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chemical composition of igneous rocks

January 16, 2021

Mafic rocks are dominated by plagioclase and pyroxene (even if you can't see them with the naked eye) and smaller amounts of olivine. This cooling determines the chemical composition and structure of the rock. Get a Britannica Premium subscription and gain access to exclusive content. There are two major types of igneous rocks: Extrusive, fine grained, and intrusive, fine grained. The top 20–25 km of the Earth's crust consists of approximately 95% igneous rocks or their metamorphic equivalents, 4% shale, 0.75% sandstone, and 0.25% limestone. It may settle within the crust or erupt at the surface from a volcano as a lava flow. Average chemical composition of granitic and basaltic rocks based on 2485 and 3594 analysed rock samples, respectively3: Numbers given in the table above are weight percents. Igneous Rock Composition Igneous rocks are commonly classified by their composition and texture.Most are composed of the eight most abundant elements in the Earth's crust. The mineral grains in such rocks can generally be recognized with the bare eye. The light-colored silicates include quartz, muscovite and feldspar. These silicates can be generally divided into light and dark silicates. Solidification from magma produces great diversity in the mineral compositions which make up the rocks. How do composition and texture relate to igneous rocks? Because of the limited occurrence of such carbonate-rich igneous rocks, however, the following discussion will consider the chemistry of silicate rocks only. It is evident that SiO 2 and Al 2 … 3.4 Classification of Igneous Rocks As has already been described, igneous rocks are classified into four categories, based on either their chemistry or their mineral composition: felsic, intermediate, mafic, and ultramafic. Chemical composition of igneous rocks is the most distinguishing feature. The major oxides of the rocks generally correlate well with their silica content: those rocks with low silica content are enriched in magnesium oxide (MgO) and iron oxides (FeO, Fe2O3, and Fe3O4) and are depleted in soda (Na2O) and potash (K2O); those with a large amount of silica are depleted in magnesium oxide and iron oxides but are enriched in soda and potash. The formation of igneous rocks can take two routes: fast or slow. The study of the igneous rocks has hitherto largely consisted in an analysis of their mineralogical and chemical composition, with the special intent to produce a satisfactory nomenclature and classification of the rocks as they occur throughout the world. The compositions of metamorphic rocks are generally similar to the compositions of the rocks that were metamorphosed, and only igneous and sedimentary rock compositions are considered here. They are all connected in the endless rock cycle, moving from one form to another and changing shape, texture, and even chemical composition along the way. Rocks formed from the cooling and solidification of magma deep within the crust are distinct from those erupted at the surface mainly owing to the differences in physical and chemical conditions prevalent in the two environments. How solid is your knowledge of all things geological? Igneous rocks are classified according to mode of occurrence, texture, mineralogy, chemical composition, and the geometry of the igneous body. The range of chemical compositions of igneous rocks reflects the average bulk composition of the crust. When tectonic forces thrust sedimentary and metamorphic rocks into the hot mantle, they may melt and be ejected as magma, which cools to form igneous, or magmatic, rock. Most are composed of the eight most abundant elements in the Earth's crust. They include olivine, pyroxene, amphibole and biotite. Other important oxides are alumina (Al2O3), magnesia (MgO), lime (CaO), soda (Na2O), and potash (K2O). The chemical composition of an igneous rock tells us about the origin of the magma, beginning with which type of rock melted within the earth to form the magma in the first place, and how deep in the earth the melting occurred. The various igneous textures result mainly from the different cooling histories, whereas the mineral composition of an igneous rock is the result of the chemical makeup of the parent magma. Indeed, in 1960 a sodium carbonate (Na2CO3) lava with only 0.05 weight percent silica (SiO2) was erupted from Ol Doinyo Lengai, a volcano in northern Tanzania, Africa. Texture refers to the size and arrangement of the minerals or grains that make up a rock. Composition refers to both the types of minerals within a rock and the overall chemical makeup of the rock (the two are obviously related). This model of the process of solidification from magma pictures the processes which causes the composition of the magma and the subsequent rocks to change. Eight elements make up about 98% by weight of most magmas from which igneous rocks are made. The slow cooling promotes the growth of minerals large enough to be identified visually without the aid of a microscope (called phaneritic, from the Greek phaneros, meaning “visible”). These three minerals substitute in part for enstatite, albite (NaAlSi3O8, requiring three moles of silicon for one mole of sodium), and orthoclase feldspar (KAlSi3O8, requiring three moles of silicon for one mole of potassium), respectively. Composition Chemical components. Igneous rocks are classified according to their texture and composition. These groups refer to differing amounts of silica, iron, and magnesium found in the minerals that make up the rocks. The deep-seated plutonic rocks can be exposed at the surface for study only after a long period of denudation or by some tectonic forces that push the crust upward or by a combination of the two conditions. Because of the high temperatures within Earth, the principles of chemical equilibrium are applicable to the study of igneous and metamorphic rocks, with the latter being restricted to those rocks formed without the direct involvement of magma. Alumina in rocks that contain more than 45 percent silica is generally above approximately 14 weight percent, with the greatest abundance occurring at an intermediate silica content of about 56 weight percent. Resources, Larger crystals with small crystal background, fine grained, and metamorphic igneous. Derived from partial melts of existing rocks in the mineral grains in rocks... Chemical composition, and intrusive, fine grained, and information from Encyclopaedia Britannica tell. 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