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 granitic masses and have been rendered massive, hard, splintery, and in some cases exceedingly tough and durable. Most hornfelses 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. 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 hornfelses (the biotite hornfelses ) are dark-brown to black with a somewhat velvety luster owing to the abundance of small crystals of shining black mica. The lime hornfelses are often white, yellow, pale-green, brown and other colors. Green and darkgreen are the prevalent tints of the hornfelses 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 garnet or andalusite scattered through the fine matrix, and these may become very prominent on the weathered faces of the rock. Metamorphism can be defined as the mineralogical, chemical and crystallographic changes in a solid-state rock, i. ... Pluton redirects here. ... Sandstone near Stadtroda, Germany Sandstone is an sedimentary rock composed mainly of feldspar and quartz and varies in colour (in a similar way to sand), through grey, yellow, red, and white. ... Shale Shale is a fine-grained sedimentary rock whose original constituents were clays or muds. ... Slate Slate is a fine-grained, homogeneous, metamorphic rock composed of clay or volcanic ash which has been metamorphosed (foliated) in layers (bedded deposits). ... Limey shale overlaid by limestone. ... Diabase is a mafic, holocrystalline, igneous rock equivalent to volcanic basalt or plutonic gabbro. ... Cleavage, in mineralogy, is the tendency of crystalline materials to split along definite planes, creating smooth surfaces, of which there are several named types: Basal cleavage: cleavage parallel to the base of a crystal, or to the plane of the lateral axes. ... A Biotite slice Biotite is a common phyllosilicate mineral that contains potassium, magnesium, iron and aluminium. ... For the file system called Lustre, see Lustre (file system) Lustre (American English: luster) is a description of the way light interacts with the surface of a crystal, rock or mineral. ... Quartz crystal A crystal is a solid in which the constituent atoms, molecules, or ions are packed in a regularly ordered, repeating pattern extending in all three spatial dimensions. ... rock with mica Mica sheet mica flakes The mica group of minerals includes several closely related materials having highly perfect basal cleavage. ... Lime has several meanings: Look up Lime in Wiktionary, the free dictionary Lime (mineral) - a group of calcium compounds and minerals in which they predominate, including: Limestone Agricultural lime - a mineral soil additive Calcium oxide (also quicklime) - a chemical compound Calcium hydroxide (also slaked lime) - a chemical compound Lime (fruit... Volcanic rock on North America Plutonic rock on North America Igneous rocks are formed when molten rock (magma) cools and solidifies, with or without crystallization, either below the surface as intrusive (plutonic) rocks or on the surface as extrusive (volcanic) rocks. ... The Garnet group of minerals show crystals with a habit of rhombic dodecahedrons and trapezohedrons. ... Andalusite-cordierite schist (Large brown crystals are Andalusite Andalusite is an alumino-silicate mineral with the chemical formula Al2SiO5. ... Weathering is the process of decomposition and/or disintegration of rocks, soils and their minerals through natural, chemical, and biological processes that is, in place. ...

The structure of the hornfelses 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. 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. This article is about minerals in the geologic sense; for nutrient minerals see dietary mineral; for the band see Mineral (band). ... Quartz is the most abundant mineral in the Earths continental crust. ... Graphite (named by Abraham Gottlob Werner in 1789, from the Greek γραφειν: to draw/write, for its use in pencils) is one of the allotropes of carbon. ... Iron oxide pigment There are a number of iron oxides: Iron oxides Iron(II) oxide or ferrous oxide (FeO) The black-coloured powder in particular can cause explosions as it readily ignites. ... Recrystallization is an essentially physical process that has meanings in chemistry and geology. ... A molecule is the smallest particle of a pure chemical substance that still retains its chemical composition and properties. ...

Slates, shales and clays yield biotite hornfelses 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. 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 hornfelses the minerals, which consist of aluminiun silicates, are commonly found; they are usually andalusite an sillimanite, but kyanite appears also in hornfelses, especially in those which 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. Quaternary clay in Estonia. ... In optics, the term dichroic has two related but distinct meanings. ... The tourmaline mineral group is chemically one of the most complicated groups of silicate minerals. ... Sillimanite: Biotite gneiss (Mesozoic and Paleozoic) Sillimanite is an alumino-sillicate mineral with the chemical formula Al2SiO5. ... Kyanite The mineral kyanite is an aluminium silicate of the sillimanite group (along with andalusite and sillimanite), also called alumino-silicate. ... Categories: Mineral stubs | Metamorphic rocks ... The mineral chiastolite is a variety of Andalusite with the chemical composition Al2OSiO4. ...

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 hornfelses, 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 hornfelses is rather remarkable, especially as it seems certain that the spots are not always of the same nature or origin. Tourmaline hornfelses 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. 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 water or other liquid. ... The mineral tourmaline is chemically one of the most complicated silicate minerals. ... General Name, Symbol, Number boron, B, 5 Chemical series metalloids Group, Period, Block 13, 2, p Appearance black/brown Atomic mass 10. ... General Name, Symbol, Number tin, Sn, 50 Chemical series poor metals Group, Period, Block 14, 5, p Appearance silvery lustrous gray Atomic mass 118. ...

A second great group of hornfelses are the calcite-silicate-hornfelses 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, 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 (andalusite, &c.) are absent from these rocks. Calcite from Brushy Creek Mine, Missouri, USA. The carbonate mineral calcite is a calcium carbonate corresponding to the formula CaCO3 and is one of the most widely distributed minerals on the Earths surface. ... In chemistry, a silicate is a compound consisting of silicon and oxygen (SixOy), one or more metals, and possibly hydrogen. ... Limey shale overlaid by limestone. ... Recrystallization is an essentially physical process that has meanings in chemistry and geology. ... Marble For the glass spheres, see marbles. ... Patterns in the sand Sand is an example of a class of materials called granular matter. ... Diopside is a monoclinic pyroxene mineral with composition MgCaSi2O6. ... Epidote is a calcium aluminium iron sorosilicate mineral, Ca2(Al, Fe)3(SiO4)3(OH), crystallizing in the monoclinic system. ... The Garnet group of minerals show crystals with a habit of rhombic dodecahedrons and trapezohedrons. ... Titanite or sphene is a calcium titanium nesosilicate mineral, CaTiSiO5. ... A green, brown, yellow, or blue metamorphic silicate mineral, Ca10Mg2Al4(SiO4)5(Si2O7)2(OH)4. ... Scapolite (Gr. ... Phlogopite is a yellow, greenish or reddish brown member of the mica family of phyllosilicates. ... The mineral pyrite, or iron pyrite, is iron disulfide, FeS2. ... Well-cleaved, dark, fine-grained chlorite-actinolite metadiabase intrudes light granitic gneiss Actinolite is an inosilicate mineral with the chemical formula Ca2(MgFe)5Si8O22(OH)2 // Mineralogy Actinolite is an intermediate member in a series between tremolite (Mg-rich) and ferro-actinolite (Fe-rich). ... Quartzite Quartzite is a hard, metamorphic rock which was originally sandstone. ... Axinite is a blue, violet-blue, brownish or reddish-brown mineral composed of calcium aluminum boro-silicate, (Ca,Fe,Mn)3Al2BO3Si4O12OH. Axinite is pyroelectric and piezoelectric Categories: Mineral stubs | Silicate minerals ... Fluorite (also called fluor-spar) is a mineral composed of calcium fluoride, CaF2. ... A borosilicate of lime commonly occuring in glassy, greenish crystals. ...

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 a reas 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. Diabase is a mafic, holocrystalline, igneous rock equivalent to volcanic basalt or plutonic gabbro. ... Basalt Basalt is an extrusive igneous rock, sometimes porphyritic, and is often both fine-grained and dense. ... A sample of andesite (dark groundmass) with amygdaloidal vesicules filled with zeolite. ... Zeolite Zeolites (Greek, zein,to boil;lithos,a stone) are minerals that have a porous structure. ... Chlorite is a group of phyllosilicate minerals often classified as clays. ... (For other meanings of Porphyr, see Porphyry) The baptismal font in the Cathedral of Magdeburg is made of rose porphyry from a site near Assuan, Egypt Porphyry is a very hard red, green or purple igneous rock consisting of large-grained crystals, such as feldspar or quartz, dispersed in a...

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 hornfelses, 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 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). 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 forniit ion 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. Banded gneiss with dike of granite orthogneiss Gneiss is a common and widely distributed type of rock formed by high grade regional metamorphic processes from preexisting formations that were originally either igneous or sedimentary rocks. ... Pressure (symbol: p) is the force per unit area acting on a surface in a direction perpendicular to that surface. ...

See also

List of rocks Banded gneiss with dike of granite orthogneiss This page is intended as a list of all rock types. ...

References

• This article incorporates text from the 1911 Encyclopædia Britannica, a publication in the public domain.