Editing Xenotime (section)

== Occurrence ==
Occurring as a minor accessory mineral, xenotime is found in [[pegmatite]]s and other [[igneous rock]]s, as well as [[gneiss]]es rich in [[mica]] and [[quartz]]. Associated minerals include [[biotite]] and other micas, [[chlorite group]] minerals, quartz, zircon, certain [[feldspar]]s, [[analcime]], [[anatase]], [[brookite]], [[rutile]], [[siderite]] and [[apatite]]. Xenotime is also known to be [[diagenesis|diagenetic]]: It may form as minute grains or as extremely thin (less than 10 [[micrometre|μ]]) coatings on detrital zircon grains in siliciclastic [[sedimentary rock]]s. The importance of these diagenetic xenotime deposits in the [[radiometric dating]] of sedimentary rocks is only beginning to be realized.<ref>{{cite web|url=http://www.geoconferences.org/grant_reports/2002_GAC/vallini.html |title=Geoconferences (WA) Inc |access-date=January 8, 2006 |url-status=dead |archive-url=https://web.archive.org/web/20061214120750/http://www.geoconferences.org/grant_reports/2002_GAC/vallini.html |archive-date=December 14, 2006 }} Daniela Vallini</ref> The formation of uranium and lead in xenotime ores classifies xenotime as a U-Pb chronometer, meaning it can be used for geological dating using U-Th-Pb [[geochronology]] techniques.<ref>{{Cite journal |last=Rasmussen |first=Birger |date=2005-01-01 |title=Radiometric dating of sedimentary rocks: the application of diagenetic xenotime geochronology |url=https://linkinghub.elsevier.com/retrieve/pii/S0012825204000558 |journal=Earth-Science Reviews |volume=68 |issue=3 |pages=197–243 |doi=10.1016/j.earscirev.2004.05.004 |bibcode=2005ESRv...68..197R |issn=0012-8252|url-access=subscription }}</ref> The spectrometry used in geochronology necessitates larger crystals of at least 10 μm, therefore SEM imaging is applied to identify crystals that meet the appropriate dimensions. After identification, there are various spectroscopy approaches and microprobes for geochronology: SIMS, EMPA, LA-ICP-MS, and ID-TIMS. Xenotime can be found in geological formations that formed from the mid-Archean age to the Mesezoic age, so geological dating using xenotime in sedimentary rocks is extensive and a useful application.

Discovered in 1824, xenotime's type locality is [[Hidra (island)|Hidra]] (Hitterø), [[Flekkefjord]], [[Vest-Agder]], [[Norway]]. Other notable localities include: [[Arendal]] and [[Tvedestrand]], Norway; [[Novo Horizonte, São Paulo]], [[Novo Horizonte, Bahia]] and [[Minas Gerais]], [[Brazil]]; [[Madagascar]] and [[California]], [[Colorado]], [[Georgia (U.S. state)|Georgia]], [[North Carolina]] and [[New Hampshire]], [[United States]]. A new discovery of gemmy, colour change (brownish to yellow) xenotime has been reported from [[Afghanistan]] and been found in [[Pakistan]]. Due to their isostructural nature, it is common for xenotime and zircon to co-crystallize together as composites; either forming crystal twins or growths over one another.<ref name="linkinghub.elsevier.com"/> In geochemistry, it is advantageous to do on site analysis of a given ore in order to determine the identities and the percentage of its compositions. A popular method of doing so is XEOL imaging, but another method has to be applied to xenotime-zircon ores because there is no way to distinguish between the intensities and color of their respective luminescence spectra, as both have green emissions at 580&nbsp;nm. The alternative method involves [[Annealing (materials science)|annealing]] of the ore followed by Cathodluminescence (CI) imaging techniques. This technique increases the intensity of only the zircon composition, allowing for ease in analysis.<ref>{{Cite journal |last=Imashuku |first=Susumu |date=2024-07-05 |title=Distinguishing xenotime and zircon in ores and estimating the xenotime content for on-site analysis |url=https://linkinghub.elsevier.com/retrieve/pii/S1386142524003822 |journal=Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy |volume=315 |pages=124216 |doi=10.1016/j.saa.2024.124216 |pmid=38581724 |bibcode=2024AcSpA.31524216I |issn=1386-1425|doi-access=free }}</ref> North of [[Mount Funabuse]] in [[Gifu Prefecture]], [[Japan]], a notable [[basalt]]ic [[rock (geology)|rock]] is quarried at a hill called Maru-Yama: crystals of xenotime and zircon arranged in a radiating, flower-like pattern are visible in polished slices of the rock, which is known as ''[[chrysanthemum]] stone'' (translated from the [[Japanese language|Japanese]] 菊石 ''kiku-ishi''). This stone is widely appreciated in Japan for its ornamental value.

Small tonnages of xenotime sand are recovered in association with Malaysian [[tin mining]], etc. and are processed commercially. The lanthanide content is typical of "yttrium earth" minerals and runs about two-thirds yttrium, with the remainder being mostly the heavy lanthanides, where the even-numbered lanthanides (such as Gd, Dy, Er, or Yb) each being present at about the 5% level, and the odd-numbered lanthanides (such as Tb, Ho, Tm, Lu) each being present at about the 1% level. Dysprosium is usually the most abundant of the even-numbered heavies, and holmium is the most abundant of the odd-numbered heavies.  The lightest lanthanides are generally better represented in monazite while the heaviest lanthanides are in xenotime.

Xenotime ores have to undergo chemical treatments to separate the rare earth elements (RREs) that make up its composition. Firstly, leaching, or dissolving of the phosphate shell is performed using [[sulfuric acid]] (H<sub>2</sub>SO<sub>4</sub>) or [[sodium hydroxide]] (NaOH), leaving behind the mixed RREs. Various techniques can be applied next to further separate the individual elements. One is the use of [[ion exchange]] methods, which encourages different elution times for different lanthanides based on ionic bonding. The quaternary ammonium anion salt trioctyl methylammonium nitrate, or commonly referred to as [[Aliquat 336]], is used to extract the lighter REEs from the heavier REEs. Yttrium is then extracted from the heavier REEs with thiocyanate salts. The remaining heavy RREs are further separated using various treatments of Aliquat 336 and nitrate salts.<ref>{{Cite journal |last1=Xie |first1=Feng |last2=Zhang |first2=Ting An |last3=Dreisinger |first3=David |last4=Doyle |first4=Fiona |date=2014-02-01 |title=A critical review on solvent extraction of rare earths from aqueous solutions |url=https://linkinghub.elsevier.com/retrieve/pii/S0892687513003452 |journal=Minerals Engineering |volume=56 |pages=10–28 |doi=10.1016/j.mineng.2013.10.021 |bibcode=2014MiEng..56...10X |issn=0892-6875|doi-access=free }}</ref>