Granulite - AbsoluteAstronomy.com

Granulites are medium to coarse–grained metamorphic rock

Metamorphic rock

Metamorphic rock is the transformation of an existing rock type, the protolith, in a process called metamorphism, which means "change in form". The protolith is subjected to heat and pressure causing profound physical and/or chemical change...

s that have experienced high temperature

Temperature

Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...

metamorphism, composed mainly of feldspar

Feldspar

Feldspars are a group of rock-forming tectosilicate minerals which make up as much as 60% of the Earth's crust....

s sometimes associated with quartz

Quartz

Quartz is the second-most-abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2. There are many different varieties of quartz,...

and anhydrous ferromagnesian minerals

Mafic

Mafic is an adjective describing a silicate mineral or rock that is rich in magnesium and iron; the term is a portmanteau of the words "magnesium" and "ferric". Most mafic minerals are dark in color and the relative density is greater than 3. Common rock-forming mafic minerals include olivine,...

, with granoblastic texture and gneissose to massive structure. They are of particular interest to geologist

Geologist

A geologist is a scientist who studies the solid and liquid matter that constitutes the Earth as well as the processes and history that has shaped it. Geologists usually engage in studying geology. Geologists, studying more of an applied science than a theoretical one, must approach Geology using...

s because many granulites represent samples of the deep continental crust

Continental crust

The continental crust is the layer of igneous, sedimentary, and metamorphic rocks which form the continents and the areas of shallow seabed close to their shores, known as continental shelves. This layer is sometimes called sial due to more felsic, or granitic, bulk composition, which lies in...

. Some granulites experienced decompression from deep in the Earth

Earth

Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...

to shallower crustal levels at high temperature; others cooled while remaining at depth in the Earth.

The minerals present in a granulite will vary depending on the parent rock of the granulite and the temperature and pressure

Pressure

Pressure is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.- Definition :...

conditions experienced during metamorphism. A common type of granulite found in high-grade metamorphic rocks of the continent

Continent

A continent is one of several very large landmasses on Earth. They are generally identified by convention rather than any strict criteria, with seven regions commonly regarded as continents—they are : Asia, Africa, North America, South America, Antarctica, Europe, and Australia.Plate tectonics is...

s contains pyroxene

Pyroxene

The pyroxenes are a group of important rock-forming inosilicate minerals found in many igneous and metamorphic rocks. They share a common structure consisting of single chains of silica tetrahedra and they crystallize in the monoclinic and orthorhombic systems...

, plagioclase

Plagioclase

Plagioclase is an important series of tectosilicate minerals within the feldspar family. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a solid solution series, more properly known as the plagioclase feldspar series...

feldspar and accessory garnet

Garnet

The garnet group includes a group of minerals that have been used since the Bronze Age as gemstones and abrasives. The name "garnet" may come from either the Middle English word gernet meaning 'dark red', or the Latin granatus , possibly a reference to the Punica granatum , a plant with red seeds...

, oxides and possibly amphibole

Amphibole

Amphibole is the name of an important group of generally dark-colored rock-forming inosilicate minerals, composed of double chain tetrahedra, linked at the vertices and generally containing ions of iron and/or magnesium in their structures.-Mineralogy:...

. Both clinopyroxene and orthopyroxene may be present, and in fact, the coexistence of clino- and orthopyroxene in a metamorphosed basalt

Basalt

Basalt is a common extrusive volcanic rock. It is usually grey to black and fine-grained due to rapid cooling of lava at the surface of a planet. It may be porphyritic containing larger crystals in a fine matrix, or vesicular, or frothy scoria. Unweathered basalt is black or grey...

defines the granulite facies.

Formation

Granulites form at high temperature conditions at a range of pressure conditions, typically during regional metamorphism. In some cases, the high temperatures are difficult to account for at the inferred depths at typical geothermal gradients. In extreme cases, granulites may form at temperatures in excess of 1000 degrees Celsius. Some granulites may represent the residues of partial melting, and in other cases represent rocks

Rock (geology)

In geology, rock or stone is a naturally occurring solid aggregate of minerals and/or mineraloids.The Earth's outer solid layer, the lithosphere, is made of rock. In general rocks are of three types, namely, igneous, sedimentary, and metamorphic...

that never melted, despite extremely high temperatures, because the minerals are anhydrous and therefore do not melt even at high temperature conditions.

In continental crustal rocks, biotite

Biotite

Biotite is a common phyllosilicate mineral within the mica group, with the approximate chemical formula . More generally, it refers to the dark mica series, primarily a solid-solution series between the iron-endmember annite, and the magnesium-endmember phlogopite; more aluminous endmembers...

may break down at high temperatures to form orthopyroxene + potassium feldspar + water

Water

Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...

, producing a granulite. Other possible minerals formed at dry conditions include sapphirine

Sapphirine

Sapphirine is a rare mineral, a silicate of magnesium and aluminium with the chemical formula 86O20 . Named for its sapphire-like colour, sapphirine is primarily of interest to researchers and collectors: well-formed crystals are treasured and occasionally cut into gemstones...

, spinel

Spinel

Spinel is the magnesium aluminium member of the larger spinel group of minerals. It has the formula MgAl2O4. Balas ruby is an old name for a rose-tinted variety.-Spinel group:...

, sillimanite

Sillimanite

Sillimanite is an alumino-silicate mineral with the chemical formula Al2SiO5. Sillimanite is named after the American chemist Benjamin Silliman . It was first described in 1824 for an occurrence in Chester, Middlesex County, Connecticut, USA....

, and osumilite

Osumilite

Osumilite a very rare hydrate potassium-sodium-iron-magnesium-aluminium silicate mineral. Osumilite is part of the milarite group of cyclosilicates.-Characteristics:...

. Some assemblages such as sapphirine + quartz

Quartz

indicate very high temperatures.

Granulite facies

The granulite facies is determined by lower temperature boundary of 700 +/- 50 °C and pressure range 5-15 kb. The most common mineral assemblage of granulite facies consists of antiperthitic plagioclase

Plagioclase

, alkali feldspar

Feldspar

Feldspars are a group of rock-forming tectosilicate minerals which make up as much as 60% of the Earth's crust....

containing up to 50% albite

Albite

Albite is a plagioclase feldspar mineral. It is the sodium endmember of the plagioclase solid solution series. As such it represents a plagioclase with less than 10% anorthite content. The pure albite endmember has the formula NaAlSi3O8. It is a tectosilicate. Its color is usually pure white, hence...

and Al₂O₃-rich pyroxene

Pyroxene

s.

Transition between amphibolite

Amphibolite

Amphibolite is the name given to a rock consisting mainly of hornblende amphibole, the use of the term being restricted, however, to metamorphic rocks. The modern terminology for a holocrystalline plutonic igneous rocks composed primarily of hornblende amphibole is a hornblendite, which are...

and granulite facies is defined by these reaction isograd

Isograd

In geology, an isograd is a plane of constant metamorphic grade in the field; it separates metamorphic zones of different metamorphic index minerals. On geologic maps focusing on metamorphic terranes , the boundaries between rocks of different metamorphic grade are commonly demarcated by isograd...

amphibole -> pyroxene + H₂O

biotite -> K-feldspar + garnet + orthopyroxene + H₂O.

Hornblende granulite subfacies is transitional coexistence region of anhydrous and hydrated ferromagnesian minerals, so above mentioned isograds mark the boundary with pyroxene granulite subfacies – facies with completely anhydrous mineral assemblages.

1911 Encyclopædia Britannica definition

Granulite (Latin

Latin

Latin is an Italic language originally spoken in Latium and Ancient Rome. It, along with most European languages, is a descendant of the ancient Proto-Indo-European language. Although it is considered a dead language, a number of scholars and members of the Christian clergy speak it fluently, and...

granulum, "a little grain") is a name used by petrographers to designate two distinct classes of rock

Rock (geology)

s. According to the terminology of the French

France

The French Republic , The French Republic , The French Republic , (commonly known as France , is a unitary semi-presidential republic in Western Europe with several overseas territories and islands located on other continents and in the Indian, Pacific, and Atlantic oceans. Metropolitan France...

school it signifies a granite

Granite

Granite is a common and widely occurring type of intrusive, felsic, igneous rock. Granite usually has a medium- to coarse-grained texture. Occasionally some individual crystals are larger than the groundmass, in which case the texture is known as porphyritic. A granitic rock with a porphyritic...

in which both kinds of mica

Mica

The mica group of sheet silicate minerals includes several closely related materials having highly perfect basal cleavage. All are monoclinic, with a tendency towards pseudohexagonal crystals, and are similar in chemical composition...

(muscovite and biotite) occur, and corresponds to the German

German language

German is a West Germanic language, related to and classified alongside English and Dutch. With an estimated 90 – 98 million native speakers, German is one of the world's major languages and is the most widely-spoken first language in the European Union....

Granit, or to the English

English language

English is a West Germanic language that arose in the Anglo-Saxon kingdoms of England and spread into what was to become south-east Scotland under the influence of the Anglian medieval kingdom of Northumbria...

muscovite biotite granite. This application has not been accepted generally. To the German petrologists granulite means a more or less banded fine-grained metamorphic rock

Metamorphic rock

, consisting mainly of quartz

Quartz

and feldspar

Feldspar

Feldspars are a group of rock-forming tectosilicate minerals which make up as much as 60% of the Earth's crust....

in very small irregular crystal

Crystal

A crystal or crystalline solid is a solid material whose constituent atoms, molecules, or ions are arranged in an orderly repeating pattern extending in all three spatial dimensions. The scientific study of crystals and crystal formation is known as crystallography...

s and usually also containing a fair number of minute, rounded, pale-red garnet

Garnet

s. Among English and American geologist

Geologist

s the term is generally employed in this sense.

The granulites are very closely allied to the gneiss

Gneiss

Gneiss is a common and widely distributed type of rock formed by high-grade regional metamorphic processes from pre-existing formations that were originally either igneous or sedimentary rocks.-Etymology:...

es, as they consist of nearly the same mineral

Mineral

A mineral is a naturally occurring solid chemical substance formed through biogeochemical processes, having characteristic chemical composition, highly ordered atomic structure, and specific physical properties. By comparison, a rock is an aggregate of minerals and/or mineraloids and does not...

s, but they are finer-grained, have usually less perfect foliation, are more frequently garnetiferous, and have some special features of microscopic structure. In the rocks of this group the minerals, as seen in a microscopic slide, occur as small rounded grains forming a closely fitted mosaic. The individual crystals never have perfect form, and indeed traces of it area rare. In some granulites they interlock, with irregular borders; in others they have been drawn out and flattened into tapering lenticles by crushing. In most cases they are somewhat rounded with smaller grains between the larger. This is especially true of the quartz and feldspar which are the predominant minerals; mica always appears as flat scales (irregular or rounded but not hexagonal). Both muscovite and biotite may be present and vary considerably in abundance; very commonly they have their flat sides parallel and give the rock a
rudimentary schist

Schist

The schists constitute a group of medium-grade metamorphic rocks, chiefly notable for the preponderance of lamellar minerals such as micas, chlorite, talc, hornblende, graphite, and others. Quartz often occurs in drawn-out grains to such an extent that a particular form called quartz schist is...

osity, and they may be aggregated into bands in which case the granulites are indistinguishable from certain varieties of gneiss. The garnets are very generally larger than the above-mentioned ingredients, and easily visible with the eye as pink spots on the broken surfaces of the rock. They usually are filled with enclosed grains of the other minerals.

The feldspar of the granulites is mostly orthoclase

Orthoclase

Orthoclase is an important tectosilicate mineral which forms igneous rock. The name is from the Greek for "straight fracture," because its two cleavage planes are at right angles to each other. Alternate names are alkali feldspar and potassium feldspar...

or cryptoperthite; microcline, oligoclase and albite

Albite

are also common. Basic feldspars occur only rarely. Among accessory minerals, in addition to apatite

Apatite

Apatite is a group of phosphate minerals, usually referring to hydroxylapatite, fluorapatite, chlorapatite and bromapatite, named for high concentrations of OH−, F−, Cl− or Br− ions, respectively, in the crystal...

, zircon

Zircon

Zircon is a mineral belonging to the group of nesosilicates. Its chemical name is zirconium silicate and its corresponding chemical formula is ZrSiO4. A common empirical formula showing some of the range of substitution in zircon is 1–x4x–y...

, and iron oxide

Iron oxide

Iron oxides are chemical compounds composed of iron and oxygen. All together, there are sixteen known iron oxides and oxyhydroxides.Iron oxides and oxide-hydroxides are widespread in nature, play an important role in many geological and biological processes, and are widely utilized by humans, e.g.,...

s, the following may be mentioned: hornblende

Hornblende

Hornblende is a complex inosilicate series of minerals .It is not a recognized mineral in its own right, but the name is used as a general or field term, to refer to a dark amphibole....

(not common), riebeckite

Riebeckite

Riebeckite is a sodium-rich member of the amphibole group of silicate minerals, chemical formula [][Na2][32][2|Si8O22]. It forms a series with magnesioriebeckite. It crystallizes in the monoclinic system, usually as long prismatic crystals showing a diamond-shaped cross section, but also in...

(rare), epidote

Epidote

Epidote is a calcium aluminium iron sorosilicate mineral, Ca2Al2O, crystallizing in the monoclinic system. Well-developed crystals are of frequent occurrence: they are commonly prismatic in habit, the direction of elongation being perpendicular to the single plane of symmetry. The faces are often...

and zoisite

Zoisite

Zoisite is a calcium aluminium hydroxy sorosilicate belonging to the epidote group of minerals. Its chemical formula is Ca2Al3O...

, calcite

Calcite

Calcite is a carbonate mineral and the most stable polymorph of calcium carbonate . The other polymorphs are the minerals aragonite and vaterite. Aragonite will change to calcite at 380-470°C, and vaterite is even less stable.-Properties:...

, sphene, andalusite

Andalusite

Andalusite is an aluminium nesosilicate mineral with the chemical formula Al2SiO5.The variety chiastolite commonly contains dark inclusions of carbon or clay which form a checker-board pattern when shown in cross-section....

, sillimanite

Sillimanite

, kyanite

Kyanite

Kyanite, whose name derives from the Greek word kuanos sometimes referred to as "kyanos", meaning deep blue, is a typically blue silicate mineral, commonly found in aluminium-rich metamorphic pegmatites and/or sedimentary rock. Kyanite in metamorphic rocks generally indicates pressures higher than...

, hercynite

Hercynite

Hercynite is a spinel mineral with the formula FeAl2O4.It occurs in high-grade metamorphosed iron rich argillaceous sediments as well as in mafic and ultramafic igneous rocks. Due to its hardness it also is found in placers....

(a green spinel), rutile

Rutile

Rutile is a mineral composed primarily of titanium dioxide, TiO2.Rutile is the most common natural form of TiO2. Two rarer polymorphs of TiO2 are known:...

, orthite and tourmaline

Tourmaline

Tourmaline is a crystal boron silicate mineral compounded with elements such as aluminium, iron, magnesium, sodium, lithium, or potassium. Tourmaline is classified as a semi-precious stone and the gem comes in a wide variety of colors...

. Though occasionally we may find larger grains of feldspar, quartz or epidote, it is more characteristic of these rocks that all the minerals are in small, nearly uniform, imperfectly shaped individuals.

On account of the minuteness with which it has been described and the important controversies on points of theoretical geology which have arisen regarding it, the granulite district of Saxony

Saxony

The Free State of Saxony is a landlocked state of Germany, contingent with Brandenburg, Saxony Anhalt, Thuringia, Bavaria, the Czech Republic and Poland. It is the tenth-largest German state in area, with of Germany's sixteen states....

(in the area of Rosswein and Penig) in Germany

Germany

Germany , officially the Federal Republic of Germany , is a federal parliamentary republic in Europe. The country consists of 16 states while the capital and largest city is Berlin. Germany covers an area of 357,021 km2 and has a largely temperate seasonal climate...

may be considered the typical region for rocks of this group. It should be remembered that though granulites are probably the commonest rocks of this country, they are mingled with granites, gneisses, gabbro

Gabbro

Gabbro refers to a large group of dark, coarse-grained, intrusive mafic igneous rocks chemically equivalent to basalt. The rocks are plutonic, formed when molten magma is trapped beneath the Earth's surface and cools into a crystalline mass....

s, amphibolite

Amphibolite

s, mica schists and many other petrographical types. All of these rocks show more-or-less metamorphism

Metamorphism

Metamorphism is the solid-state recrystallization of pre-existing rocks due to changes in physical and chemical conditions, primarily heat, pressure, and the introduction of chemically active fluids. Mineralogical, chemical and crystallographic changes can occur during this process...

either of a thermal character or due to pressure and crushing. The granites pass into gneiss and granulite; the gabbros into flaser gabbro and amphibolite; the slate

Slate

Slate is a fine-grained, foliated, homogeneous metamorphic rock derived from an original shale-type sedimentary rock composed of clay or volcanic ash through low-grade regional metamorphism. The result is a foliated rock in which the foliation may not correspond to the original sedimentary layering...

s often contain andalusite or chiastolite

Chiastolite

The mineral chiastolite is a variety of andalusite with the chemical composition Al2SiO5. It is noted for distinctive cross-shaped black inclusions of graphite. In areas around Georgetown, California, metamorphosed sediments contained andalusite and chiastolite in a graphite rich metasediment...

, and show transitions to mica schists. At one time these rocks were regarded as Archean gneisses of a special type. Johannes Georg Lehmann propounded the hypothesis that their present state was due principally to crushing
acting on them in a solid condition, grinding them down and breaking up their minerals, while the pressure to which they were subjected welded them together into coherent rock. It is now believed, however, that they are comparatively recent and include sedimentary rock

Sedimentary rock

Sedimentary rock are types of rock that are formed by the deposition of material at the Earth's surface and within bodies of water. Sedimentation is the collective name for processes that cause mineral and/or organic particles to settle and accumulate or minerals to precipitate from a solution....

s, partly of Palaeozoic age, and intrusive masses which may be nearly massive or may have gneissose, flaser or granulitic structures. These have been developed largely by the injection of semi-consolidated highly viscous

Viscosity

Viscosity is a measure of the resistance of a fluid which is being deformed by either shear or tensile stress. In everyday terms , viscosity is "thickness" or "internal friction". Thus, water is "thin", having a lower viscosity, while honey is "thick", having a higher viscosity...

intrusions, and the varieties of texture are original or were produced very shortly after the crystallization of the rocks. Meanwhile, however, Lehmanns advocacy of post-consolidation crushing as a factor in the development of granulites has been so successful that the terms granulitization and granulitic structures are widely employed to indicate the results of dynamometamorphism acting on rocks at a period long after their solidification.

The Saxon granulites are apparently for the most part igneous and correspond in composition to granites and porphyries

Porphyry (geology)

Porphyry is a variety of igneous rock consisting of large-grained crystals, such as feldspar or quartz, dispersed in a fine-grained feldspathic matrix or groundmass. The larger crystals are called phenocrysts...

. There are, however, many granulites which undoubtedly were originally sediment

Sediment

Sediment is naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of fluids such as wind, water, or ice, and/or by the force of gravity acting on the particle itself....

s (arkose

Arkose

Arkose is a detrital sedimentary rock, specifically a type of sandstone containing at least 25% feldspar. Arkosic sand is sand that is similarly rich in feldspar, and thus the potential precursor of arkose....

s, grits and sandstone

Sandstone

Sandstone is a sedimentary rock composed mainly of sand-sized minerals or rock grains.Most sandstone is composed of quartz and/or feldspar because these are the most common minerals in the Earth's crust. Like sand, sandstone may be any colour, but the most common colours are tan, brown, yellow,...

s). A large part of the highlands of Scotland

Scotland

Scotland is a country that is part of the United Kingdom. Occupying the northern third of the island of Great Britain, it shares a border with England to the south and is bounded by the North Sea to the east, the Atlantic Ocean to the north and west, and the North Channel and Irish Sea to the...

consists of paragranulites of this kind, which have received the group name of Moine gneisses.

Along with the typical acid

Acid

An acid is a substance which reacts with a base. Commonly, acids can be identified as tasting sour, reacting with metals such as calcium, and bases like sodium carbonate. Aqueous acids have a pH of less than 7, where an acid of lower pH is typically stronger, and turn blue litmus paper red...

granulites above described, in Saxony, India

India

India , officially the Republic of India , is a country in South Asia. It is the seventh-largest country by geographical area, the second-most populous country with over 1.2 billion people, and the most populous democracy in the world...

, Scotland and other countries there occur dark-colored basic granulites (trap granulites). These are fine-grained rocks, not usually banded, nearly black in color with small red spots of garnet. Their essential minerals are pyroxene, plagioclase and garnet: chemically they resemble the gabbros. Green augite and hypersthene

Hypersthene

Hypersthene is a common rock-forming inosilicate mineral belonging to the group of orthorhombic pyroxenes. Many references have formally abandoned this term, preferring to categorise this mineral as enstatite or ferrosilite. It is found in igneous and some metamorphic rocks as well as in stony and...

form a considerable part of these rocks, they may contain also biotite, hornblende and quartz. Around the garnets there is often a radial grouping of small grains of pyroxene and hornblende in a clear matrix of feldspar: these centric structures are frequent in granulites. The rocks of this group accompany gabbro and serpentine, but the exact conditions under which they are formed and the significance of their structures is not very clearly understood.

Formation

Granulite facies

1911 Encyclopædia Britannica definition

See also