Editing Thorium (section)

===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>