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{{short description|Science that applies chemistry to analyze geological systems}}
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'''Geochemistry''' is the [[science]] that uses the tools and principles of [[chemistry]] to explain the mechanisms behind major geological systems such as the [[Earth's crust]] and its [[ocean]]s.<ref name=Albarede/>{{rp|1}} The realm of geochemistry extends beyond the [[Earth]], encompassing the entire [[Solar System]],<ref name=cosmo/> and has made important contributions to the understanding of a number of processes including [[mantle convection]], the formation of [[planet]]s and the origins of [[granite]] and [[basalt]].<ref name=Albarede/>{{rp|1}} It is an integrated field of chemistry and [[geology]]. 
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==History==
[[File:Victorgoldschmidt0006MA8633570-0001.jpg|thumb|[[Victor Goldschmidt]] (1909)]]
The term ''geochemistry'' was first used by the Swiss-German chemist [[Christian Friedrich Schönbein]] in 1838: "a comparative geochemistry ought to be launched, before geognosy can become geology, and before the mystery of the genesis of our planets and their inorganic matter may be revealed."<ref name=Buildingbridges>{{cite book|first= Helge|last= Kragh|chapter= From geochemistry to cosmochemistry: The origin of a scientific discipline, 1915–1955|title= Chemical Sciences in the 20th Century: Bridging Boundaries|editor-last= Reinhardt|editor-first= Carsten|date= 2008|publisher= John Wiley & Sons|isbn= 978-3-527-30271-0|pages= 160–192|url= https://books.google.com/books?id=gIOK5EUm5ysC|access-date= 2020-05-03|archive-date= 2023-01-17|archive-url= https://web.archive.org/web/20230117100436/https://books.google.com/books?id=gIOK5EUm5ysC|url-status= live}}</ref> However, for the rest of the century the more common term was "chemical geology", and there was little contact between [[geologist]]s and [[chemist]]s.<ref name=Buildingbridges/>

Geochemistry emerged as a separate discipline after major laboratories were established, starting with the [[United States Geological Survey]] (USGS) in 1884, which began systematic surveys of the chemistry of rocks and minerals. The chief USGS chemist, [[Frank Wigglesworth Clarke]], noted that the elements generally decrease in abundance as their atomic weights increase, and summarized the work on elemental abundance in ''The Data of Geochemistry''.<ref name=Buildingbridges/><ref name=McSween>{{cite book|last1=McSween|first1=Harry Y. Jr.|last2=Richardson|first2=Steven M.|last3=Uhle|first3=Maria E.|title=Geochemistry pathways and processes|date=2003|publisher=Columbia University|location=New York|isbn=9780231509039|edition=2nd}}</ref>{{rp|2}}

The composition of [[meteorites]] was investigated and compared to terrestrial rocks as early as 1850. In 1901, [[Oliver C. Farrington]] hypothesised that, although there were differences, the relative abundances should still be the same.<ref name=Buildingbridges/>  This was the beginnings of the field of [[cosmochemistry]] and has contributed much of what we know about the formation of the Earth and the Solar System.<ref name="Cornell">{{cite book|title= Geochemistry (Unpublished)|last= White|first= William M.|page= 1|url= https://www.geo.cornell.edu/geology/classes/geo455/Chapters.HTML|access-date= 14 March 2012|archive-date= 16 June 2014|archive-url= https://web.archive.org/web/20140616205643/http://www.geo.cornell.edu/geology/classes/geo455/Chapters.HTML|url-status= live}}</ref>

In the early 20th century, [[Max von Laue]] and [[William L. Bragg]] showed that X-ray scattering could be used to determine the structures of crystals. In the 1920s and 1930s, [[Victor Goldschmidt]] and associates at the [[University of Oslo]] applied these methods to many common minerals and formulated a set of rules for how elements are grouped. Goldschmidt published this work in the series  ''Geochemische Verteilungsgesetze der Elemente'' [Geochemical Laws of the Distribution of Elements].<ref name=McSween/>{{rp|2}}<ref>{{cite book|last1=Mason|first1=Brian|title=Victor Moritz Goldschmidt : father of modern geochemistry|date=1992|publisher=Geochemical Society|location=San Antonio, Tex.|isbn=0-941809-03-X}}</ref>

The research of [[Manfred Schidlowski]] from the 1960s to around the year 2002 was concerned with the biochemistry of the [[Early Earth]] with a focus on isotope-biogeochemistry and the evidence of the earliest life processes in [[Precambrian]].<ref>[[Manfred Schidlowski]]: ''[https://www.sciencedirect.com/science/article/abs/pii/S0301926800001285?via%3Dihub Carbon isotopes as biochemical recorders of life over 3.8 Ga of Earth history: Evolution of a concept] {{Webarchive|url=https://web.archive.org/web/20211208230235/https://www.sciencedirect.com/science/article/abs/pii/S0301926800001285?via%3Dihub |date=2021-12-08 }}''. In: Precambrian Research. Vol. 106, Issues 1-2, 1 February 2001, pages 117-134.</ref><ref>Harald Strauss: ’’[https://e-docs.geo-leo.de/bitstream/handle/11858/8243/Gmit_50.pdf?sequence=3&isAllowed=y Obituary] {{Webarchive|url=https://web.archive.org/web/20211208230237/https://e-docs.geo-leo.de/bitstream/handle/11858/8243/Gmit_50.pdf?sequence=3&isAllowed=y |date=2021-12-08 }}’’. In: Geowissenschaftiche Mitteilungen, Nr. 50, december 2012, page 102-103</ref>

==Subfields==
Some subfields of geochemistry are:<ref>{{cite web|title=Welcome to GPS Geochemistry|url=http://web.gps.caltech.edu/options/geochemistry/|website=GPS Research Program|publisher=California Institute of Technology|access-date=2 October 2017|archive-date=21 September 2017|archive-url=https://web.archive.org/web/20170921225209/http://web.gps.caltech.edu/options/geochemistry/|url-status=dead}}</ref>
*[[Aqueous geochemistry]] studies the role of various elements in watersheds, including [[copper]], [[sulfur]], [[Mercury (element)|mercury]], and how elemental fluxes are exchanged through atmospheric-terrestrial-aquatic interactions.<ref>{{cite book|last1=Langmuir|first1=Donald|title=Aqueous environmental geochemistry|date=1997|publisher=Prentice Hall|location=Upper Saddle River, N.J.|isbn=9780023674129}}</ref>
*[[Biogeochemistry]] is the field of study focusing on the effect of life on the chemistry of the Earth.<ref>{{cite book|last1=Schlesinger|first1=William H.|last2=Bernhardt|first2=Emily S.|title=Biogeochemistry : an analysis of global change|date=2013|publisher=Academic Press|isbn=9780123858740|edition=Third}}</ref>{{rp|3}}
*[[Cosmochemistry]] includes the analysis of the distribution of elements and their isotopes in the [[universe|cosmos]].<ref name=cosmo>{{cite book|last1=McSween|first1=Harry Y. Jr.|last2=Huss|first2=Gary R.|title=Cosmochemistry|date=2010|publisher=Cambridge University Press|isbn=9781139489461}}</ref>{{rp|1}}
*[[Isotope geochemistry]] involves the determination of the relative and absolute concentrations of the [[chemical element|element]]s and their [[isotope]]s in the Earth and on Earth's surface.<ref>{{cite book|last1=Kendall|first1=Carol|author-link1=Carol Kendall (scientist)|last2=Caldwell|first2=Eric A.|chapter=Chapter 2: Fundamentals of Isotope Geochemistry|title=Isotope Tracers in Catchment Hydrology|editor-last1=Kendall|editor-first1=C.|editor-link1=Carol Kendall (scientist)|editor-last2=McDonnell|editor-first2=J. J.|date=1998|publisher=Elsevier Science|location=Amsterdam|pages=51–86|chapter-url=https://wwwrcamnl.wr.usgs.gov/isoig/isopubs/itchch2.html|access-date=3 October 2017|archive-date=3 May 2019|archive-url=https://web.archive.org/web/20190503212721/https://wwwrcamnl.wr.usgs.gov/isoig/isopubs/itchch2.html|url-status=live}}</ref>
*[[Organic geochemistry]], the study of the role of processes and compounds that are derived from living or once-living organisms.<ref>{{cite book|last1=Killops|first1=Stephen D.|last2=Killops|first2=Vanessa J.|title=Introduction to Organic Geochemistry|date=2013|publisher=John Wiley & Sons|isbn=9781118697207}}</ref>
*[[Photogeochemistry]] is the study of light-induced chemical reactions that occur or may occur among natural components of the Earth's surface.<ref>{{cite journal | pmc = 5307419 | pmid=28246525 | doi=10.1186/s12932-017-0039-y | volume=18 | title=A survey of photogeochemistry | journal=Geochem Trans | page=1 | last1 = Doane | first1 = TA| year=2017 | issue=1 | bibcode=2017GeoTr..18....1D | doi-access=free }}</ref>
*[[Regional geochemistry]] includes applications to environmental, hydrological and mineral exploration studies.<ref>{{cite journal|last1=Garrett|first1=R.G.|last2=Reimann|first2=C.|last3=Smith|first3=D.B.|last4=Xie|first4=X.|title=From geochemical prospecting to international geochemical mapping: a historical overview: Table 1|journal=Geochemistry: Exploration, Environment, Analysis|date=November 2008|volume=8|issue=3–4|pages=205–217|doi=10.1144/1467-7873/08-174|s2cid=130836294}}</ref>

==Chemical elements==
The building blocks of materials are the [[chemical element]]s. These can be identified by their [[atomic number]] Z, which is the number of [[proton]]s in the [[Atomic nucleus|nucleus]]. An element can have more than one value for N, the [[neutron number|number of neutrons]] in the nucleus. The sum of these is the [[mass number]], which is roughly equal to the [[atomic mass]]. Atoms with the same atomic number but different neutron numbers are called [[isotope]]s. A given isotope is identified by a letter for the element preceded by a superscript for the mass number. For example, two common isotopes of [[chlorine]] are <sup>35</sup>Cl and <sup>37</sup>Cl. There are about 1700 known combinations of Z and N, of which only about 260 are stable. However, most of the unstable isotopes do not occur in nature. In geochemistry, stable isotopes are used to trace chemical pathways and reactions, while radioactive isotopes are primarily used to date samples.<ref name=McSween/>{{rp|13–17}}

The chemical behavior of an atom – its [[Chemical affinity|affinity for other elements]] and the type of [[chemical bond|bonds]] it forms – is determined by the arrangement of electrons in [[Atomic orbital|orbitals]], particularly the outermost ([[valence electron|valence]]) electrons. These arrangements are reflected in the position of elements in the [[periodic table]].<ref name=McSween/>{{rp|13–17}} Based on position, the elements fall into the broad groups of [[alkali metal]]s, [[alkaline earth metal]]s, [[transition metal]]s, semi-metals (also known as [[metalloid]]s), [[halogen]]s, [[noble gas]]es, [[lanthanide]]s and [[actinide]]s.<ref name=McSween/>{{rp|20–23}}

Another useful classification scheme for geochemistry is the [[Goldschmidt classification]], which places the elements into four main groups. ''Lithophiles'' combine easily with oxygen. These elements, which include [[sodium|Na]], [[potassium|K]], [[silicon|Si]], [[aluminium|Al]], [[titanium|Ti]], [[magnesium|Mg]] and [[calcium|Ca]], dominate in the [[Earth's crust]], forming [[silicate]]s and other oxides. ''Siderophile'' elements ([[iron|Fe]], [[cobalt|Co]], [[nickel|Ni]], [[platinum|Pt]], [[rhenium|Re]], [[osmium|Os]]) have an affinity for [[iron]] and tend to concentrate in the [[Earth's core|core]]. ''Chalcophile'' elements ([[copper|Cu]], [[silver|Ag]], [[zinc|Zn]], [[lead|Pb]], [[sulfur|S]]) form [[sulfide]]s; and ''atmophile'' elements ([[oxygen|O]], [[nitrogen|N]], [[hydrogen|H]] and noble gases) dominate the atmosphere. Within each group, some elements are [[refractory]], remaining stable at high temperatures, while others are [[Volatility (chemistry)|volatile]], evaporating more easily, so heating can separate them.<ref name=Albarede>{{cite book|last1=Albarède|first1=Francis|title=Geochemistry : an introduction|date=2007|publisher=Cambridge Univ. Press|location=Cambridge|isbn=9780521891486|edition=5th|others=Translated from the French.}}</ref>{{rp|17}}<ref name=McSween/>{{rp|23}}

==Differentiation and mixing==
The chemical composition of the Earth and other bodies is determined by two opposing processes: differentiation and mixing. In the Earth's [[mantle (geology)|mantle]], differentiation occurs at [[mid-ocean ridge]]s through [[partial melting]], with more refractory materials remaining at the base of the [[lithosphere]] while the remainder rises to form [[basalt]]. After an oceanic plate descends into the mantle, [[convection]] eventually mixes the two parts together. [[Erosion]] differentiates [[granite]], separating it into [[clay]] on the ocean floor, [[sandstone]] on the edge of the continent, and dissolved minerals in ocean waters. [[Metamorphism]] and [[anatexis]] (partial melting of crustal rocks) can mix these elements together again. In the ocean, biological organisms can cause chemical differentiation, while dissolution of the organisms and their wastes can mix the materials again.<ref name=Albarede/>{{rp|23&ndash;24}}

===Fractionation===
{{See also|magmatic differentiation}}
A major source of differentiation is [[fractionation]], an unequal distribution of elements and isotopes. This can be the result of chemical reactions, [[Phase transition|phase changes]], kinetic effects, or [[radioactivity]].<ref name=Albarede/>{{rp|2&ndash;3}}
On the largest scale, ''[[planetary differentiation]]'' is a physical and chemical separation of a planet into chemically distinct regions. For example, the terrestrial planets formed iron-rich cores and silicate-rich mantles and crusts.<ref name="cosmo"/>{{rp|218}} In the Earth's mantle, the primary source of chemical differentiation is [[partial melting]], particularly near mid-ocean ridges.<ref>{{cite book|last1=Olson|first1=Gerald Schubert; Donald L. Turcotte; Peter|title=Mantle convection in the earth and planets|date=2001|publisher=Cambridge Univ. Press|location=Cambridge|isbn=9780521798365}}</ref>{{rp|68,153}} This can occur when the solid is heterogeneous or a [[solid solution]], and part of the melt is separated from the solid. The process is known as ''equilibrium'' or ''batch'' melting if the solid and melt remain in equilibrium until the moment that the melt is removed, and ''fractional'' or ''Rayleigh'' melting if it is removed continuously.<ref>{{cite book|last1=Wilson|first1=Marjorie|title=Igneous petrogenesis|date=2007|publisher=Springer|location=Dordrecht|isbn=9789401093880}}</ref>

[[Isotopic fractionation]] can have mass-dependent and mass-independent forms. Molecules with heavier isotopes have lower [[Zero-point energy|ground state energies]] and are therefore more stable. As a result, chemical reactions show a small isotope dependence, with heavier isotopes preferring species or compounds with a higher oxidation state; and in phase changes, heavier isotopes tend to concentrate in the heavier phases.<ref name=Kendall>{{cite book|last1=Kendall|first1=Carol|author-link1=Carol Kendall (scientist)|last2=Caldwell|first2=Eric A.|chapter=Chapter 2: Fundamentals of Isotope Geochemistry|editor-last1=Kendall|editor-first1=Carol|editor-last2=McDonnell|editor-first2=J. J.|title=Isotope tracers in catchment hydrology|date=2000|publisher=Elsevier|location=Amsterdam|isbn=9780444501554|pages=51&ndash;86|chapter-url=https://wwwrcamnl.wr.usgs.gov/isoig/isopubs/itchch2.html|access-date=24 October 2017|archive-date=14 March 2008|archive-url=https://web.archive.org/web/20080314192517/http://wwwrcamnl.wr.usgs.gov/isoig/isopubs/itchch2.html|url-status=live}}</ref> Mass-dependent fractionation is largest in light elements because the difference in masses is a larger fraction of the total mass.<ref name=Hoefs>{{cite book|last1=Hoefs|first1=Jochen|title=Stable Isotope Geochemistry |chapter=Isotope Fractionation Processes of Selected Elements |date=2015|pages=47&ndash;134|doi=10.1007/978-3-319-19716-6_2|isbn=978-3-319-19715-9|s2cid=100690717 }}</ref>{{rp|47}}

Ratios between isotopes are generally compared to a standard. For example, sulfur has four stable isotopes, of which the two most common are <sup>32</sup>S and <sup>34</sup>S.<ref name=Hoefs/>{{rp|98}} The ratio of their concentrations, {{math|''R''{{=}}<sup>34</sup>S/<sup>32</sup>S}}, is reported as
:<math>\delta{}^{34}\mathrm{S} = 1000\left(\frac{R}{R_\mathrm{s}}-1\right),</math>
where {{math|''R''<sub>s</sub>}} is the same ratio for a standard. Because the differences are small, the ratio is multiplied by 1000 to make it parts per thousand (referred to as parts per mil). This is represented by the symbol {{math|‰}}.<ref name=Kendall/>{{rp|55}}

====Equilibrium====
''[[Equilibrium fractionation]]'' occurs between chemicals or phases that are in equilibrium with each other. In equilibrium fractionation between phases, heavier phases prefer the heavier isotopes. For two phases A and B, the effect can be represented by the factor
:<math> a_\mathrm{A-B} = \frac{R_\mathrm{A}}{R_\mathrm{B}}. </math>
In the liquid-vapor phase transition for water, {{math|''a''<sub>l-v</sub>}} at 20 degrees [[Celsius]] is 1.0098 for <sup>18</sup>O and 1.084 for <sup>2</sup>H. In general, fractionation is greater at lower temperatures. At 0&nbsp;°C, the factors are 1.0117 and 1.111.<ref name=Kendall/>{{rp|59}}

====Kinetic====
When there is no equilibrium between phases or chemical compounds, ''[[kinetic fractionation]]'' can occur. For example, at interfaces between liquid water and air, the forward reaction is enhanced if the humidity of the air is less than 100% or the water vapor is moved by a wind. Kinetic fractionation generally is enhanced compared to equilibrium fractionation and depends on factors such as reaction rate, reaction pathway and bond energy. Since lighter isotopes generally have weaker bonds, they tend to react faster and enrich the reaction products.<ref name=Kendall/>{{rp|60}}

Biological fractionation is a form of kinetic fractionation since reactions tend to be in one direction. Biological organisms prefer lighter isotopes because there is a lower energy cost in breaking energy bonds. In addition to the previously mentioned factors, the environment and species of the organism can have a large effect on the fractionation.<ref name=Kendall/>{{rp|70}}

== Cycles ==
{{Main|Geochemical cycle}}
{{See also|Climate model#Box models}}
Through a variety of physical and chemical processes, chemical elements change in concentration and move around in what are called ''geochemical cycles''. An understanding of these changes requires both detailed observation and theoretical models. Each chemical compound, element or isotope has a concentration that is a function {{math|''C''('''r''',''t'')}} of position and time, but it is impractical to model the full variability. Instead, in an approach borrowed from [[chemical engineering]],<ref name=Albarede/>{{rp|81}} geochemists average the concentration over regions of the Earth called ''geochemical reservoirs''. The choice of reservoir depends on the problem; for example, the ocean may be a single reservoir or be split into multiple reservoirs.<ref name=Lasaga>{{cite journal|last1=Lasaga|first1=Antonio C.|last2=Berner|first2=Robert A.|title=Fundamental aspects of quantitative models for geochemical cycles|journal=Chemical Geology|date=April 1998|volume=145|issue=3–4|pages=161–175|doi=10.1016/S0009-2541(97)00142-3|bibcode=1998ChGeo.145..161L|doi-access=free}}</ref> In a type of model called a ''box model'', a reservoir is represented by a box with inputs and outputs.<ref name=Albarede/>{{rp|81}}<ref name=Lasaga/>

Geochemical models generally involve feedback. In the simplest case of a linear cycle, either the input or the output from a reservoir is proportional to the concentration. For example, [[salt]] is removed from the ocean by formation of [[evaporite]]s, and given a constant rate of evaporation in evaporite basins, the rate of removal of salt should be proportional to its concentration. For a given component {{math|''C''}}, if the input to a reservoir is a constant {{math|''a''}} and the output is {{math|''kC''}} for some constant {{math|''k''}}, then the ''[[mass balance]]'' equation is
{{NumBlk2|:|<math>\frac{d C}{d t} = a - kC.</math>|1}}
This expresses the fact that any change in mass must be balanced by changes in the input or output. On a time scale of {{math|''t'' {{=}} 1/k}}, the system approaches a [[steady state]] in which {{math|''C''<sup>steady</sup> {{=}} ''a''/''k''}}. The ''[[residence time]]'' is defined as
:<math>\tau_\mathrm{res} = C^\text{steady}/I = C^\text{steady}/O,</math>
where {{math|''I''}} and {{math|''O''}} are the input and output rates. In the above example, the steady-state input and output rates are both equal to {{math|''a''}}, so {{math|&tau;<sub>res</sub> {{=}} 1/''k''}}.<ref name=Lasaga/>

If the input and output rates are nonlinear functions of {{math|'' C''}}, they may still be closely balanced over time scales much greater than the residence time; otherwise, there will be large fluctuations in {{math|'' C''}}. In that case, the system is always close to a steady-state and the lowest order expansion of the mass balance equation will lead to a linear equation like Equation ({{EquationNote|1}}). In most systems, one or both of the input and output depend on {{math|''C''}}, resulting in feedback that tends to maintain the steady-state. If an external forcing perturbs the system, it will return to the steady-state on a time scale of {{math|1/''k''}}.<ref name=Lasaga/>

==Abundance of elements==
{{main|Abundance of the chemical elements}}

===Solar System===
[[File:ElementalAbundance.svg|thumb|upright=1.7|Abundances of Solar System elements.<ref>Data from table 6 of {{cite journal|last1=Cameron|first1=A.G.W.|title=Abundances of the elements in the solar system|journal=Space Science Reviews|date=September 1973|volume=15|issue=1|pages=121|doi=10.1007/BF00172440|bibcode=1973SSRv...15..121C|s2cid=120201972}}</ref>]]
The composition of the [[Solar System]] is similar to that of many other stars, and aside from small anomalies it can be assumed to have formed from a [[solar nebula]] that had a uniform composition, and the composition of the [[Sun]]'s [[photosphere]] is similar to that of the rest of the Solar System. The composition of the photosphere is determined by fitting the [[Spectral line|absorption lines]] in its [[spectrum]] to models of the Sun's atmosphere.<ref name=Palme>{{cite book|first1=H.|last1=Palme|first2=A.|last2=Jones|chapter=1.03 &ndash; Solar system abundance of the elements|editor-last1=Holland|editor-first1=H.D.|editor-last2=Turekian|editor-first2=K.K.|title=Treatise on Geochemistry|volume=1: Meteorites, Comets and Planets|date=2003|publisher=Elsevier Science|location=Oxford|isbn=9780080437514|doi=10.1016/B0-08-043751-6/01060-4|pages=41&ndash;61|edition=1st|chapter-url=https://www.elsevier.ca/brochures/treatiseongeochemistry/contents/sample1.pdf|access-date=3 October 2017|archive-date=3 October 2017|archive-url=https://web.archive.org/web/20171003225547/https://www.elsevier.ca/brochures/treatiseongeochemistry/contents/sample1.pdf|url-status=dead}}</ref> By far the largest two elements by fraction of total mass are hydrogen (74.9%) and [[helium]] (23.8%), with all the remaining elements contributing just 1.3%.<ref>{{cite journal|last1=Lodders|first1=Katharina|author1-link=Katharina Lodders|title=Solar System Abundances and Condensation Temperatures of the Elements|journal=The Astrophysical Journal|date=10 July 2003|volume=591|issue=2|pages=1220–1247|doi=10.1086/375492|bibcode=2003ApJ...591.1220L|citeseerx=10.1.1.695.5451|s2cid=42498829 }}</ref> There is a general trend of [[Exponential decay|exponential decrease]] in abundance with increasing atomic number, although elements with even atomic number are more common than their odd-numbered neighbors (the [[Oddo–Harkins rule]]). Compared to the overall trend, [[lithium]], [[boron]] and [[beryllium]] are depleted and iron is anomalously enriched.<ref>{{cite book|last1=Middlemost|first1=Eric A. K.|title=Magmas, Rocks and Planetary Development: A Survey of Magma/Igneous Rock Systems|date=2014|publisher=Routledge|isbn=9781317892649}}</ref>{{rp|284&ndash;285}}

The pattern of elemental abundance is mainly due to two factors. The hydrogen, helium, and some of the lithium were [[Big Bang nucleosynthesis|formed in about 20 minutes after the Big Bang]], while the rest were [[Stellar nucleosynthesis|created in the interiors of stars]].<ref name=McSween/>{{rp|316&ndash;317}}

===Meteorites===
[[Meteorite]]s come in a variety of compositions, but chemical analysis can determine whether they were once in [[planetesimal]]s that [[melting|melted]] or [[Planetary differentiation|differentiated]].<ref name=Palme/>{{rp|45}} [[Chondrite]]s are undifferentiated and have round mineral inclusions called [[chondrule]]s. With the ages of 4.56 billion years, they date to the [[Formation and evolution of the Solar System|early solar system]]. A particular kind, the [[CI chondrite]], has a composition that closely matches that of the Sun's photosphere, except for depletion of some volatiles (H, He, C, N, O) and a group of elements (Li, B, Be) that are destroyed by nucleosynthesis in the Sun.<ref name=McSween/>{{rp|318}}<ref name=Palme/> Because of the latter group, CI chondrites are considered a better match for the composition of the early Solar System. Moreover, the chemical analysis of CI chondrites is more accurate than for the photosphere, so it is generally used as the source for chemical abundance, despite their rareness (only five have been recovered on Earth).<ref name=Palme/>

===Giant planets===
[[File: Gas Giant Interiors.jpg|thumb|upright=2|Cutaways illustrating models of the interiors of the giant planets.]]
The planets of the Solar System are divided into two groups: the four inner planets are the [[terrestrial planet]]s ([[Mercury (planet)|Mercury]], [[Venus]], [[Earth]] and [[Mars]]), with relatively small sizes and rocky surfaces. The four outer planets are the [[giant planets]], which are dominated by hydrogen and helium and have lower mean densities. These can be further subdivided into the [[gas giant]]s ([[Jupiter]] and [[Saturn]]) and the [[ice giant]]s ([[Uranus]] and [[Neptune]]) that have large icy cores.<ref>{{cite book|last1=Encrenaz|first1=Therese|author1-link=Thérèse Encrenaz|last2=Bibring|first2=Jean-Pierre|last3=Blanc|first3=M.|last4=Barucci|first4=Maria-Antonietta|last5=Roques|first5=Francoise|last6=Zarka|first6=Philippe|title=The solar system|date=2004|publisher=Springer|location=Berlin|isbn=9783540002413|edition=3rd}}</ref>{{rp|26&ndash;27,283&ndash;284}}

Most of our direct information on the composition of the giant planets is from [[spectroscopy]]. Since the 1930s, Jupiter was known to contain hydrogen, [[methane]] and [[ammonium]]. In the 1960s, [[interferometry]] greatly increased the resolution and sensitivity of spectral analysis, allowing the identification of a much greater collection of molecules including [[ethane]], [[acetylene]], water and [[carbon monoxide]].<ref name=Lewis>{{cite book|last1=Lewis|first1=John|title=Physics and Chemistry of the Solar System|date=1995|publisher=Elsevier Science|location=Burlington|isbn=9780323145848}}</ref>{{rp|138&ndash;139}} However, Earth-based spectroscopy becomes increasingly difficult with more remote planets, since the reflected light of the Sun is much dimmer; and spectroscopic analysis of light from the planets can only be used to detect vibrations of molecules, which are in the [[infrared]] frequency range. This constrains the abundances of the elements H, C and N.<ref name=Lewis/>{{rp|130}} Two other elements are detected: phosphorus in the gas [[phosphine]] (PH<sub>3</sub>) and germanium in [[germane]] (GeH<sub>4</sub>).<ref name=Lewis/>{{rp|131}}

The helium atom has vibrations in the [[ultraviolet]] range, which is strongly absorbed by the atmospheres of the outer planets and Earth. Thus, despite its abundance, helium was only detected once spacecraft were sent to the outer planets, and then only indirectly through collision-induced absorption in hydrogen molecules.<ref name=Lewis/>{{rp|209}} Further information on Jupiter was obtained from the [[Galileo Probe|''Galileo'' probe]] when it was sent into the atmosphere in 1995;<ref>{{cite journal|last1=Atreya|first1=S.K|last2=Mahaffy|first2=P.R|last3=Niemann|first3=H.B|last4=Wong|first4=M.H|last5=Owen|first5=T.C|title=Composition and origin of the atmosphere of Jupiter—an update, and implications for the extrasolar giant planets|journal=Planetary and Space Science|date=February 2003|volume=51|issue=2|pages=105–112|doi=10.1016/S0032-0633(02)00144-7|bibcode=2003P&SS...51..105A}}</ref><ref name=Fortney>{{cite journal|last1=Fortney|first1=Jonathan|title=Viewpoint: Peering into Jupiter|journal=Physics|volume=3|page=26|date=22 March 2010|doi=10.1103/Physics.3.26|doi-access=free}}</ref> and the [[Cassini retirement|final mission]] of the [[Cassini–Huygens|Cassini probe]] in 2017 was to enter the atmosphere of Saturn.<ref name="Bittersweet ending">{{Cite news |url=http://www.latimes.com/science/sciencenow/la-sci-sn-cassini-ends-scene-20170915-story.html |title=As NASA's Cassini mission flames out over Saturn, scientists mark bittersweet end of mission |last=Netburn |first=Deborah |date=15 September 2017 |newspaper=The Los Angeles Times |access-date=10 October 2017 |archive-date=16 November 2017 |archive-url=https://web.archive.org/web/20171116091911/http://www.latimes.com/science/sciencenow/la-sci-sn-cassini-ends-scene-20170915-story.html |url-status=live }}</ref> In the atmosphere of Jupiter, He was found to be depleted by a factor of 2 compared to solar composition and Ne by a factor of 10, a surprising result since the other noble gases and the elements C, N and S were enhanced by factors of 2 to 4 (oxygen was also depleted but this was attributed to the unusually dry region that Galileo sampled).<ref name=Fortney/>

Spectroscopic methods only penetrate the atmospheres of Jupiter and Saturn to depths where the pressure is about equal to 1 [[bar (unit)|bar]], approximately Earth's [[atmospheric pressure]] at [[sea level]].<ref name=Lewis/>{{rp|131}} The Galileo probe penetrated to 22 bars.<ref name=Fortney/> This is a small fraction of the planet, which is expected to reach pressures of over 40 Mbar. To constrain the composition in the interior, thermodynamic models are constructed using the information on temperature from infrared emission spectra and equations of state for the likely compositions.<ref name=Lewis/>{{rp|131}} High-pressure experiments predict that hydrogen will be a metallic liquid in the interior of Jupiter and Saturn, while in Uranus and Neptune it remains in the molecular state.<ref name=Lewis/>{{rp|135&ndash;136}} Estimates also depend on models for the formation of the planets. Condensation of the presolar nebula would result in a gaseous planet with the same composition as the Sun, but the planets could also have formed when a solid core captured nebular gas.<ref name=Lewis/>{{rp|136}}

In current models, the four giant planets have cores of rock and ice that are roughly the same size, but the proportion of hydrogen and helium decreases from about 300 Earth masses in Jupiter to 75 in Saturn and just a few in Uranus and Neptune.<ref name=Lewis/>{{rp|220}} Thus, while the gas giants are primarily composed of hydrogen and helium, the ice giants are primarily composed of heavier elements (O, C, N, S), primarily in the form of water, methane, and ammonia. The surfaces are cold enough for molecular hydrogen to be liquid, so much of each planet is likely a hydrogen ocean overlaying one of heavier compounds.<ref>{{cite book|chapter=11. Uranus and Neptune|last1=Lang|first1=Kenneth R.|title=NASA's Cosmos|date=2010|publisher=Tufts University|chapter-url=https://ase.tufts.edu/cosmos/print_chapter.asp?id=11|access-date=11 October 2017|archive-date=9 September 2018|archive-url=https://web.archive.org/web/20180909093838/https://ase.tufts.edu/cosmos/print_chapter.asp?id=11|url-status=live}}</ref> Outside the core, Jupiter has a mantle of liquid metallic hydrogen and an atmosphere of molecular hydrogen and helium. Metallic hydrogen does not mix well with helium, and in Saturn, it may form a separate layer below the metallic hydrogen.<ref name=Lewis/>{{rp|138}}

===Terrestrial planets===
{{See also|Composition of Mars}}
Terrestrial planets are believed to have come from the same nebular material as the giant planets, but they have lost most of the lighter elements and have different histories. Planets closer to the Sun might be expected to have a higher fraction of refractory elements, but if their later stages of formation involved collisions of large objects with orbits that sampled different parts of the Solar System, there could be little systematic dependence on position.<ref name=Anderson>{{cite book|last1=Anderson|first1=Don L.|title=New Theory of the Earth|date=2007|publisher=Cambridge University Press|isbn=9781139462082}}</ref>{{rp|3&ndash;4}}

Direct information on Mars, Venus and Mercury largely comes from spacecraft missions. Using [[gamma-ray spectrometer]]s, the composition of the crust of Mars has been measured by the [[Mars Odyssey]] orbiter,<ref>{{cite web|title=GRS|url=https://mars.nasa.gov/odyssey/mission/instruments/grs/|website=Jet Propulsion Laboratory|publisher=USA.gov|access-date=17 October 2017|archive-date=8 February 2018|archive-url=https://web.archive.org/web/20180208132702/https://mars.nasa.gov/odyssey/mission/instruments/grs/|url-status=live}}</ref> the crust of Venus by some of the [[Venera]] missions to Venus,<ref name=Anderson/> and the crust of Mercury by the ''[[MESSENGER]]'' spacecraft.<ref>{{cite journal|last1=Rhodes|first1=Edgar A.|last2=Evans|first2=Larry G.|last3=Nittler|first3=Larry R.|last4=Starr|first4=Richard D.|last5=Sprague|first5=Ann L.|last6=Lawrence|first6=David J.|last7=McCoy|first7=Timothy J.|last8=Stockstill-Cahill|first8=Karen R.|last9=Goldsten|first9=John O.|last10=Peplowski|first10=Patrick N.|last11=Hamara|first11=David K.|last12=Boynton|first12=William V.|last13=Solomon|first13=Sean C.|title=Analysis of MESSENGER Gamma-Ray Spectrometer data from the Mercury flybys|journal=Planetary and Space Science|date=December 2011|volume=59|issue=15|pages=1829–1841|doi=10.1016/j.pss.2011.07.018|bibcode=2011P&SS...59.1829R}}</ref> Additional information on Mars comes from meteorites that have landed on Earth (the [[Shergottite]]s, [[Nakhlite]]s, and [[Chassignite]]s, collectively known as SNC meteorites).<ref name=Kieffer>{{cite book|editor-last1=Kieffer|editor-first1=Hugh H.|title=Mars|date=1994|publisher=University of Arizona Press|location=Tucson|isbn=9780816512577|edition=2nd|url-access=registration|url=https://archive.org/details/mars0000unse}}</ref>{{rp|124}} Abundances are also constrained by the masses of the planets, while the internal distribution of elements is constrained by their moments of inertia.<ref name=McSween/>{{rp|334}}

The planets condensed from the solar nebula, and much of the details of their composition are determined by fractionation as they cooled. The phases that condense fall into five groups. First to condense are materials rich in refractory elements such as Ca and Al. These are followed by nickel and iron, then [[Talc|magnesium silicates]]. Below about 700 [[kelvin]]s (700 K), [[Iron(II) sulfide|FeS]] and volatile-rich metals and silicates form a fourth group, and in the fifth group [[Iron(II) oxide|FeO]] enter the magnesium silicates.<ref name=Morgan1980>{{cite journal|last1=Morgan|first1=John W.|last2=Anders|first2=Edward|title=Chemical composition of Earth, Venus, and Mercury|journal=Proceedings of the National Academy of Sciences of the United States of America|date=December 1980|volume=77|issue=12|pages=6973&ndash;6977|jstor=9538|bibcode=1980PNAS...77.6973M|doi=10.1073/pnas.77.12.6973|pmid=16592930|pmc=350422|doi-access=free}}</ref> The compositions of the planets and the Moon are ''chondritic'', meaning that within each group the ratios between elements are the same as in carbonaceous chondrites.<ref name=McSween/>{{rp|334}}

The estimates of planetary compositions depend on the model used. In the ''equilibrium condensation'' model, each planet was formed from a ''feeding zone'' in which the compositions of solids were determined by the temperature in that zone. Thus, Mercury formed at 1400 K, where iron remained in a pure metallic form and there was little magnesium or silicon in solid form; Venus at 900 K, so all the magnesium and silicon condensed; Earth at 600 K, so it contains FeS and silicates; and Mars at 450 K, so FeO was incorporated into magnesium silicates. The greatest problem with this theory is that volatiles would not condense, so the planets would have no atmospheres and Earth no atmosphere.<ref name=McSween/>{{rp|335&ndash;336}}

In ''chondritic mixing'' models, the compositions of chondrites are used to estimate planetary compositions. For example, one model mixes two components, one with the composition of C1 chondrites and one with just the refractory components of C1 chondrites.<ref name=McSween/>{{rp|337}} In another model, the abundances of the five fractionation groups are estimated using an index element for each group. For the most refractory group, [[uranium]] is used; iron for the second; the ratios of potassium and [[thallium]] to uranium for the next two; and the molar ratio FeO/(FeO+[[Magnesium oxide|MgO]]) for the last. Using thermal and seismic models along with heat flow and density, Fe can be constrained to within 10 percent on Earth, Venus, and Mercury. U can be constrained within about 30% on Earth, but its abundance on other planets is based on "educated guesses". One difficulty with this model is that there may be significant errors in its prediction of volatile abundances because some volatiles are only partially condensed.<ref name=Morgan1980/><ref name=McSween/>{{rp|337&ndash;338}}

==Earth's crust==
{{See also|Abundance of elements in Earth's crust}}
The more common rock constituents are nearly all [[oxide]]s; [[chloride]]s, [[sulfide]]s and [[fluoride]]s are the only important exceptions to this and their total amount in any rock is usually much less than 1%. By 1911, [[Frank Wigglesworth Clarke|F. W. Clarke]] had calculated that a little more than 47% of the Earth's crust consists of [[oxygen]]. It occurs principally in combination as oxides, of which the chief are [[silica]], [[alumina]], [[iron oxide]]s, and various [[carbonate]]s ([[calcium carbonate]], [[magnesium carbonate]],  [[sodium carbonate]], and [[potassium carbonate]]). The silica functions principally as an acid, forming silicates, and all the commonest minerals of [[igneous rock]]s 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 of the Earth's crust: SiO<sub>2</sub>=59.71, Al<sub>2</sub>O<sub>3</sub>=15.41, Fe<sub>2</sub>O<sub>3</sub>=2.63, FeO=3.52, MgO=4.36, CaO=4.90, Na<sub>2</sub>O=3.55, K<sub>2</sub>O=2.80, H<sub>2</sub>O=1.52, TiO<sub>2</sub>=0.60, P<sub>2</sub>O<sub>5</sub>=0.22, (total 99.22%). All the other constituents occur only in very small quantities, usually much less than 1%.<ref name=EB1911/>

These oxides combine in a haphazard way. For example, [[potash]] (potassium carbonate) and soda ([[sodium carbonate]]) combine to produce [[feldspar]]s. In some cases, they may take other forms, such as [[nepheline]], [[leucite]], and [[muscovite]], but in the great majority of instances they are found as feldspar. [[Phosphoric acid]] with [[Lime (material)|lime]] (calcium carbonate) forms [[apatite]]. [[Titanium dioxide]] with [[ferrous oxide]] gives rise to [[ilmenite]]. Part of the lime forms lime feldspar. Magnesium carbonate and iron oxides with silica crystallize as [[olivine]] or [[enstatite]], or with alumina and lime form the complex ferromagnesian silicates of which the [[pyroxene]]s, [[amphibole]]s, and [[biotite]]s are the chief. Any excess of silica above what is required to neutralize the [[Base (chemistry)|base]]s will separate out as [[quartz]]; excess of alumina crystallizes as [[corundum]]. 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.<ref name=EB1911/>

===Mineral constitution===
Except in [[Acidic rock|acid]] or siliceous igneous rocks containing greater than 66% of [[Silicon dioxide|silica]], known as [[felsic]] rocks, quartz is not abundant in igneous rocks. In basic rocks (containing 20% of silica or less) it is rare for them to contain as much silicon, these are referred to as [[mafic]] rocks. If [[magnesium]] and [[iron]] are above average while silica is low, [[olivine]] may be expected; where silica is present in greater quantity over ferromagnesian 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 [[Alkali metal|alkalis]], soda-bearing [[pyroxenes]] and [[amphiboles]] may be present. The lower the percentage of silica and alkali's, the greater is the prevalence of [[plagioclase|plagioclase feldspar]] as contracted with soda or potash feldspar.<ref name=EB1911/>

Earth's crust is composed of 90% silicate minerals and their abundance in the Earth is as follows: [[plagioclase feldspar]] (39%), [[alkali feldspar]] (12%), quartz (12%), [[pyroxene]] (11%), [[amphiboles]] (5%), [[micas]] (5%), [[clay minerals]] (5%); the remaining silicate minerals make up another 3% of Earth's crust. Only 8% of the Earth is composed of non-silicate minerals such as [[Carbonate minerals|carbonates]], [[Oxide minerals|oxides]], and [[Sulfide minerals|sulfides]].<ref>According to [http://www.sandatlas.org/minerals/] {{Webarchive|url=https://web.archive.org/web/20140428104415/http://www.sandatlas.org/minerals/|date=2014-04-28}}, which cites this: Klein, C., Hurlbut, C. S. (1993) ''Manual of Mineralogy'', 21st Edition. John Wiley & Sons.</ref>

The other determining factor, namely the physical conditions attending consolidation, plays, on the whole, a smaller part, yet is by no means negligible. 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 [[camptonite]]s of the same locality that contain feldspar 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.<ref name=EB1911/>

===Felsic, intermediate and mafic igneous rocks===
Those rocks that contain the most silica, and on crystallizing yield free quartz, form a group generally designated the "felsic" rocks. Those again that contain the least silica and most magnesia and iron, so that quartz is absent while [[olivine]] is usually abundant, form the "mafic" 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]] 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 mafic rocks. Lastly, a small sub-group rich in olivine and without feldspar has been called the "ultramafic" rocks. They have very low percentages of silica but much iron and magnesia.

Except these last, practically all rocks contain felspars or feldspathoid minerals. In the acid rocks, the common feldspars are orthoclase, perthite, microcline, and oligoclase—all having much silica and alkalis. In the mafic rocks labradorite, anorthite, and bytownite prevail, being rich in lime and poor in silica, potash, and soda. Augite is the most common ferromagnesian in mafic rocks, but biotite and hornblende are on the whole more frequent in felsic rocks.<ref name=EB1911/>

{| class="wikitable"
!rowspan=2 | Most Common Minerals
! Felsic
!colspan=2 | Intermediate
! Mafic
! Ultramafic
|-
! Quartz <br /> Orthoclase (and Oligoclase), Mica, Hornblende, Augite
! Little or no Quartz: <br /> Orthoclase hornblende, Augite, Biotite
! Little or no Quartz: <br /> Plagioclase Hornblende, Augite, Biotite
! No Quartz <br /> Plagioclase Augite, Olivine
! No Felspar <br /> Augite, Hornblende, Olivine
|-
| Plutonic or Abyssal type
| [[Granite]] || [[Syenite]] || [[Diorite]] || [[Gabbro]] || [[Peridotite]]
|-
| Intrusive or [[Hypabyssal]] type
| [[Quartz-porphyry]] || [[Orthoclase-porphyry]] || Porphyrite || [[Dolerite]] || [[Picrite]]
|-
| Lavas or Effusive type
| [[Rhyolite]], [[Obsidian]] || [[Trachyte]] || [[Andesite]] || [[Basalt]] || [[Komatiite]]
|}

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 mafic character. We might in consequence regard them as varieties of syenite, diorite, gabbro, etc., in which feldspathoid 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 "alkali" 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.<ref name=EB1911/>

{| class="wikitable"
|+Nepheline and Leucite-bearing Rocks
|-
! Most Common Minerals
! Alkali Feldspar, Nepheline or Leucite, Augite, Hornblend, Biotite
! Soda Lime Feldspar, Nepheline or Leucite, Augite, Hornblende (Olivine)
! Nepheline or Leucite, Augite, Hornblende, Olivine
|-
| Plutonic type
| Nepheline-syenite, Leucite-syenite, Nepheline-porphyry
| Essexite and Theralite
| Ijolite and Missourite
|-
| Effusive type or Lavas
| Phonolite, Leucitophyre
| Tephrite and Basanite
| Nepheline-basalt, Leucite-basalt
|}

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, but it has grown up with the growth 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 [[Aggregate (geology)|aggregate]]s 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 monzonites between syenite and diorite, norites and hyperites between diorite and gabbro, and so on.<ref name=EB1911>{{EB1911|wstitle=Petrology |inline=1|volume=21|pages=328–333 |first=John Smith |last=Flett}}</ref>

==Trace metals in the ocean==
[[Trace metal]]s readily form [[Complex (chemistry)|complexes]] with major [[ion]]s in the ocean, including [[hydroxide]], [[carbonate]], and [[chloride]] and their chemical speciation changes depending on whether the environment is [[redox|oxidized or reduced]].<ref>{{cite journal|last1=Nameroff|first1=T|last2=Balistrieri|first2=L|last3=Murray|first3=J|title=Suboxic Trace Metal Geochemistry in the Eastern Tropic North Pacific|journal=Geochimica et Cosmochimica Acta|volume=66|issue=7|pages=1139–1158|date=2002|doi=10.1016/s0016-7037(01)00843-2|bibcode=2002GeCoA..66.1139N}}</ref>  Benjamin (2002) defines complexes of metals with more than one type of [[ligand]], other than water, as mixed-ligand-complexes. In some cases, a ligand contains more than one ''donor'' atom, forming very strong complexes, also called [[chelate]]s (the ligand is the chelator). One of the most common chelators is EDTA ([[ethylenediaminetetraacetic acid]]), which can replace six molecules of water and form strong bonds with metals that have a plus two charge.<ref name=Benjamin>{{cite book|last1=Benjamin|first1=M|title=Water Chemistry|date=2002|publisher=University of Washington|isbn=1-57766-667-4}}</ref>  With stronger complexation, lower [[Thermodynamic activity|activity]] of the free metal ion is observed. One consequence of the lower reactivity of complexed metals compared to the same concentration of free metal is that the chelation tends to stabilize metals in the aqueous solution instead of in solids.<ref name=Benjamin/>
 
[[Chemical concentration|Concentrations]] of the trace metals [[cadmium]], [[copper]], [[molybdenum]], [[manganese]], [[rhenium]], [[uranium]] and [[vanadium]] in sediments record the redox history of the oceans.<ref name=":0">{{Cite journal |last1=Ferriday |first1=Tim |last2=Montenari |first2=Michael |date=2016 |title=Chemostratigraphy and Chemofacies of Source Rock Analogues: A High-Resolution Analysis of Black Shale Successions from the Lower Silurian Formigoso Formation (Cantabrian Mountains, NW Spain) |url=https://www.sciencedirect.com/science/article/abs/pii/S2468517816300053 |journal=Stratigraphy & Timescales |volume=1 |pages=123–255 |doi=10.1016/bs.sats.2020.07.001 |s2cid=229217907 |via=Elsevier Science Direct |access-date=2022-06-20 |archive-date=2022-06-20 |archive-url=https://web.archive.org/web/20220620140355/https://www.sciencedirect.com/science/article/abs/pii/S2468517816300053 |url-status=live |url-access=subscription }}</ref> Within aquatic environments, cadmium(II) can either be in the form CdCl<sup>+</sup><sub>(aq)</sub> in [[oxic]] waters or CdS(s) in a reduced environment. Thus, higher concentrations of Cd in marine sediments may indicate low redox potential conditions in the past. For copper(II), a prevalent form is CuCl<sup>+</sup>(aq) within oxic environments and CuS(s) and Cu<sub>2</sub>S within reduced environments. The reduced seawater environment leads to two possible oxidation states of copper, Cu(I) and Cu(II).<ref name=":0" /> [[Molybdenum]] is present as the Mo(VI) oxidation state as MoO<sub>4</sub><sup>2−</sup><sub>(aq)</sub> in oxic environments. Mo(V) and Mo(IV) are present in reduced environments in the forms MoO<sub>2</sub><sup>+</sup><sub>(aq)</sub> and MoS<sub>2(s)</sub>.<ref name=":0" /> [[Rhenium]] is present as the Re(VII) oxidation state as ReO<sub>4</sub><sup>−</sup> within oxic conditions, but is reduced to Re(IV) which may form ReO<sub>2</sub> or ReS<sub>2</sub>. Uranium is in oxidation state VI in UO<sub>2</sub>(CO<sub>3</sub>)<sub>3</sub><sup>4−</sup>(aq) and is found in the reduced form UO<sub>2</sub>(s).<ref name=":0" /> [[Vanadium]] is in several forms in oxidation state V(V); HVO<sub>4</sub><sup>2−</sup> and H<sub>2</sub>VO<sub>4</sub><sup>−</sup>. Its reduced forms can include VO<sub>2</sub><sup>+</sup>, VO(OH)<sub>3</sub><sup>−</sup>, and V(OH)<sub>3</sub>.<ref name=":0" /> These relative dominance of these species depends on [[pH]].

In the water column of the ocean or deep lakes, vertical profiles of dissolved trace metals are characterized as following ''conservative–type'', ''nutrient–type'', or ''scavenged–type'' distributions. Across these three distributions, trace metals have different residence times and are used to varying extents by [[plankton]]ic microorganisms. Trace metals with conservative-type distributions have high concentrations relative to their biological use. One example of a trace metal with a conservative-type distribution is molybdenum. It has a residence time within the oceans of around 8 x 10<sup>5</sup> years and is generally present as the [[molybdate]] anion (MoO<sub>4</sub><sup>2−</sup>). Molybdenum interacts weakly with particles and displays an almost uniform vertical profile in the ocean. Relative to the abundance of molybdenum in the ocean, the amount required as a metal cofactor for [[enzyme]]s in marine [[phytoplankton]] is negligible.<ref name=Bruland>{{cite book |last1=Bruland |first1=K |last2=Lohan |first2=M |chapter= 6.02 &ndash; Controls on Trace Metals in Seawater|editor-last1=Holland |editor-first1=H.D. |editor-last2= Turekian |editor-first2=K.K. |title= Treatise on Geochemistry |volume= 6: The Oceans and Marine Geochemistry |date=2003 |pages=23–47 |doi= 10.1016/B0-08-043751-6/06105-3|bibcode=2003TrGeo...6...23B}}</ref>

Trace metals with nutrient-type distributions are strongly associated with the internal cycles of particulate organic matter, especially the assimilation by plankton. The lowest dissolved concentrations of these metals are at the surface of the ocean, where they are assimilated by [[plankton]]. As dissolution and decomposition occur at greater depths, concentrations of these trace metals increase. Residence times of these metals, such as zinc, are several thousand to one hundred thousand years. Finally, an example of a scavenged-type trace metal is [[aluminium]], which has strong interactions with particles as well as a short residence time in the ocean. The residence times of scavenged-type trace metals are around 100 to 1000 years. The concentrations of these metals are highest around bottom sediments, [[hydrothermal vent]]s, and rivers. For aluminium, atmospheric dust provides the greatest source of external inputs into the ocean.<ref name=Bruland/>

Iron and copper show hybrid distributions in the ocean. They are influenced by recycling and intense scavenging. Iron is a limiting nutrient in vast areas of the oceans and is found in high abundance along with manganese near hydrothermal vents. Here, many iron precipitates are found, mostly in the forms of iron sulfides and oxidized iron oxyhydroxide compounds. Concentrations of iron near hydrothermal vents can be up to one million times the concentrations found in the open ocean.<ref name=Bruland/>

Using electrochemical techniques, it is possible to show that bioactive trace metals (zinc, cobalt, cadmium, iron, and copper) are bound by organic ligands in surface seawater. These ligand complexes serve to lower the bioavailability of trace metals within the ocean. For example, copper, which may be toxic to open ocean phytoplankton and bacteria, can form organic complexes. The formation of these complexes reduces the concentrations of bioavailable inorganic complexes of copper that could be toxic to sea life at high concentrations. Unlike copper, zinc toxicity in marine phytoplankton is low and there is no advantage to increasing the organic binding of Zn<sup>2+</sup>. In [[high-nutrient, low-chlorophyll regions]], iron is the limiting nutrient, with the dominant species being strong organic complexes of Fe(III).<ref name=Bruland/>

==See also==
{{div col|colwidth=30em}}
* [[Biogeochemical cycle]]
* [[Geochemical cycle]]
* [[Petrology]]
* [[Radiometric dating]]
* [[Tephrochronology]]
* [[Volcanic gas]]
{{div col end}}

== References ==
{{Reflist|30em}}

== Further reading ==
{{Refbegin}}
*{{cite book|last1=Faure|first1=Gunter|author1-link=Gunter Faure|last2=Mensing|first2=Teresa M.|title=Isotopes : principles and applications|date=2005|publisher=Wiley|location=New Jersey|isbn=0471384372|edition=3rd}}
*{{cite book|editor-last1=Holland|editor-first1=H.D.|editor-last2=Turekian|editor-first2=K.K.|title=Treatise on geochemistry|date=2003|publisher=Elsevier Science|location=Oxford|isbn=978-0-08-043751-4|edition=1st}}
*{{cite book|editor-last1=Marshall|editor-first1=C.P.|editor-last2=Fairbridge|editor-first2=R.W.|title=Geochemistry|date=2006|publisher=SpringerLink|location=Berlin|isbn=1-4020-4496-8}}
*{{cite web|last1=Natural Environment Research Council|title=Geochemistry data model|url=http://www.earthdatamodels.org/designs/geochemistry_BGS.html|website=EarthDataModels.org|access-date=9 October 2017|archive-date=23 December 2014|archive-url=https://web.archive.org/web/20141223170825/http://www.earthdatamodels.org/designs/geochemistry_BGS.html|url-status=live}}
*{{cite book|last1=Rollinson|first1=Hugh R.|title=Using geochemical data : evaluation, presentation, interpretation|date=1996|publisher=Longman|location=Harlow|isbn=978-0-582-06701-1|edition=Repr.}}
*{{cite book|title= Geochemistry (Unpublished)|last= White|first= William M.|page= 1|url= https://www.geo.cornell.edu/geology/classes/geo455/Chapters.HTML|access-date= 14 March 2012|archive-date= 16 June 2014|archive-url= https://web.archive.org/web/20140616205643/http://www.geo.cornell.edu/geology/classes/geo455/Chapters.HTML|url-status= live}}
{{Refend}}

==External links==
*[http://www.Geokem.com ''The Geochemistry of Igneous Rocks''] (Gunn Interactive Ltd.)

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{{BranchesofChemistry}}

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[[Category:Geochemistry| ]]
[[Category:Earth sciences]]