Editing Goldschmidt classification (section)

==Siderophile elements==
{{Redirect|Siderophile|the bacteria|Siderophilic bacteria|the disease "siderophilia"|Iron overload}}
{{see also|Earth's inner core#Composition}}
[[File:Elemental abundances.svg|thumb|350px|alt=see text|Abundance ([[atom fraction]]) of the chemical elements in Earth's upper [[continental crust]] as a function of atomic number. The rarest elements in the crust (shown in yellow) are not the heaviest, but are rather the siderophile (iron-loving) elements in the Goldschmidt classification of elements. These have been depleted by being relocated into the [[Earth's core]]. Their abundance in [[meteoroid]] materials is relatively higher. Additionally, tellurium and selenium have been depleted from the crust due to formation of volatile hydrides.]]

Siderophile elements ({{ety|grc|''{{lang|grc|σίδηρος}}'' ({{Transliteration|grc|sídēros}})|iron}}) are the [[transition metals]] which tend to sink towards the core during [[planetary differentiation]], because they dissolve readily in iron either as [[solid solution]]s or in the molten state. Some sources<ref name=walker2014>Richard J. Walker (2014), [https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4128271/ "Siderophile element constraints on the origin of the Moon"], ''[[Philosophical Transactions of the Royal Society A]]'', accessed 1 December 2015.</ref> include elements which are not transition metals in their list of siderophiles, such as [[germanium]]. Other sources may also differ in their list based on the temperature being discussed{{snd}}[[niobium]], [[vanadium]], [[chromium]], and [[manganese]] may be considered siderophiles or not, depending on the assumed temperature and pressure.<ref name=nature2001>{{cite journal|last1=Ball|first1=Philip|title=Earth scientists iron out their differences|url=http://www.nature.com/news/2001/010104/full/news010104-6.html|journal=[[Nature (journal)|Nature]]|year=2001|publisher=Macmillan Publishers Limited|doi=10.1038/news010104-6|access-date=5 June 2017|url-access=subscription}}</ref> Also confusing the issue is that some elements, such as the aforementioned [[manganese]], as well as [[molybdenum]], form strong bonds with oxygen, but in the free state (as they existed on the early Earth [[Geological history of oxygen|when free oxygen did not exist]]) can mix so easily with iron that they do not concentrate in the siliceous crust, as do true lithophile elements. [[Iron]], meanwhile, is simply ''everywhere''.

The siderophile elements include the highly siderophilic [[ruthenium]], [[rhodium]], [[palladium]], [[rhenium]], [[osmium]], [[iridium]], [[platinum]], and [[gold]], the moderately siderophilic [[cobalt]] and [[nickel]], in addition to the "disputed" elements mentioned earlier{{snd}}some sources<ref name=walker2014 /> even include [[tungsten]] and [[silver]].<ref>{{cite book|last1=Ramanathan|first1=A. L.|last2=Bhattacharya|first2=Prosun|last3=Dittmar|first3=Thorsten|last4=Prasad|first4=B.|last5=Neupane|first5=B.|title=Management and Sustainable Development of Coastal Zone Environments|date=2010|publisher=Springer Science & Business Media|isbn=9789048130689|page=166|url=https://books.google.com/books?id=q1gm2YGe25wC&q=manganese+siderophile&pg=PA166|access-date=5 June 2017}}</ref>

Most siderophile elements have practically no affinity for oxygen: indeed oxides of gold are [[Chemical stability|thermodynamically unstable]]. They form stronger bonds with [[carbon]] or [[sulfur]], but even these are not strong enough to separate out with the chalcophile elements. Thus, siderophile elements are bound with iron through [[metallic bonding]] in the Earth's core, where pressures may be high enough to keep the iron solid. Manganese, iron, and molybdenum ''do'' form strong bonds with oxygen, but in the free state (as on the early Earth) can mix so easily with iron that they do not concentrate in the siliceous crust, as do true lithophile elements. However, ores of manganese are found in much the same sites as are those of aluminium and titanium, owing to manganese's great reactivity towards oxygen.

Because they are so concentrated in the dense core, siderophile elements are known for their rarity in the Earth's crust. Most of them have always been known as [[precious metal]]s because of this. Iridium is the rarest transition metal occurring within the Earth's crust, with an [[abundance by mass]] of less than one part per billion. Mineable [[Deposition (geology)|deposits]] of [[precious metals]] usually form as a result of the [[erosion]] of [[ultramafic rock]]s, but are not highly concentrated even compared to their [[Abundance of elements in Earth's crust|crustal abundances]], which are typically several orders of magnitude below their solar abundances. However, because they are concentrated in the [[Earth's mantle]] and [[Earth's core]], siderophile elements are believed to be present in the Earth as a whole (including the core) in something approaching their solar abundances.