The field of geochemistry involves study of the chemical Chemistry is the science of matter and the changes it undergoes. The science of matter is also addressed by physics, but while physics takes a more general and fundamental approach, chemistry is more specialized, being concerned with the composition, behavior, structure, and properties of matter, as well as the changes it undergoes during chemical composition of the 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. It is sometimes referred to as the World, the Blue Planet,[note 6] or by its Latin name, Terra.[note 7] and other planets A planet is a celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared its neighbouring region of planetesimals.[a], chemical processes and reactions that govern the composition of rocks In geology, rock is a naturally occurring solid aggregate of minerals and/or mineraloids and soils Soil is a natural body consisting of layers of mineral constituents of variable thicknesses, which differ from the parent materials in their morphological, physical, chemical, and mineralogical characteristics, and the cycles of matter and energy that transport the Earth's chemical components in time and space, and their interaction with the hydrosphere A hydrosphere in physical geography describes the combined mass of water found on, under, and over the surface of a planet and the atmosphere An atmosphere is a layer of gases that may surround a material body of sufficient mass,and that is held in place by the gravity of the body. An atmosphere may be retained for a longer duration, if the gravity is high and the atmosphere's temperature is low. Some planets consist mainly of various gases, but only their outer layer is their.

The most important fields of geochemistry are:

1. Isotope geochemistry Isotope geochemistry is an aspect of geology based upon study of the relative and absolute concentrations of the elements and their isotopes in the Earth. Variations in the abundance of these isotopes, typically measured with an isotope ratio mass spectrometer or an accelerator mass spectrometer, can reveal information about the age of a rock or:Determination of the relative and absolute concentrations of the elements A chemical element is a pure chemical substance consisting of one type of atom distinguished by its atomic number, which is the number of protons in its nucleus. The term is also used to refer to a pure chemical substance composed of atoms with the same number of protons. Common examples of elements are iron, copper, silver, gold, hydrogen, carbon, and their isotopes Isotopes are different types of atoms of the same chemical element, each having a different number of neutrons. In a corresponding manner, isotopes differ in mass number (or number of nucleons) but never in atomic number. The number of protons (the atomic number) is the same because that is what characterizes a chemical element. For example, in the earth and on earth's surface.
2. Examination of the distribution and movements of elements in different parts of the earth (crust, mantle, hydrosphere etc.) and in minerals with the goal to determine the underlying system of distribution and movement.
3. Cosmochemistry Cosmochemistry or chemical cosmology is concerned with the origin and development of the substances of the universe. The main focus are the elements and their isotopes, primarily within Solar System objects. The term was coined by Harold Urey.[dubious – discuss] Closely related fields are astrochemistry, a branch of astronomy concerned with: Analysis of the distribution of elements and their isotopes in the cosmos The Universe is commonly defined as the totality of everything that exists, including all physical matter and energy, the planets, stars, galaxies, and the contents of intergalactic space, although this usage may differ with the context . The term Universe may be used in slightly different contextual senses, denoting such concepts as the cosmos,.
4. Biogeochemistry Biogeochemistry is the scientific discipline that involves the study of the chemical, physical, geological, and biological processes and reactions that govern the composition of the natural environment . In particular, biogeochemistry is the study of the cycles of chemical elements, such as carbon and nitrogen, and their interactions with and: Field of study focusing on the effect of life on the chemistry of the earth.
5. Organic geochemistry Organic geochemistry is the study of the impacts and processes that organisms have had on the Earth. The study of organic geochemistry is usually traced to the work of Alfred E. Treibs, "the father of organic geochemistry." Treibs first isolated metalloporphyrins from petroleum. This discovery established the biological origin of: A study of the role of processes and compounds that are derived from living or once-living organisms.
6. Regional, environmental and exploration geochemistry: Applications to environmental, hydrological and mineral exploration studies.

Victor Goldschmidt is considered by most to be the father of modern geochemistry and the ideas of the subject were formed by him in a series of publications from 1922 under the title ‘Geochemische Verteilungsgesetze der Elemente’.

Chemical characteristics

The more common rock constituents are nearly all oxides An oxide is a chemical compound containing at least one oxygen atom as well as at least one other element. Most of the Earth's crust consists of oxides. Oxides result when elements are oxidized by oxygen in air. Combustion of hydrocarbons affords the two principal oxides of carbon, carbon monoxide and carbon dioxide. Even materials that are; chlorine Chlorine (pronounced /ˈklɔəriːn/ KLOR-een, from the Greek word 'χλωρóς' , is the chemical element with atomic number 17 and symbol Cl. It is a halogen, found in the periodic table in group 17 (formerly VII, VIIa, or VIIb). As the chloride ion, which is part of common salt and other compounds, it is abundant in nature and necessary to, sulfur Sulfur or sulphur is the chemical element that has the atomic number 16. It is denoted with the symbol S. It is an abundant, multivalent non-metal. Sulfur, in its native form, is a bright yellow crystalline solid. In nature, it can be found as the pure element and as sulfide and sulfate minerals. It is an essential element for life and is found in and fluorine Fluorine is the chemical element with atomic number 9, represented by the symbol F. Fluorine forms a single bond with itself in elemental form, resulting in the diatomic F2 molecule. F2 is a supremely reactive, poisonous, pale, yellowish brown gas. Elemental fluorine is the most chemically reactive and electronegative of all the elements. For are the only important exceptions to this and their total amount in any rock is usually much less than 1%. F. W. Clarke has calculated that a little more than 47% of the Earth's crust consists of oxygen Oxygen (pronounced /ˈɒksɨdʒɨn/, OK-si-jin, from the Greek roots ὀξύς (acid, literally "sharp", from the taste of acids) and -γενής (-genēs) (producer, literally begetter), is the element with atomic number 8 and represented by the symbol O. It is a member of the chalcogen group on the periodic table, and is a highly. It occurs principally in combination as oxides, of which the chief are silica The chemical compound silicon dioxide, also known as silica , is an oxide of silicon with a chemical formula of Si , alumina Aluminium oxide is the family of inorganic compounds with the chemical formula Al2O3. It is an amphoteric oxide and is commonly referred to as alumina, corundum as well as many other names, reflecting its widespread occurrence in nature and industry. Its most significant use is in the production of aluminium metal, although it is also used as an, iron oxides Iron oxides are chemical compounds composed of iron and oxygen. Altogether, there are sixteen known iron oxides and oxyhydroxides, and various carbonates In chemistry, a carbonate is a salt of carbonic acid, characterized by the presence of the carbonate ion, CO2−3. The name may also mean an ester of carbonic acid, an organic compound containing the carbonate group O=C2 (calcium carbonate Calcium carbonate is a chemical compound with the chemical formula Ca , magnesium carbonate Magnesium carbonate, MgCO3, is a white solid that occurs in nature as a mineral. Several hydrated and basic forms of magnesium carbonate also exist as minerals. In addition, MgCO3 has a variety of uses, sodium carbonate Sodium Carbonate , Na2CO3, is a sodium salt of carbonic acid. It most commonly occurs as a crystalline heptahydrate, which readily effloresces to form a white powder, the monohydrate; and is domestically well known for its everyday use as a water softener. It has a cooling alkaline taste, and can be extracted from the ashes of many plants. It is, and potassium carbonate Potassium carbonate is a white salt, soluble in water (insoluble in alcohol), which forms a strongly alkaline solution. It can be made as the product of potassium hydroxide's absorbent reaction with carbon dioxide. It is deliquescent, often appearing a damp or wet solid. Potassium carbonate is used in the production of soap and glass). The silica functions principally as an acid, forming silicates, and all the commonest minerals of igneous rocks are of this nature. From a computation based on 1672 analyses of numerous kinds of rocks Clarke arrived at the following as the average percentage composition: SiO₂=59.71, Al₂O₃=15.41, Fe₂O₃=2.63, FeO=3.52, MgO=4.36, CaO=4.90, Na₂O=3.55, K₂O=2.80, H₂O=1.52, TiO₂=0.60, P₂O₅=0.22, total 99.22%). All the other constituents occur only in very small quantities, usually much less than 1%.

These oxides combine in a haphazard way. For example, potash Potash is the common name for potassium carbonate and various mined and manufactured salts that contain the element potassium in water-soluble form. In some rare cases, potash can be formed with traces of organic materials such as plant remains (potassium carbonate) and soda (sodium carbonate Sodium Carbonate , Na2CO3, is a sodium salt of carbonic acid. It most commonly occurs as a crystalline heptahydrate, which readily effloresces to form a white powder, the monohydrate; and is domestically well known for its everyday use as a water softener. It has a cooling alkaline taste, and can be extracted from the ashes of many plants. It is) combine to produce feldspars Feldspars crystallize from magma in both intrusive and extrusive igneous rocks, as veins, and are also present in many types of metamorphic rock. Rock formed almost entirely of calcic plagioclase feldspar is known as anorthosite. Feldspars are also found in many types of sedimentary rock. In some cases they may take other forms, such as nepheline Nepheline, also called nephelite , is a feldspathoid: a silica-undersaturated aluminosilicate, Na3K , leucite, and muscovite Muscovite is a phyllosilicate mineral of aluminium and potassium with formula K , but in the great majority of instances they are found as feldspar. Phosphoric acid Phosphoric acid, also known as orthophosphoric acid or phosphoric acid, is a mineral (inorganic) acid having the chemical formula H3P with lime Lime is a general term for calcium-containing inorganic materials, in which carbonates, oxides and hydroxides predominate. Strictly speaking, lime is calcium oxide or calcium hydroxide. It is also the name for a single mineral of the CaO composition, occurring very rarely. The word 'lime' originates with its earliest use as building mortar and has (calcium carbonate) forms apatite Apatite is a group of phosphate minerals, usually referring to hydroxyapatite, fluorapatite, chlorapatite and bromapatite, named for high concentrations of OH−, F−, Cl− or Br−ions, respectively, in the crystal. The formula of the admixture of the four most common endmembers is written as Ca106(OH, F, Cl, Br)2, and the crystal unit cell. Titanium dioxide Titanium dioxide, also known as titanium oxide or titania, is the naturally occurring oxide of titanium, chemical formula Ti with ferrous oxide Iron oxide, also known as ferrous oxide, iron oxide/oxidized iron or more commonly rusted iron, is one of the iron oxides. It is a black-colored powder with the chemical formula Fe gives rise to ilmenite Ilmenite is a weakly magnetic titanium-iron oxide mineral which is iron-black or steel-gray. It is a crystalline iron titanium oxide . It crystallizes in the trigonal system, and it has the same crystal structure as corundum and hematite. Part of the lime forms lime feldspar. Magnesium carbonate and iron oxides with silica crystallize as olivine The mineral olivine is a magnesium iron silicate with the formula (Mg,Fe)2Si or enstatite Enstatite is the magnesium endmember of the pyroxene silicate mineral series enstatite - ferrosilite (FeSiO3). The magnesium rich members of the solid solution series are common rock-forming minerals found in igneous and metamorphic rocks. The intermediate composition, (Mg,Fe)SiO3, has historically been known as hypersthene, although this name has, or with alumina and lime form the complex ferro-magnesian silicates of which the pyroxenes The pyroxenes are a group of important rock-forming silicate 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. Pyroxenes have the general formula XY2O6 (where X represents calcium, sodium, iron+2 and, amphiboles Amphibole defines an important group of generally dark-colored rock-forming inosilicate minerals, composed of double chain SiO4 tetrahedra, linked at the vertices and generally containing ions of iron and/or magnesium in their structures. Amphiboles crystallize into two crystal systems, monoclinic and orthorhombic. In chemical composition and, and biotites Biotite is a common phyllosilicate mineral within the mica group, with the approximate chemical formula K3AlSi3O10(F,OH)2. 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 include siderophyllite. Biotite was are the chief. Any excess of silica above what is required to neutralize the bases will separate out as 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; excess of alumina crystallizes as corundum Corundum is a crystalline form of aluminium oxide with traces of iron, titanium and chromium. It is a rock-forming mineral. It is one of the naturally clear transparent materials, but can have different colors when impurities are present. Transparent specimens are used as gems, called ruby if red, while all other colors are called sapphire. These must be regarded only as general tendencies. It is possible, by rock analysis, to say approximately what minerals the rock contains, but there are numerous exceptions to any rule.

Mineral constitution

Hence we may say that except in acid or siliceous rocks containing 66% of silica and over, quartz will not be abundant. In basic rocks (containing 20% of silica or less) it is rare and accidental. If magnesia and iron be above the average while silica is low, olivine may be expected; where silica is present in greater quantity over ferro-magnesian minerals, such as augite, hornblende, enstatite or biotite, occur rather than olivine. Unless potash is high and silica relatively low, leucite will not be present, for leucite does not occur with free quartz. Nepheline, likewise, is usually found in rocks with much soda and comparatively little silica. With high alcalis, soda-bearing pyroxenes and amphiboles may be present. The lower the percentage of silica and the alkalis, the greater is the prevalence of t lime felspar as contracted with soda or potash felspar. Clarke has calculated the relative abundance of the principal rock-forming minerals with the following results: Apatite=0.6, titanium minerals=1.5, quartz=12.0, felspars=59.5, biotite=3.8, hornblende and pyroxene=16.8, total=94.2%. This, however, can only be a rough approximation.

The other determining factor, namely the physical conditions attending consolidation, plays on the whole a smaller part, yet is by no means negligible, as a few instances will prove. Certain minerals are practically confined to deep-seated intrusive rocks, e.g., microcline, muscovite, diallage. Leucite is very rare in plutonic masses; many minerals have special peculiarities in microscopic character according to whether they crystallized in depth or near the surface, e.g., hypersthene, orthoclase, quartz. There are some curious instances of rocks having the same chemical composition, but consisting of entirely different minerals, e.g., the hornblendite of Gran, in Norway, which contains only hornblende, has the same composition as some of the camptonites of the same locality that contain felspar and hornblende of a different variety. In this connection we may repeat what has been said above about the corrosion of porphyritic minerals in igneous rocks. In rhyolites and trachytes, early crystals of hornblende and biotite may be found in great numbers partially converted into augite and magnetite. Hornblende and biotite were stable under the pressures and other conditions below the surface, but unstable at higher levels. In the ground-mass of these rocks, augite is almost universally present. But the plutonic representatives of the same magma, granite and syenite contain biotite and hornblende far more commonly than augite.

Acid, intermediate and basic igneous rocks

Those rocks that contain the most silica, and on crystallizing yield free quartz, form a group generally designated the "acid" rocks. Those again that contain least silica and most magnesia and iron, so that quartz is absent while olivine The mineral olivine is a magnesium iron silicate with the formula (Mg,Fe)2Si is usually abundant, form the "basic" group. The "intermediate" rocks include those characterized by the general absence of both quartz and olivine. An important subdivision of these contains a very high percentage of alkalis, especially soda, and consequently has minerals such as nepheline Nepheline, also called nephelite , is a feldspathoid: a silica-undersaturated aluminosilicate, Na3K and leucite not common in other rocks. It is often separated from the others as the "alkali" or "soda" rocks, and there is a corresponding series of basic rocks. Lastly a small sub-group rich in olivine and without felspar has been called the "ultrabasic" rocks. They have very low percentages of silica but much iron and magnesia.

Except these last, practically all rocks contain felspars or felspathoid minerals. In the acid rocks the common felspars are orthoclase, perthite, microcline, and oligoclase—all having much silica and alkalis. In the basic rocks labradorite, anorthite and bytownite prevail, being rich in lime and poor in silica, potash and soda. Augite is the commonest ferro-magnesian of the basic rocks, but biotite and hornblende are on the whole more frequent in the acid.

Rocks that contain leucite or nepheline, either partly or wholly replacing felspar, are not included in this table. They are essentially of intermediate or of basic character. We might in consequence regard them as varieties of syenite, diorite, gabbro, etc., in which felspathoid minerals occur, and indeed there are many transitions between syenites of ordinary type and nepheline — or leucite — syenite, and between gabbro or dolerite and theralite or essexite. But, as many minerals develop in these "alcali" rocks that are uncommon elsewhere, it is convenient in a purely formal classification like that outlined here to treat the whole assemblage as a distinct series.

This classification is based essentially on the mineralogical constitution of the igneous rocks. Any chemical distinctions between the different groups, though implied, are relegated to a subordinate position. It is admittedly artificial by it has grown up with the grown of the science and is still adopted as the basis on which more minute subdivisions are erected. The subdivisions are by no means of equal value. The syenites, for example, and the peridotites, are far less important than the granites, diorites and gabbros. Moreover, the effusive andesites do not always correspond to the plutonic diorites but partly also to the gabbros. As the different kinds of rock, regarded as aggregates of minerals, pass gradually into one another, transitional types are very common and are often so important as to receive special names. The quartz-syenites and nordmarkites may be interposed between granite and syenite, the tonalites and adamellites between granite and diorite, the monzoaites between syenite and diorite, norites and hyperites between diorite and gabbro, and so on.^[1].

See also

• Important publications in geochemistry
• Petrology
• Tephrochronology

Further reading

• Holland, H.D., & Turekian, K.K. (2004). Treatise on Geochemistry. 9 Volumes. Elsevier
• Marshall, C., & Fairbridge, R. (2006). Encyclopedia of Geochemistry. ISBN 1-4020-4496-8. Berlin: Springer.
• Bernard Gunn: The Geochemistry of Igneous Rocks
• Gunter Faure (1986). Principles of Isotope Geology, John Wiley & Sons. ISBN 0-471-86412-9
• H.R. Rollinson (1993), Using Geochemical Data: evaluation, presentation, interpretation (Longman). ISBN 978-0582067011
• W.M. White: Geochemistry (Free Download)

References

1. ^ This article incorporates text from the article "Petrology" in the Encyclopædia Britannica, Eleventh Edition, a publication now in the public domain.

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