of West Greenland. It has also been reported from a syenite which intruded limestone in the Sweetgrass Hills, Montana, and within hornfels and marble xenoliths in the alkalic intrusive of Mont Saint-Hilaire, Quebec. It occurs associated with aegirine, microcline, albite, elpidite, epididymite, taeniolite, pectolite, calcite, galena and quartz.

Nokhu Crags

Nokhu Crags is a rock formation and mountain summit in the Never Summer Mountains range of the Rocky Mountains of North America. The name is derived from the Arapaho language, Neaha-no-xhu, meaning "Eagles Nest." The 12,490-foot (3,807 m) peak is located in State Forest State Park, 2.5 miles (4.0 km) south (bearing 181°) of Cameron Pass in Jackson County, Colorado, United States. The summit lies just northwest of the Continental Divide and Rocky Mountain National Park, near the headwaters of the Michigan River. The peak is prominently visible from State Highway 14 and can be seen throughout the southern North Park basin where it is known also known as "the Crags" or "Sleeping Indian" for its resemblance to the form of a supine chief. To the east lie the shallow basins of Snow Lake and the Michigan or American Lakes; to the north lies a snow filled couloir; to the west the mountain descends directly into the deep waters of Lake Agnes; and to the south lie Static Peak, Mount Richthofen, and the remainder of the Never Summer Mountain Range.

Pyroxenite

Pyroxenite is an ultramafic igneous rock consisting essentially of minerals of the pyroxene group, such as augite, diopside, hypersthene, bronzite or enstatite. Pyroxenites are classified into clinopyroxenites, orthopyroxenites, and the websterites which contain both types of pyroxenes (see diagram below). Closely allied to this group are the hornblendites, consisting essentially of hornblende and other amphiboles.

They are essentially of igneous origin, though some pyroxenites are included in the metamorphic Lewisian complex of Scotland. The pyroxene-rich rocks, which result from the type of contact metamorphism known as pyroxene-hornfels facies, have siliceous sediment or basaltic protoliths, and are respectively metapelites and metabasites.

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