Hornfels (German, meaning "hornstone") is the group designation for a series of contact metamorphic rocks that have been baked and indurated by the heat of intrusive igneous masses and have been rendered massive, hard, splintery, and in some cases exceedingly tough and durable. The German language (de ''Deutsch'') is a West Germanic language and one of the world's major languages. Metamorphism can be defined as the solid state recrystallisation of pre-existing rocks due to changes in heat and/or pressure and/or introduction of fluids i In Geology, an intrusion is a body of Igneous rock that has Crystallized from molten Magma below the surface of the Earth. Most hornfels are fine-grained, and while the original rocks (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, this structure is effaced or rendered inoperative in the hornfels. Sandstone is a Sedimentary rock composed mainly of Sand -size Mineral or rock grains. Shale (also called mudstone) is a fine-grained Sedimentary rock whose original constituents were Clay minerals or Muds It is characterized by Slate is a fine-grained foliated homogeneous, Metamorphic rock derived from an original Shale -type Sedimentary rock composed of Clay Limestone is a Sedimentary rock composed largely of the Mineral Calcite ( Calcium carbonate: CaCO3 Diabase (ˈdaɪəbeɪs or Dolerite is a Mafic, Holocrystalline, Igneous rock equivalent to Volcanic Basalt or plutonic Cleavage, in Mineralogy, is the tendency of crystalline materials to split along definite planes creating smooth surfaces of which there are several named types 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 hornfelses ) are dark-brown to black with a somewhat velvety luster owing to the abundance of small crystals of shining black mica. Biotite is a common phyllosilicate Mineral within the Mica group with the approximate chemical formula K(Mg Fe3AlSi3O10(F Lustre (or luster) is a description of the way light interacts with the surface of a Crystal, rock, or Mineral. In Materials science, a crystal is a Solid in which the constituent Atoms Molecules or Ions are packed in a regularly ordered repeating The word "mica" is thought to be derived from the Latin word la micare, "glitteren" in reference to the brilliant appearance of this mineral (especially The lime hornfels are often white, yellow, pale-green, brown and other colors. Lime is a general term for various naturally occurring Minerals and materials derived from them in which Carbonates Oxides and Hydroxides of Green and darkgreen are the prevalent tints of the hornfels produced by the alteration of igneous rocks. Igneous rocks (etymology from Latin ignis, fire are rocks formed by solidification of cooled Magma (molten rock Although for the most part the constituent grains are too small to be determined by the unaided eye, there are often larger crystals of garnet or andalusite scattered through the fine matrix, and these may become very prominent on the weathered faces of the rock. The garnet group includes a group of minerals that have been used since the Bronze Age as gemstones and abrasives Andalusite is an Aluminium nesosilicate Mineral with the chemical formula Al2SiO5 Weathering is the decomposition of earth rocks, Soils and their Minerals through direct contact with the planet's Atmosphere.
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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 and from the resemblance to rough pavement work. This has been called pfiaster or pavement structure. 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. A mineral is a naturally occurring substance formed through geological processes that has a characteristic chemical composition a highly ordered atomic structure and specific Quartz (from German) is the most abundant Mineral in the Earth 's Continental crust (although Feldspar is more common in The Mineral graphite, as with Diamond and Fullerene, is one of the Allotropes of carbon. Altogether there are sixteen known Iron Oxides and oxyhydroxides 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. Recrystallization (see also Crystallization) is a physical process that has meanings in Chemistry, Metallurgy and Geology. In Chemistry, a molecule is defined as a sufficiently stable electrically neutral group of at least two Atoms in a definite arrangement held together by 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
Slates, shales and clays yield biotite hornfels in which the most conspicuous mineral is black mica, the small scales of which are transparent under the microscope and have a dark reddish brown color and strong dichroism. Clay is a naturally occurring material composed primarily of fine-grained Minerals which show plasticity through a variable range of Water content, and Dichroic redirects here For the filter see Dichroic filter. For the glass see Dichroic glass. There is also quartz, and often a considerable amount of feldspar, while graphite, tourmaline and iron oxides frequently occur in lesser quantity. Tourmaline is a Crystal Silicate mineral compounded with elements such as Aluminium, Iron, Magnesium, Sodium, Lithium In these biotite hornfels the minerals, which consist of aluminiun silicates, are commonly found; they are usually andalusite an sillimanite, but kyanite appears also in hornfels, especially in those which have a schistose character. Sillimanite also called Bucholzite is an alumino-silicate Mineral with the Chemical formula Al2SiO5 Kyanite, whose name derives from the Greek word kyanos, meaning blue is a typically blue Silicate Mineral, commonly found in Aluminium -rich The schists form a group of medium-grade Metamorphic rocks chiefly notable for the preponderance of lamellar Minerals such as Micas chlorite 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. Pleochroism is an Optical phenomenon in which grains of a rock appear to be different colors when observed at different angles under a Petrographic microscope. The Mineral chiastolite is a variety of Andalusite with the chemical composition Al2SiO5 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. Sediment is any particulate matter that can be transported by fluid flow and which eventually is deposited as a layer of solid particles on the bed or bottom of a body of 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. Tourmaline is a Crystal Silicate mineral compounded with elements such as Aluminium, Iron, Magnesium, Sodium, Lithium As the tourmaline contains boron, there must have been some permeation of vapors from the granite into the sediments. Boron (ˈbɔərɒn is a Chemical element with Atomic number 5 and the chemical symbol B. Rocks of this group are often seen in the Cornish tin-mining districts, especially near the ludes. Tin is a Chemical element with the symbol Sn (stannum and Atomic number 50
A second great group of hornfels are the calcite-silicate-hornfels that arise from the thermal alteration of impure limestone. Calcite is a carbonate mineral and the most stable polymorph of Calcium carbonate ( Ca[[carbon C]] O 3 For the Artificial intelligence Androids of the 1990s Science fiction series Space Above and Beyond, see Silicate (AI Limestone is a Sedimentary rock composed largely of the Mineral Calcite ( Calcium carbonate: CaCO3 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, scapolite; with these phlogopite, various feldspars, pyrites, quartz and actinolite often occur. Recrystallization (see also Crystallization) is a physical process that has meanings in Chemistry, Metallurgy and Geology. Marble is a nonfoliated Metamorphic rock resulting from the Metamorphism of Limestone, composed mostly of Calcite (a crystalline form of Sand is a naturally occurring Granular material composed of finely divided rock and Mineral particles Diopside is a Monoclinic Pyroxene Mineral with composition MgCaSi2O6 Epidote is a Calcium Aluminium Iron sorosilicate Mineral, Ca2Al2(Fe3+Al(SiO4(Si2O7O(OH The garnet group includes a group of minerals that have been used since the Bronze Age as gemstones and abrasives Titanite or sphene is a Calcium Titanium nesosilicate Mineral, Ca[[titanium Ti]] Si[[oxygen O]]5 Vesuvianite, also known as idocrase is a green brown yellow or blue silicate mineral. Scapolite (Gr asairos, rod stone is a group of rock-forming Silicate minerals composed of Aluminium, Calcium and Sodium Silicate Phlogopite is a yellow greenish or reddish-brown member of the Mica family of phyllosilicates. For the community in Canada see Actinolite Ontario. For the Sailor Moon character 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. Quartzite (from German Quarzit) not to be confused with the Mineral Quartz, is a hard Metamorphic rock which was originally When perfused with boric and fluoric vapors from the granite they may contain much axinite, fluorite and datolite, but the altiminous silicates (andalusite, &c. Axinite is a brown to violet-brown or reddish-brown bladed Mineral composed of Calcium Aluminium boro - silicate, (CaFeMn3Al2BO3Si4O12OH Fluorite (also called fluorspar) is a Mineral composed of Calcium fluoride, Ca[[Fluorine F2]] Datolite is a Calcium Boron Hydroxide nesosilicate, Ca[[boron B]] Si[[oxygen O]]4(O H) ) are absent from these rocks.
From diabases, basalts, andesites and other igneous rocks a third type of hornfels is produced. Diabase (ˈdaɪəbeɪs or Dolerite is a Mafic, Holocrystalline, Igneous rock equivalent to Volcanic Basalt or plutonic Basalt (bəˈsɔːlt ˈbeisɔːlt ˈbæsɔːlt is a common Extrusive Volcanic rock. For the extinct cephalopod genus see Andesites. Andesite (ˈændəsaɪt is an igneous, Volcanic rock, of intermediate 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 a reas or irregular streaks containing a suite of new minerals, which may resemble those of the calcium-silicate hornfelses above described. Zeolites (Greek zein, "to boil" lithos, "a stone" are hydrated Aluminosilicate Minerals and have a micro-porous structure The chlorites are a group of phyllosilicate Minerals Chlorites can be described by the following four endmembers based on their chemistry via substitution 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. Porphyry is a variety of Igneous rock consisting of large-grained Crystals such as Feldspar or Quartz, dispersed in a fine-grained
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. Gneiss (ˈnaɪs is a common and widely distributed type of rock formed by high-grade regional metamorphic processes from preexisting formations that were originally 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. Marble is a nonfoliated Metamorphic rock resulting from the Metamorphism of Limestone, composed mostly of Calcite (a crystalline form of That these are sediments that have undergone thermal alteration is generally admitted, but the exact conditions under which they were formed is 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). Pressure (symbol 'p' is the force per unit Area applied to an object in a direction perpendicular to the surface 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.
See also
References
- This article incorporates text from the Encyclopædia Britannica Eleventh Edition, a publication now in the public domain. This page is intended as a list of all rock types A Amphibolite Andesite Anorthosite Anthracite The Encyclopædia Britannica Eleventh Edition (1910–1911 is a 29-volume reference work that marked the beginning of the Encyclopædia Britannica The public domain is a range of abstract materials &ndash commonly referred to as Intellectual property &ndash which are not owned or controlled by anyone
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