Editing Thorium (section)

==Occurrence==
{{Main|Occurrence of thorium}}

===Formation===
<sup>232</sup>Th is a primordial nuclide, having existed in its current form for over ten billion years; it was formed during the [[r-process]], which probably occurs in [[supernova]]e and [[neutron star merger]]s. These violent events scattered it across the galaxy.<ref name="Cameron">{{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 |page=121 |doi=10.1007/BF00172440 |bibcode= 1973SSRv...15..121C |s2cid=120201972 }}</ref><ref>{{cite journal |last1=Frebel |first1=Anna |last2=Beers |first2=Timothy C. |title=The formation of the heaviest elements |journal=Physics Today |date=January 2018 |volume=71 |issue=1 |pages=30–37 |doi=10.1063/pt.3.3815 |arxiv=1801.01190 |bibcode=2018PhT....71a..30F |s2cid=4295865 }}</ref> The letter "r" stands for "rapid neutron capture", and occurs in core-collapse supernovae, where heavy seed nuclei such as [[iron-56|<sup>56</sup>Fe]] rapidly capture neutrons, running up against the [[neutron drip line]], as neutrons are captured much faster than the resulting nuclides can beta decay back toward stability. Neutron capture is the only way for stars to synthesise elements beyond iron because of the increased [[Coulomb barrier]]s that make interactions between charged particles difficult at high atomic numbers and the fact that fusion beyond <sup>56</sup>Fe is [[Endothermic process|endothermic]].<ref name="nucleosynthesis">{{cite journal |last1=Roederer |first1=I. U. |last2=Kratz |first2=K.-L. |first3=A. |last3=Frebel |display-authors=3 |first4=Norbert |last4=Christlieb |first5=Bernd |last5=Pfeiffer |first6=John J. |last6=Cowan |first7=Christopher |last7=Sneden |date=2009 |title=The End of Nucleosynthesis: Production of Lead and Thorium in the Early Galaxy |journal=The Astrophysical Journal |volume=698 |issue=2 |pages=1963–1980 |doi=10.1088/0004-637X/698/2/1963 |arxiv=0904.3105 |bibcode=2009ApJ...698.1963R|s2cid=14814446 }}</ref> Because of the abrupt loss of stability past <sup>209</sup>Bi, the r-process is the only process of stellar nucleosynthesis that can create thorium and uranium; all other processes are too slow and the intermediate nuclei alpha decay before they capture enough neutrons to reach these elements.<ref name="Cameron" /><ref name="B2FH">{{cite journal |last1=Burbidge |first1=E. Margaret |last2=Burbidge |first2=G. R. |last3=Fowler |first3=William A. |last4=Hoyle |first4=F. |title=Synthesis of the Elements in Stars |journal=Reviews of Modern Physics |date=1 October 1957 |volume=29 |issue=4 |pages=547–650 |doi=10.1103/RevModPhys.29.547 |bibcode=1957RvMP...29..547B |doi-access=free }}</ref><ref>{{cite book|last=Clayton|first=D. D.|author-link=Donald D. Clayton|title=Principles of Stellar Evolution and Nucleosynthesis|url=https://archive.org/details/principlesofstel00clay|url-access=registration|publisher=McGraw-Hill Education|date=1968|pages=[https://archive.org/details/principlesofstel00clay/page/577 577–591]|isbn=978-0-226-10953-4}}</ref>

=== Abundance ===
In the universe, thorium is among the rarest of the primordial elements at rank 77th in cosmic abundance<ref name=Cameron/><ref>{{Cite web |last=Helmenstine |first=Anne |date=28 June 2022 |title=Composition of the Universe – Element Abundance |url=https://sciencenotes.org/composition-of-the-universe-element-abundance/ |access-date=13 June 2024 |website=Science Notes and Projects |language=en-US |archive-date=24 May 2024 |archive-url=https://web.archive.org/web/20240524071239/https://sciencenotes.org/composition-of-the-universe-element-abundance/ |url-status=live }}</ref> because it is one of the two elements that can be produced only in the r-process (the other being uranium), and also because it has slowly been decaying away from the moment it formed. The only primordial elements rarer than thorium are [[thulium]], [[lutetium]], tantalum, and rhenium, the odd-numbered elements just before the third peak of r-process abundances around the heavy platinum group metals, as well as uranium.<ref name="Cameron" /><ref name="nucleosynthesis" />{{efn|[[Even and odd atomic nuclei|An even number of either protons or neutrons]] generally increases nuclear stability of isotopes, compared to isotopes with odd numbers. Elements with odd atomic numbers have no more than two stable isotopes; even-numbered elements have multiple stable isotopes, with tin (element 50) having ten.<ref name="NUBASE" />}} In the distant past the abundances of thorium and uranium were enriched by the decay of plutonium and curium isotopes, and thorium was enriched relative to uranium by the decay of <sup>236</sup>U to <sup>232</sup>Th and the natural depletion of <sup>235</sup>U, but these sources have long since decayed and no longer contribute.{{sfn|Stoll|2005|p=2}}

In the Earth's crust, thorium is much more abundant: with an [[Abundance of elements in Earth's crust|abundance]] of 8.1&nbsp;g/[[tonne]], it is one of the most abundant of the heavy elements, almost as abundant as lead (13&nbsp;g/tonne) and more abundant than tin (2.1&nbsp;g/tonne).{{sfn|Greenwood|Earnshaw|1997|p=1294}} This is because thorium is likely to form oxide minerals that do not sink into the core; it is classified as a [[Goldschmidt classification|lithophile]] under the [[Goldschmidt classification]], meaning that it is generally found combined with oxygen. Common thorium compounds are also poorly soluble in water. Thus, even though the [[Refractory metals|refractory elements]] have the same relative abundances in the Earth as in the Solar System as a whole, there is more accessible thorium than heavy platinum group metals in the crust.<ref name="albarede">{{cite book |title= Geochemistry: an introduction |page= 17 |publisher= [[Cambridge University Press]] |year= 2003 |isbn= 978-0-521-89148-6 |first= F. |last= Albarède}}</ref>

[[File:Evolution of Earth's radiogenic heat-no total.svg|thumb|upright=1.25|alt=Heat produced by the decay of K-40, Th-232, U-235, U-238 within the Earth over time|The [[radiogenic heat]] from the decay of <sup>232</sup>Th (violet) is a major contributor to the [[earth's internal heat budget]]. Of the four major nuclides providing this heat, <sup>232</sup>Th has grown to provide the most heat as the other ones decayed faster than thorium.<ref name="thoruranium">{{cite journal |last1=Trenn |first1=T. J. |date=1978 |title=Thoruranium (U-236) as the extinct natural parent of thorium: The premature falsification of an essentially correct theory |journal=Annals of Science |volume=35 |issue=6 |pages=581–597 |doi=10.1080/00033797800200441}}</ref><ref>{{cite journal |last1=Diamond |first1=H. |last2=Friedman |first2=A. M. |last3=Gindler |first3=J. E. |display-authors=3 |last4=Fields |first4=P. R. |date=1956 |title=Possible Existence of Cm<sup>247</sup> or Its Daughters in Nature |journal=Physical Review |volume=105 |issue=2 |pages=679–680 |doi=10.1103/PhysRev.105.679|bibcode=1957PhRv..105..679D}}</ref><ref>{{cite journal |last1=Rao |first1=M. N. |last2=Gopalan |first2=K. |date=1973 |title=Curium-248 in the Early Solar System |journal=Nature |volume=245 |issue=5424 |pages=304–307 |doi=10.1038/245304a0|bibcode=1973Natur.245..304R|s2cid=4226393 }}</ref><ref>{{cite journal |last1=Rosenblatt |first1=D. B. |date=1953 |title=Effects of a Primeval Endowment of U<sup>236</sup> |journal=Physical Review |volume=91 |issue=6 |pages=1474–1475 |doi=10.1103/PhysRev.91.1474|bibcode=1953PhRv...91.1474R}}</ref>]]

===On Earth===
Natural thorium is usually almost pure <sup>232</sup>Th, which is the longest-lived and most stable isotope of thorium, having a half-life comparable to the age of the universe.{{sfn|Wickleder|Fourest|Dorhout|2006|pp=53–55}} Its radioactive decay is the largest single contributor to the [[Earth#Heat|Earth's internal heat]]; the other major contributors are the shorter-lived primordial radionuclides, which are <sup>238</sup>U, <sup>40</sup>K, and <sup>235</sup>U in descending order of their contribution. (At the time of the Earth's formation, <sup>40</sup>K and <sup>235</sup>U contributed much more by virtue of their short half-lives, but they have decayed more quickly, leaving the contribution from <sup>232</sup>Th and <sup>238</sup>U predominant.)<ref name="NGJuly11">{{cite journal |last1=Gando |first1=A. |last2=Gando |first2=Y. |last3=Ichimura |first3=K. |last4=Ikeda |first4=H. |last5=Inoue |first5=K. |last6=Kibe |first6=Y. |last7=Kishimoto |first7=Y. |last8=Koga |first8=M. |last9=Minekawa |first9=Y. |last10=Mitsui |first10=T. |last11=Morikawa |first11=T. |last12=Nagai |first12=N. |last13=Nakajima |first13=K. |last14=Nakamura |first14=K. |last15=Narita |first15=K. |last16=Shimizu |first16=I. |last17=Shimizu |first17=Y. |last18=Shirai |first18=J. |last19=Suekane |first19=F. |last20=Suzuki |first20=A. |last21=Takahashi |first21=H. |last22=Takahashi |first22=N. |last23=Takemoto |first23=Y. |last24=Tamae |first24=K. |last25=Watanabe |first25=H. |last26=Xu |first26=B. D. |last27=Yabumoto |first27=H. |last28=Yoshida |first28=H. |last29=Yoshida |first29=S. |last30=Enomoto |first30=S. |last31=Kozlov |first31=A. |last32=Murayama |first32=H. |last33=Grant |first33=C. |last34=Keefer |first34=G. |last35=Piepke |first35=A. |last36=Banks |first36=T. I. |last37=Bloxham |first37=T. |last38=Detwiler |first38=J. A. |last39=Freedman |first39=S. J. |last40=Fujikawa |first40=B. K. |last41=Han |first41=K. |last42=Kadel |first42=R. |last43=O'Donnell |first43=T. |last44=Steiner |first44=H. M. |last45=Dwyer |first45=D. A. |last46=McKeown |first46=R. D. |last47=Zhang |first47=C. |last48=Berger |first48=B. E. |last49=Lane |first49=C. E. |last50=Maricic |first50=J. |last51=Miletic |first51=T. |last52=Batygov |first52=M. |last53=Learned |first53=J. G. |last54=Matsuno |first54=S. |last55=Sakai |first55=M. |last56=Horton-Smith |first56=G. A. |last57=Downum |first57=K. E. |last58=Gratta |first58=G. |last59=Tolich |first59=K. |last60=Efremenko |first60=Y. |last61=Perevozchikov |first61=O. |last62=Karwowski |first62=H. J. |last63=Markoff |first63=D. M. |last64=Tornow |first64=W. |last65=Heeger |first65=K. M. |last66=Decowski |first66=M. P. |title=Partial radiogenic heat model for Earth revealed by geoneutrino measurements |journal=Nature Geoscience |date=September 2011 |volume=4 |issue=9 |pages=647–651 |doi=10.1038/ngeo1205 |bibcode=2011NatGe...4..647K |url=https://authors.library.caltech.edu/25422/ |access-date=3 February 2019 |archive-date=17 April 2023 |archive-url=https://web.archive.org/web/20230417202330/https://authors.library.caltech.edu/25422/ }}</ref> Its decay accounts for a gradual decrease of thorium content of the Earth: the planet currently has around 85% of the amount present at the formation of the Earth.<ref name="Emsley2011" /> The other natural thorium isotopes are much shorter-lived; of them, only <sup>230</sup>Th is usually detectable, occurring in [[secular equilibrium]] with its parent <sup>238</sup>U, and making up at most 0.04% of natural thorium.{{sfn|Wickleder|Fourest|Dorhout|2006|pp=53–55}}{{efn|Other isotopes may occur alongside <sup>232</sup>Th, but only in trace quantities. If the source contains no uranium, the only other thorium isotope present would be <sup>228</sup>Th, which occurs in the [[decay chain]] of <sup>232</sup>Th (the [[thorium series]]): the ratio of <sup>228</sup>Th to <sup>232</sup>Th would be under 10<sup>−10</sup>.{{sfn|Wickleder|Fourest|Dorhout|2006|pp=53–55}} If uranium is present, tiny traces of several other isotopes will also be present: <sup>231</sup>Th and <sup>227</sup>Th from the decay chain of <sup>235</sup>U (the [[actinium series]]), and slightly larger but still tiny traces of <sup>234</sup>Th and <sup>230</sup>Th from the decay chain of <sup>238</sup>U (the [[uranium series]]).{{sfn|Wickleder|Fourest|Dorhout|2006|pp=53–55}} <sup>229</sup>Th is also been produced in the decay chain of <sup>237</sup>Np (the [[neptunium series]]): all primordial <sup>237</sup>Np is [[extinct radionuclide|extinct]], but it is still produced as a result of nuclear reactions in uranium ores.<ref>{{cite journal |last1=Peppard |first1=D. F. |last2=Mason |first2=G. W. |first3=P. R. |last3=Gray |display-authors=3 |first4=J. F. |last4=Mech |date=1952 |title=Occurrence of the (4''n'' + 1) Series in Nature |journal=Journal of the American Chemical Society |volume=74 |issue=23 |pages=6081–6084 |doi=10.1021/ja01143a074 |bibcode=1952JAChS..74.6081P |url=https://digital.library.unt.edu/ark:/67531/metadc172698/ |archive-date=28 July 2019 |access-date=3 February 2019 |archive-url=https://web.archive.org/web/20190728065436/https://digital.library.unt.edu/ark:/67531/metadc172698/ |url-status=live }}</ref> <sup>229</sup>Th is mostly produced as a [[decay product|daughter]] of artificial <sup>233</sup>U made by [[neutron irradiation]] of <sup>232</sup>Th, and is extremely rare in nature.{{sfn|Wickleder|Fourest|Dorhout|2006|pp=53–55}}}}

Thorium only occurs as a minor constituent of most minerals, and was for this reason previously thought to be rare.{{sfn|Wickleder|Fourest|Dorhout|2006|pp=55–56}}<ref>{{cite report |url=http://www.atsdr.cdc.gov/tfacts147.pdf |title=Thorium |author=[[Agency for Toxic Substances and Disease Registry]] |year=2016 |access-date=30 September 2017 |archive-date=12 April 2021 |archive-url=https://web.archive.org/web/20210412041611/https://www.atsdr.cdc.gov/tfacts147.pdf }}</ref> In fact, it is the 37th most abundant element in the Earth's crust with an abundance of 12 parts per million.<ref>{{Cite book |last=Emsley |first=John |url=https://books.google.com/books?id=dGZaDwAAQBAJ&dq=%2237th+most+abundant+element%22+thorium&pg=PA547 |title=Nature's Building Blocks: An A-Z Guide to the Elements |date=25 August 2011 |publisher=Oxford University Press |isbn=978-0-19-257046-8 |language=en}}</ref> In nature, thorium occurs in the +4 oxidation state, together with uranium(IV), [[zirconium]](IV), hafnium(IV), and cerium(IV), and also with [[scandium]], [[yttrium]], and the trivalent lanthanides which have similar [[ionic radius|ionic radii]].{{sfn|Wickleder|Fourest|Dorhout|2006|pp=55–56}} Because of thorium's radioactivity, minerals containing it are often [[metamictization|metamict]] (amorphous), their crystal structure having been damaged by the alpha radiation produced by thorium.<ref name="Woodhead">{{cite journal |last1=Woodhead |first1=James A. |last2=Rossman |first2=George R. |last3=Silver |first3=Leon T. |title=The metamictization of zircon: Radiation dose-dependent structural characteristics |journal=American Mineralogist |date=1 February 1991 |volume=76 |issue=1–2 |pages=74–82 |url=https://pubs.geoscienceworld.org/msa/ammin/article-abstract/76/1-2/74/42514/The-metamictization-of-zircon-Radiation-dose |archive-date=13 April 2023 |access-date=10 October 2021 |archive-url=https://web.archive.org/web/20230413105622/https://pubs.geoscienceworld.org/msa/ammin/article-abstract/76/1-2/74/42514/The-metamictization-of-zircon-Radiation-dose |url-status=live }}</ref> An extreme example is [[ekanite]], {{chem2|(Ca,Fe,Pb)2(Th,U)Si8O20}}, which almost never occurs in nonmetamict form due to the thorium it contains.<ref name="ekanite">{{cite journal |last1=Szymanski |first1=J. T. |last2=Owens |first2=D. R. |last3=Roberts |first3=A. C. |last4=Ansell |first4=H. G. |last5=Chao |first5=George Y. |title=A mineralogical study and crystal-structure determination of nonmetamict ekanite, ThCa<sub>2</sub>Si<sub>8</sub>O<sub>20</sub> |journal=The Canadian Mineralogist |date=1 February 1982 |volume=20 |issue=1 |pages=65–75 |url=https://pubs.geoscienceworld.org/canmin/article/20/1/65/11549/A-mineralogical-study-and-crystal-structure |archive-date=24 October 2021 |access-date=10 October 2021 |archive-url=https://web.archive.org/web/20211024193133/https://pubs.geoscienceworld.org/canmin/article/20/1/65/11549/A-mineralogical-study-and-crystal-structure |url-status=live }}</ref>

[[Monazite]] (chiefly phosphates of various rare-earth elements) is the most important commercial source of thorium because it occurs in large deposits worldwide, principally in India, South Africa, Brazil, Australia, and [[Malaysia]]. It contains around 2.5% thorium on average, although some deposits may contain up to 20%.{{sfn|Wickleder|Fourest|Dorhout|2006|pp=55–56}}{{sfn|Greenwood|Earnshaw|1997|p=1255}} Monazite is a chemically unreactive mineral that is found as yellow or brown sand; its low reactivity makes it difficult to extract thorium from it.{{sfn|Wickleder|Fourest|Dorhout|2006|pp=55–56}} [[Allanite]] (chiefly silicates-hydroxides of various metals) can have 0.1–2% thorium and [[zircon]] (chiefly [[zirconium silicate]], {{chem2|ZrSiO4}}) up to 0.4% thorium.{{sfn|Wickleder|Fourest|Dorhout|2006|pp=55–56}}

Thorium dioxide occurs as the rare mineral [[thorianite]]. Due to its being isotypic with [[uranium dioxide]], these two common actinide dioxides can form solid-state solutions and the name of the mineral changes according to the {{chem2|ThO2}} content.{{sfn|Wickleder|Fourest|Dorhout|2006|pp=55–56}}{{efn|Thorianite refers to minerals with 75–100&nbsp;mol%&nbsp;{{chem2|ThO2}}; uranothorianite, 25–75&nbsp;mol%&nbsp;{{chem2|ThO2}}; thorian uraninite, 15–25&nbsp;mol%&nbsp;{{chem2|ThO2}}; [[uraninite]], 0–15&nbsp;mol%&nbsp;{{chem2|ThO2}}.{{sfn|Wickleder|Fourest|Dorhout|2006|pp=55–56}}}} [[Thorite]] (chiefly [[thorium silicate]], {{chem2|ThSiO4}}), also has a high thorium content and is the mineral in which thorium was first discovered.{{sfn|Wickleder|Fourest|Dorhout|2006|pp=55–56}} In thorium silicate minerals, the {{chem2|Th(4+)}} and {{chem2|SiO4(4-)}} ions are often replaced with {{chem2|M(3+)}} (where M = Sc, Y, or Ln) and phosphate ({{chem2|PO4(3-)}}) ions respectively.{{sfn|Wickleder|Fourest|Dorhout|2006|pp=55–56}} Because of the great insolubility of thorium dioxide, thorium does not usually spread quickly through the environment when released. The {{chem2|Th(4+)}} ion is soluble, especially in acidic soils, and in such conditions the thorium concentration can be higher.<ref name="Emsley2011">{{cite book| pages=544–548| title=Nature's building blocks: an A–Z guide to the elements|first= J.|last=Emsley|author-link=John Emsley|publisher=[[Oxford University Press]]| isbn= 978-0-19-960563-7| date=2011}}</ref>