Editing Silicon (section)

==Occurrence==
[[File:Olivine-gem7-10a.jpg|thumb|upright|Olivine]]
Silicon is the eighth most abundant element in the universe, coming after [[hydrogen]], [[helium]], [[carbon]], [[nitrogen]], [[oxygen]], [[iron]], and [[neon]]. These abundances are not replicated well on Earth due to substantial separation of the elements taking place during the formation of the [[Solar System]]. Silicon makes up 27.2% of the Earth's crust by weight, second only to oxygen at 45.5%, with which it always is associated in nature. Further fractionation took place in the formation of the Earth by [[planetary differentiation]]: [[Earth's core]], which makes up 31.5% of the mass of the Earth, has approximate composition {{chem|Fe|25|Ni|2|Co|0.1|S|3}}; the [[Mantle (geology)|mantle]] makes up 68.1% of the Earth's mass and is composed mostly of denser oxides and silicates, an example being [[olivine]], {{chem|(Mg,Fe)|2|SiO|4}}; while the lighter siliceous minerals such as [[aluminosilicate]]s rise to the surface and form the crust, making up 0.4% of the Earth's mass.{{sfn|Greenwood|Earnshaw|1997|p=329}}<ref>{{harvnb|Greenwood|Earnshaw|1997|pp=329–330}}</ref>

The crystallisation of [[igneous rock]]s from magma depends on a number of factors; among them are the chemical composition of the magma, the cooling rate, and some properties of the individual minerals to be formed, such as [[lattice energy]], melting point, and complexity of their crystal structure. As magma is cooled, [[olivine]] appears first, followed by [[pyroxene]], [[amphibole]], [[biotite]] mica, [[orthoclase feldspar]], [[muscovite mica]], [[quartz]], [[zeolite]]s, and finally, hydrothermal minerals. This sequence shows a trend toward increasingly complex silicate units with cooling, and the introduction of [[hydroxide]] and [[fluoride]] anions in addition to oxides. Many metals may substitute for silicon. After these igneous rocks undergo [[weathering]], transport, and deposition, [[sedimentary rock]]s like clay, shale, and sandstone are formed. [[Metamorphism]] also may occur at high temperatures and pressures, creating an even vaster variety of minerals.{{sfn|Greenwood|Earnshaw|1997|p=329}}

There are four sources for silicon fluxes into the ocean: chemical weathering of continental rocks, river transport, dissolution of continental terrigenous silicates, and the reaction between submarine basalts and hydrothermal fluid which release dissolved silicon. All four of these fluxes are interconnected in the ocean's biogeochemical cycle as they all were initially formed from the weathering of Earth's crust.<ref name="The World Ocean Silica Cycle">{{cite journal |last1=Tréguer |first1=Paul J. |last2=De La Rocha |first2=Christina L. |title=The World Ocean Silica Cycle |journal=Annual Review of Marine Science |date=3 January 2013 |volume=5 |issue=1 |pages=477–501 |doi=10.1146/annurev-marine-121211-172346|pmid=22809182 }}</ref>

Approximately 300–900 megatonnes of [[Dust#Atmospheric|aeolian dust]] is deposited into the world's oceans each year. Of that value, 80–240 megatonnes are in the form of particulate silicon. The total amount of particulate silicon deposition into the ocean is still less than the amount of silicon influx into the ocean via riverine transportation.<ref name="Atmospheric transport of silicon">{{cite book |last1=Tegen |first1=Ina |last2=Kohfeld |first2=Karen |title=Atmospheric transport of silicon |date=2006 |publisher=Island Press |isbn=1-59726-115-7 |pages=81–91}}</ref> Aeolian inputs of particulate lithogenic silicon into the North Atlantic and Western North Pacific oceans are the result of dust settling on the oceans from the Sahara and Gobi Desert, respectively.<ref name="The World Ocean Silica Cycle" /> Riverine transports are the major source of silicon influx into the ocean in coastal regions, while silicon deposition in the open ocean is greatly influenced by the settling of aeolian dust.<ref name="Atmospheric transport of silicon" />