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68-412: Members of the biotite group are sheet silicates . Iron , magnesium , aluminium , silicon , oxygen , and hydrogen form sheets that are weakly bound together by potassium ions . The term "iron mica" is sometimes used for iron-rich biotite, but the term also refers to a flaky micaceous form of haematite , and the field term Lepidomelane for unanalysed iron-rich Biotite avoids this ambiguity. Biotite

136-427: A pseudohexagonal crystal. Although not easily seen because of the cleavage and sheets, fracture is uneven. It appears greenish to brown or black, and even yellow when weathered . It can be transparent to opaque, has a vitreous to pearly luster , and a grey-white streak . When biotite crystals are found in large chunks, they are called "books" because they resemble books with pages of many sheets. The color of biotite

204-486: A change in composition. Solidification into rock occurs either below the surface as intrusive rocks or on the surface as extrusive rocks. Igneous rock may form with crystallization to form granular, crystalline rocks, or without crystallization to form natural glasses . Igneous rocks occur in a wide range of geological settings: shields, platforms, orogens, basins, large igneous provinces, extended crust and oceanic crust. Igneous and metamorphic rocks make up 90–95% of

272-459: A combination of these processes. Other mechanisms, such as melting from a meteorite impact , are less important today, but impacts during the accretion of the Earth led to extensive melting, and the outer several hundred kilometres of our early Earth was probably an ocean of magma. Impacts of large meteorites in the last few hundred million years have been proposed as one mechanism responsible for

340-405: A crystalline basement formed of a great variety of metamorphic and igneous rocks, including granulite and granite. Oceanic crust is composed primarily of basalt and gabbro . Both continental and oceanic crust rest on peridotite of the mantle. Rocks may melt in response to a decrease in pressure, to a change in composition (such as an addition of water), to an increase in temperature, or to

408-399: A hexagonal sheet. The remaining oxygen ion (the apical oxygen ion) is available to bond with the octahedral sheet. The octahedral sheet in biotite is a trioctahedral sheet having the structure of a sheet of the mineral brucite , with magnesium or ferrous iron being the usual cations. Apical oxygens take the place of some of the hydroxyl ions that would be present in a brucite sheet, bonding

476-527: A huge mass of analytical data—over 230,000 rock analyses can be accessed on the web through a site sponsored by the U. S. National Science Foundation (see the External Link to EarthChem). The single most important component is silica, SiO 2 , whether occurring as quartz or combined with other oxides as feldspars or other minerals. Both intrusive and volcanic rocks are grouped chemically by total silica content into broad categories. This classification

544-417: A major constituent of deep ocean sediment , and of diatomaceous earth . A silicate mineral is generally an inorganic compound consisting of subunits with the formula [SiO 2+ n ] . Although depicted as such, the description of silicates as anions is a simplification. Balancing the charges of the silicate anions are metal cations, M . Typical cations are Mg , Fe , and Na . The Si-O-M linkage between

612-653: A microscope for fine-grained volcanic rock, and may be impossible for glassy volcanic rock. The rock must then be classified chemically. Mineralogical classification of an intrusive rock begins by determining if the rock is ultramafic, a carbonatite, or a lamprophyre . An ultramafic rock contains more than 90% of iron- and magnesium-rich minerals such as hornblende, pyroxene, or olivine, and such rocks have their own classification scheme. Likewise, rocks containing more than 50% carbonate minerals are classified as carbonatites, while lamprophyres are rare ultrapotassic rocks. Both are further classified based on detailed mineralogy. In

680-438: A pale to deep greenish brown or brown color, with moderate to strong pleochroism . Biotite has a high birefringence which can be partially masked by its deep intrinsic color. Under cross-polarized light , biotite exhibits extinction approximately parallel to cleavage lines, and can have characteristic bird's eye maple extinction , a mottled appearance caused by the distortion of the mineral's flexible lamellae during grinding of

748-455: A result of the processes that have been forming and re-working the crust for billions of years. These processes include partial melting , crystallization , fractionation , metamorphism , weathering , and diagenesis . Living organisms also contribute to this geologic cycle . For example, a type of plankton known as diatoms construct their exoskeletons ("frustules") from silica extracted from seawater . The frustules of dead diatoms are

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816-420: A shared oxygen vertex—a silicon:oxygen ratio of 2:7. The Nickel–Strunz classification is 09.B. Examples include: Cyclosilicates (from Greek κύκλος kýklos 'circle'), or ring silicates, have three or more tetrahedra linked in a ring. The general formula is (Si x O 3 x ) , where one or more silicon atoms can be replaced by other 4-coordinated atom(s). The silicon:oxygen ratio is 1:3. Double rings have

884-553: A simplified compositional classification, igneous rock types are categorized into felsic or mafic based on the abundance of silicate minerals in the Bowen's Series. Rocks dominated by quartz, plagioclase, alkali feldspar and muscovite are felsic. Mafic rocks are primarily composed of biotite, hornblende, pyroxene and olivine. Generally, felsic rocks are light colored and mafic rocks are darker colored. For textural classification, igneous rocks that have crystals large enough to be seen by

952-514: A three-dimensional framework of silicate tetrahedra with SiO 2 in a 1:2 ratio. This group comprises nearly 75% of the crust of the Earth . Tectosilicates, with the exception of the quartz group, are aluminosilicates . The Nickel–Strunz classifications are 09.F and 09.G, 04.DA (Quartz/ silica family). Examples include: Igneous rock Igneous rock ( igneous from Latin igneus  'fiery'), or magmatic rock ,

1020-462: A viscosity similar to thick, cold molasses or even rubber when erupted. Felsic magma, such as rhyolite , is usually erupted at low temperature and is up to 10,000 times as viscous as basalt. Volcanoes with rhyolitic magma commonly erupt explosively, and rhyolitic lava flows are typically of limited extent and have steep margins because the magma is so viscous. Felsic and intermediate magmas that erupt often do so violently, with explosions driven by

1088-401: Is 09.D – examples include: Phyllosilicates (from Greek φύλλον phýllon 'leaf'), or sheet silicates, form parallel sheets of silicate tetrahedra with Si 2 O 5 or a 2:5 ratio. The Nickel–Strunz classification is 09.E. All phyllosilicate minerals are hydrated , with either water or hydroxyl groups attached. Examples include: Tectosilicates, or "framework silicates," have

1156-470: Is also sometimes called "black mica" as opposed to "white mica" ( muscovite ) – both form in the same rocks , and in some instances side by side. Like other mica minerals, biotite has a highly perfect basal cleavage , and consists of flexible sheets, or lamellae , which easily flake off. It has a monoclinic crystal system , with tabular to prismatic crystals with an obvious pinacoid termination. It has four prism faces and two pinacoid faces to form

1224-584: Is an example. The molten rock, which typically contains suspended crystals and dissolved gases, is called magma . It rises because it is less dense than the rock from which it was extracted. When magma reaches the surface, it is called lava . Eruptions of volcanoes into air are termed subaerial , whereas those occurring underneath the ocean are termed submarine . Black smokers and mid-ocean ridge basalt are examples of submarine volcanic activity. The volume of extrusive rock erupted annually by volcanoes varies with plate tectonic setting. Extrusive rock

1292-728: Is distinguishable from the other two on the TAS diagram, being higher in total alkali oxides for a given silica content, but the tholeiitic and calc-alkaline series occupy approximately the same part of the TAS diagram. They are distinguished by comparing total alkali with iron and magnesium content. These three magma series occur in a range of plate tectonic settings. Tholeiitic magma series rocks are found, for example, at mid-ocean ridges, back-arc basins , oceanic islands formed by hotspots, island arcs and continental large igneous provinces . All three series are found in relatively close proximity to each other at subduction zones where their distribution

1360-595: Is expressed differently for major and minor elements and for trace elements. Contents of major and minor elements are conventionally expressed as weight percent oxides (e.g., 51% SiO 2 , and 1.50% TiO 2 ). Abundances of trace elements are conventionally expressed as parts per million by weight (e.g., 420 ppm Ni, and 5.1 ppm Sm). The term "trace element" is typically used for elements present in most rocks at abundances less than 100 ppm or so, but some trace elements may be present in some rocks at abundances exceeding 1,000 ppm. The diversity of rock compositions has been defined by

1428-506: Is formed by the cooling of molten magma on the earth's surface. The magma, which is brought to the surface through fissures or volcanic eruptions , rapidly solidifies. Hence such rocks are fine-grained ( aphanitic ) or even glassy. Basalt is the most common extrusive igneous rock and forms lava flows, lava sheets and lava plateaus. Some kinds of basalt solidify to form long polygonal columns . The Giant's Causeway in Antrim, Northern Ireland

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1496-436: Is most often used to classify plutonic rocks. Chemical classifications are preferred to classify volcanic rocks, with phenocryst species used as a prefix, e.g. "olivine-bearing picrite" or "orthoclase-phyric rhyolite". The IUGS recommends classifying igneous rocks by their mineral composition whenever possible. This is straightforward for coarse-grained intrusive igneous rock, but may require examination of thin sections under

1564-486: Is occasionally found in large cleavable crystals, especially in pegmatite veins, as in New England , Virginia and North Carolina USA. Other notable occurrences include Bancroft and Sudbury , Ontario Canada. It is an essential constituent of many metamorphic schists , and it forms in suitable compositions over a wide range of pressure and temperature . It has been estimated that biotite comprises up to 7% of

1632-409: Is one of the three main rock types , the others being sedimentary and metamorphic . Igneous rocks are formed through the cooling and solidification of magma or lava . The magma can be derived from partial melts of existing rocks in either a planet 's mantle or crust . Typically, the melting is caused by one or more of three processes: an increase in temperature, a decrease in pressure , or

1700-496: Is produced in the following proportions: The behaviour of lava depends upon its viscosity , which is determined by temperature, composition, and crystal content. High-temperature magma, most of which is basaltic in composition, behaves in a manner similar to thick oil and, as it cools, treacle . Long, thin basalt flows with pahoehoe surfaces are common. Intermediate composition magma, such as andesite , tends to form cinder cones of intermingled ash , tuff and lava, and may have

1768-402: Is related to depth and the age of the subduction zone. The tholeiitic magma series is well represented above young subduction zones formed by magma from relatively shallow depth. The calc-alkaline and alkaline series are seen in mature subduction zones, and are related to magma of greater depths. Andesite and basaltic andesite are the most abundant volcanic rock in island arc which is indicative of

1836-487: Is summarized in the following table: The percentage of alkali metal oxides ( Na 2 O plus K 2 O ) is second only to silica in its importance for chemically classifying volcanic rock. The silica and alkali metal oxide percentages are used to place volcanic rock on the TAS diagram , which is sufficient to immediately classify most volcanic rocks. Rocks in some fields, such as the trachyandesite field, are further classified by

1904-482: Is usually black and the mineral has a hardness of 2.5–3 on the Mohs scale of mineral hardness . Biotite dissolves in both acid and alkaline aqueous solutions , with the highest dissolution rates at low pH . However, biotite dissolution is highly anisotropic with crystal edge surfaces ( h k 0 ) reacting 45 to 132 times faster than basal surfaces ( 001 ). In thin section , biotite exhibits moderate relief and

1972-479: The IUGS , this is often impractical, and chemical classification is done instead using the TAS classification . Igneous rocks are classified according to mode of occurrence, texture, mineralogy, chemical composition, and the geometry of the igneous body. The classification of the many types of igneous rocks can provide important information about the conditions under which they formed. Two important variables used for

2040-453: The convection of solid mantle, it will cool slightly as it expands in an adiabatic process , but the cooling is only about 0.3 °C per kilometre. Experimental studies of appropriate peridotite samples document that the solidus temperatures increase by 3 °C to 4 °C per kilometre. If the rock rises far enough, it will begin to melt. Melt droplets can coalesce into larger volumes and be intruded upwards. This process of melting from

2108-473: The 1640s and is derived either from French granit or Italian granito , meaning simply "granulate rock". The term rhyolite was introduced in 1860 by the German traveler and geologist Ferdinand von Richthofen The naming of new rock types accelerated in the 19th century and peaked in the early 20th century. Much of the early classification of igneous rocks was based on the geological age and occurrence of

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2176-434: The 1960s. However, the concept of normative mineralogy has endured, and the work of Cross and his coinvestigators inspired a flurry of new classification schemes. Among these was the classification scheme of M.A. Peacock, which divided igneous rocks into four series: the alkalic, the alkali-calcic, the calc-alkali, and the calcic series. His definition of the alkali series, and the term calc-alkali, continue in use as part of

2244-520: The Earth's surface. Intrusive igneous rocks that form at depth within the crust are termed plutonic (or abyssal ) rocks and are usually coarse-grained. Intrusive igneous rocks that form near the surface are termed subvolcanic or hypabyssal rocks and they are usually much finer-grained, often resembling volcanic rock. Hypabyssal rocks are less common than plutonic or volcanic rocks and often form dikes, sills, laccoliths, lopoliths , or phacoliths . Extrusive igneous rock, also known as volcanic rock,

2312-453: The TOT layer is neutralized by the interlayer potassium ions. Because the hexagons in the T and O sheets are slightly different in size, the sheets are slightly distorted when they bond into a TOT layer. This breaks the hexagonal symmetry and reduces it to monoclinic symmetry. However, the original hexahedral symmetry is discernible in the pseudohexagonal character of biotite crystals. Members of

2380-456: The anion would be just neutral silica [SiO 2 ] n . Replacement of one in every four silicon atoms by an aluminum atom results in the anion [AlSi 3 O 8 ] n , whose charge is neutralized by the potassium cations K . In mineralogy , silicate minerals are classified into seven major groups according to the structure of their silicate anion: Tectosilicates can only have additional cations if some of

2448-450: The basic TAS classification include: In older terminology, silica oversaturated rocks were called silicic or acidic where the SiO 2 was greater than 66% and the family term quartzolite was applied to the most silicic. A normative feldspathoid classifies a rock as silica-undersaturated; an example is nephelinite . Magmas are further divided into three series: The alkaline series

2516-417: The biotite crystal structure at high temperatures, these methods may provide only minimum ages for many rocks. Biotite is also useful in assessing temperature histories of metamorphic rocks, because the partitioning of iron and magnesium between biotite and garnet is sensitive to temperature. Sheet silicate Silicate minerals are rock-forming minerals made up of silicate groups. They are

2584-463: The biotite group are found in a wide variety of igneous and metamorphic rocks . For instance, biotite occurs in the lava of Mount Vesuvius and in the Monzoni intrusive complex of the western Dolomites . Biotite in granite tends to be poorer in magnesium than the biotite found in its volcanic equivalent, rhyolite . Biotite is an essential phenocryst in some varieties of lamprophyre . Biotite

2652-568: The calc-alkaline magmas. Some island arcs have distributed volcanic series as can be seen in the Japanese island arc system where the volcanic rocks change from tholeiite—calc-alkaline—alkaline with increasing distance from the trench. Some igneous rock names date to before the modern era of geology. For example, basalt as a description of a particular composition of lava-derived rock dates to Georgius Agricola in 1546 in his work De Natura Fossilium . The word granite goes back at least to

2720-429: The chemical composition of an igneous rock was its most fundamental characteristic, it should be elevated to prime position. Geological occurrence, structure, mineralogical constitution—the hitherto accepted criteria for the discrimination of rock species—were relegated to the background. The completed rock analysis is first to be interpreted in terms of the rock-forming minerals which might be expected to be formed when

2788-625: The classification of igneous rocks are particle size, which largely depends on the cooling history, and the mineral composition of the rock. Feldspars , quartz or feldspathoids , olivines , pyroxenes , amphiboles , and micas are all important minerals in the formation of almost all igneous rocks, and they are basic to the classification of these rocks. All other minerals present are regarded as nonessential in almost all igneous rocks and are called accessory minerals . Types of igneous rocks with other essential minerals are very rare, but include carbonatites , which contain essential carbonates . In

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2856-405: The crust of a planet. Bodies of intrusive rock are known as intrusions and are surrounded by pre-existing rock (called country rock ). The country rock is an excellent thermal insulator , so the magma cools slowly, and intrusive rocks are coarse-grained ( phaneritic ). The mineral grains in such rocks can generally be identified with the naked eye. Intrusions can be classified according to

2924-402: The different types of extrusive igneous rocks than between different types of intrusive igneous rocks. Generally, the mineral constituents of fine-grained extrusive igneous rocks can only be determined by examination of thin sections of the rock under a microscope , so only an approximate classification can usually be made in the field . Although classification by mineral makeup is preferred by

2992-585: The exposed continental crust. An igneous rock composed almost entirely of dark mica (biotite or phlogopite) is known as a glimmerite or biotitite . Biotite may be found in association with its common alteration product chlorite . The largest documented single crystals of biotite were approximately 7 m (75 sq ft) sheets found in Iveland , Norway. Biotite is used extensively to constrain ages of rocks, by either potassium-argon dating or argon–argon dating . Because argon escapes readily from

3060-433: The extensive basalt magmatism of several large igneous provinces. Decompression melting occurs because of a decrease in pressure. The solidus temperatures of most rocks (the temperatures below which they are completely solid) increase with increasing pressure in the absence of water. Peridotite at depth in the Earth's mantle may be hotter than its solidus temperature at some shallower level. If such rock rises during

3128-585: The formula (Si 2 x O 5 x ) or a 2:5 ratio. The Nickel–Strunz classification is 09.C. Possible ring sizes include: Some example minerals are: The ring in axinite contains two B and four Si tetrahedra and is highly distorted compared to the other 6-member ring cyclosilicates. Inosilicates (from Greek ἴς is [genitive: ἰνός inos ] 'fibre'), or chain silicates, have interlocking chains of silicate tetrahedra with either SiO 3 , 1:3 ratio, for single chains or Si 4 O 11 , 4:11 ratio, for double chains. The Nickel–Strunz classification

3196-486: The great majority of cases, the rock has a more typical mineral composition, with significant quartz, feldspars, or feldspathoids. Classification is based on the percentages of quartz, alkali feldspar, plagioclase, and feldspathoid out of the total fraction of the rock composed of these minerals, ignoring all other minerals present. These percentages place the rock somewhere on the QAPF diagram , which often immediately determines

3264-433: The larger crystals, called phenocrysts, grow to considerable size before the main mass of the magma crystallizes as finer-grained, uniform material called groundmass. Grain size in igneous rocks results from cooling time so porphyritic rocks are created when the magma has two distinct phases of cooling. Igneous rocks are classified on the basis of texture and composition. Texture refers to the size, shape, and arrangement of

3332-409: The largest and most important class of minerals and make up approximately 90 percent of Earth's crust . In mineralogy , silica (silicon dioxide, SiO 2 ) is usually considered a silicate mineral rather than an oxide mineral . Silica is found in nature as the mineral quartz , and its polymorphs . On Earth, a wide variety of silicate minerals occur in an even wider range of combinations as

3400-399: The magma crystallizes, e.g., quartz feldspars, olivine , akermannite, Feldspathoids , magnetite , corundum , and so on, and the rocks are divided into groups strictly according to the relative proportion of these minerals to one another. This new classification scheme created a sensation, but was criticized for its lack of utility in fieldwork, and the classification scheme was abandoned by

3468-457: The majority of minerals will be visible to the naked eye or at least using a hand lens, magnifying glass or microscope. Plutonic rocks also tend to be less texturally varied and less prone to showing distinctive structural fabrics. Textural terms can be used to differentiate different intrusive phases of large plutons, for instance porphyritic margins to large intrusive bodies, porphyry stocks and subvolcanic dikes . Mineralogical classification

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3536-697: The mineral grains or crystals of which the rock is composed. Texture is an important criterion for the naming of volcanic rocks. The texture of volcanic rocks, including the size, shape, orientation, and distribution of mineral grains and the intergrain relationships, will determine whether the rock is termed a tuff , a pyroclastic lava or a simple lava . However, the texture is only a subordinate part of classifying volcanic rocks, as most often there needs to be chemical information gleaned from rocks with extremely fine-grained groundmass or from airfall tuffs, which may be formed from volcanic ash. Textural criteria are less critical in classifying intrusive rocks where

3604-415: The naked eye are called phaneritic ; those with crystals too small to be seen are called aphanitic . Generally speaking, phaneritic implies an intrusive origin or plutonic, indicating slow cooling; aphanitic are extrusive or volcanic, indicating rapid cooling. An igneous rock with larger, clearly discernible crystals embedded in a finer-grained matrix is termed porphyry . Porphyritic texture develops when

3672-510: The oxide has a coordination number of two. Some silicon centers may be replaced by atoms of other elements, still bound to the four corner oxygen corners. If the substituted atom is not normally tetravalent, it usually contributes extra charge to the anion, which then requires extra cations . For example, in the mineral orthoclase [KAlSi 3 O 8 ] n , the anion is a tridimensional network of tetrahedra in which all oxygen corners are shared. If all tetrahedra had silicon centers,

3740-441: The ratio of potassium to sodium (so that potassic trachyandesites are latites and sodic trachyandesites are benmoreites). Some of the more mafic fields are further subdivided or defined by normative mineralogy , in which an idealized mineral composition is calculated for the rock based on its chemical composition. For example, basanite is distinguished from tephrite by having a high normative olivine content. Other refinements to

3808-403: The release of dissolved gases—typically water vapour, but also carbon dioxide . Explosively erupted pyroclastic material is called tephra and includes tuff , agglomerate and ignimbrite . Fine volcanic ash is also erupted and forms ash tuff deposits, which can often cover vast areas. Because volcanic rocks are mostly fine-grained or glassy, it is much more difficult to distinguish between

3876-473: The rock must be classified chemically. There are relatively few minerals that are important in the formation of common igneous rocks, because the magma from which the minerals crystallize is rich in only certain elements: silicon , oxygen , aluminium, sodium , potassium , calcium , iron, and magnesium . These are the elements that combine to form the silicate minerals , which account for over ninety percent of all igneous rocks. The chemistry of igneous rocks

3944-424: The rock type. In a few cases, such as the diorite-gabbro-anorthite field, additional mineralogical criteria must be applied to determine the final classification. Where the mineralogy of an volcanic rock can be determined, it is classified using the same procedure, but with a modified QAPF diagram whose fields correspond to volcanic rock types. When it is impractical to classify a volcanic rock by mineralogy,

4012-467: The rocks. However, in 1902, the American petrologists Charles Whitman Cross , Joseph P. Iddings , Louis V. Pirsson , and Henry Stephens Washington proposed that all existing classifications of igneous rocks should be discarded and replaced by a "quantitative" classification based on chemical analysis. They showed how vague, and often unscientific, much of the existing terminology was and argued that as

4080-435: The shape and size of the intrusive body and its relation to the bedding of the country rock into which it intrudes. Typical intrusive bodies are batholiths , stocks , laccoliths , sills and dikes . Common intrusive rocks are granite , gabbro , or diorite . The central cores of major mountain ranges consist of intrusive igneous rocks. When exposed by erosion, these cores (called batholiths ) may occupy huge areas of

4148-441: The silicates and the metals are strong, polar-covalent bonds. Silicate anions ([SiO 2+ n ] ) are invariably colorless, or when crushed to a fine powder, white. The colors of silicate minerals arise from the metal component, commonly iron. In most silicate minerals, silicon is tetrahedral, being surrounded by four oxides. The coordination number of the oxides is variable except when it bridges two silicon centers, in which case

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4216-541: The silicon is replaced by an atom of lower valence such as aluminum. Al for Si substitution is common. Nesosilicates (from Greek νῆσος nēsos 'island'), or orthosilicates, have the orthosilicate ion , present as isolated (insular) [SiO 4 ] tetrahedra connected only by interstitial cations . The Nickel–Strunz classification is 09.A –examples include: Sorosilicates (from Greek σωρός sōros 'heap, mound') have isolated pyrosilicate anions Si 2 O 7 , consisting of double tetrahedra with

4284-467: The tetrahedral sheets tightly to the octahedral sheet. Tetrahedral sheets have a strong negative charge, since their bulk composition is AlSi 3 O 10 . The trioctahedral sheet has a positive charge, since its bulk composition is M 3 (OH) 2 (M represents a divalent ion such as ferrous iron or magnesium) The combined TOT layer has a residual negative charge, since its bulk composition is M 3 (AlSi 3 O 10 )(OH) 2 . The remaining negative charge of

4352-425: The thin section. Basal sections of biotite in thin section are typically approximately hexagonal in shape and usually appear isotropic under cross-polarized light. Like other micas, biotite has a crystal structure described as TOT-c , meaning that it is composed of parallel TOT layers weakly bonded to each other by cations ( c ). The TOT layers in turn consist of two tetrahedral sheets ( T ) strongly bonded to

4420-475: The top 16 kilometres (9.9 mi) of the Earth's crust by volume. Igneous rocks form about 15% of the Earth's current land surface. Most of the Earth's oceanic crust is made of igneous rock. Igneous rocks are also geologically important because: Igneous rocks can be either intrusive ( plutonic and hypabyssal) or extrusive ( volcanic ). Intrusive igneous rocks make up the majority of igneous rocks and are formed from magma that cools and solidifies within

4488-434: The two faces of a single octahedral sheet ( O ). It is the relatively weak ionic bonding between TOT layers that gives biotite its perfect basal cleavage. The tetrahedral sheets consist of silica tetrahedra, which are silicon ions surrounded by four oxygen ions. In biotite, one in four silicon ions is replaced by an aluminium ion. The tetrahedra each share three of their four oxygen ions with neighboring tetrahedra to produce

4556-586: The widely used Irvine-Barager classification, along with W.Q. Kennedy's tholeiitic series. By 1958, there were some 12 separate classification schemes and at least 1637 rock type names in use. In that year, Albert Streckeisen wrote a review article on igneous rock classification that ultimately led to the formation of the IUGG Subcommission of the Systematics of Igneous Rocks. By 1989 a single system of classification had been agreed upon, which

4624-467: Was further revised in 2005. The number of recommended rock names was reduced to 316. These included a number of new names promulgated by the Subcommission. The Earth's crust averages about 35 kilometres (22 mi) thick under the continents , but averages only some 7–10 kilometres (4.3–6.2 mi) beneath the oceans . The continental crust is composed primarily of sedimentary rocks resting on

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