| Xenotime | |
|---|---|
 Xenotime withrutile | |
| General | |
| Category | Phosphate minerals |
| Formula | YPO4 |
| IMA symbol | Xtm[1] |
| Strunz classification | 8.AD.35 |
| Crystal system | Tetragonal |
| Crystal class | Dipyramidal (4/mmm) H-M symbol: (4/m) |
| Space group | I41/a |
| Identification | |
| Color | Brown, brownish yellow, gray |
| Crystal habit | Prismatic, radial aggregates, granular |
| Cleavage | Perfect [100] |
| Fracture | Uneven to splintery |
| Mohs scale hardness | 4.5 |
| Luster | Vitreous to resinous |
| Streak | Pale brown, yellowish or reddish, to white |
| Diaphaneity | Translucent to opaque |
| Specific gravity | 4.4–5.1 |
| Refractive index | 1.720–1.815 |
| Birefringence | δ = 0.096 |
| Pleochroism | Dichroic |
| Other characteristics | Not radioactive or luminescent |
| References | [2][3][4][5] |
Xenotime is arare-earthphosphate mineral, the major component of which isyttrium orthophosphate (YPO4). The phosphate ions are described by a tetrahedral shape and coordinate to the center Y3+ metal ion in a way that closely resembles the structure ofzircon (ZrSiO4).[6] It forms a solid solution series withchernovite-(Y) (YAsO4) and therefore may contain traceimpurities ofarsenic, as well assilicon dioxide andcalcium. Other iso-structural ions that undergo exchanges with PO4 are VO4 and NbO4 ions, contributing to the list of possible co-occurring elements that may be in need of separation.[7] Therare-earth elementsdysprosium,erbium,terbium andytterbium, as well as metal elements such asthorium anduranium (all replacing yttrium) are the expressive secondary components of xenotime. Due to uranium and thorium impurities, some xenotime specimens may be weakly to stronglyradioactive.Lithiophyllite,monazite andpurpurite are sometimes grouped with xenotime in the informal "anhydrous phosphates" group. Xenotime is used chiefly as a source of yttrium and heavylanthanide metals (dysprosium, ytterbium, erbium and gadolinium). Occasionally,gemstones are also cut from the finest xenotime crystals.
The namexenotime is written originallykenotime from theGreek wordskenós (κενός) 'vain' andtimē (τιμή) 'honor', akin to 'vainglory'. It was coined by French mineralogistFrançois Sulpice Beudant as a rebuke of another scientist, Swedish chemistJöns Jacob Berzelius, for the latter's premature claim to have found in the mineral a newchemical element (he named itThorium, but later understood to be a compound of a previously discovered yttrium). The criticism was blunted, as over timekenotime was misread and misprintedxenotime[2][3][5] with the error suggesting the etymologyxénos (ξένος) +timē (τιμή) as 'different honor'. Xenotime was first described for an occurrence inVest-Agder,Norway in 1824.[3]
Crystallising in thetetragonal (I41/amd)crystal system, xenotime is typically translucent to opaque (rarely transparent) in shades of brown to brownish yellow (most common) but also reddish to greenish brown and gray. Xenotime has a variablehabit: It may be prismatic (stubby or slender and elongate) with dipyramidal terminations, in radial or granular aggregates, or rosettes. A soft mineral (Mohs hardness 4.5), xenotime is—in comparison to most other translucent minerals—fairly dense, with aspecific gravity between 4.4–5.1. Itslustre, which may be vitreous to resinous, together with its crystal system, may lead to a confusion withzircon (ZrSiO4), the latter having a similar crystal structure and with which xenotime may sometimes occur.
Xenotime has two directions of perfect prismaticcleavage and itsfracture is uneven to irregular (sometimes splintery). It is considered brittle and itsstreak is white. Therefractive index of xenotime is 1.720–1.815 with abirefringence of 0.095 (uniaxial positive). Xenotime isdichroic with pink, yellow or yellowish brown seen in the extraordinary ray and brownish yellow, grayish brown or greenish brown seen in the ordinary ray. There is no reaction underultraviolet light. While xenotime may contain significant amounts of thorium or uranium, the mineral does not undergometamictization likesphene or zircon would.
Occurring as a minor accessory mineral, xenotime is found inpegmatites and otherigneous rocks, as well asgneisses rich inmica andquartz. Associated minerals includebiotite and other micas,chlorite group minerals, quartz, zircon, certainfeldspars,analcime,anatase,brookite,rutile,siderite andapatite. Xenotime is also known to bediagenetic: It may form as minute grains or as extremely thin (less than 10μ) coatings on detrital zircon grains in siliciclasticsedimentary rocks. The importance of these diagenetic xenotime deposits in theradiometric dating of sedimentary rocks is only beginning to be realized.[8] 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-Pbgeochronology techniques.[9] 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 Mesozoic age, so geological dating using xenotime in sedimentary rocks is extensive and a useful application.
Discovered in 1824, xenotime's type locality isHidra (Hitterø),Flekkefjord,Vest-Agder,Norway. Other notable localities include:Arendal andTvedestrand, Norway;Novo Horizonte, São Paulo,Novo Horizonte, Bahia andMinas Gerais,Brazil;Madagascar andCalifornia,Colorado,Georgia,North Carolina andNew Hampshire, United States. A new discovery of gemmy, colour change (brownish to yellow) xenotime has been reported fromAfghanistan and been found inPakistan. 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.[7] 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 nm. The alternative method involvesannealing of the ore followed by Cathodluminescence (CI) imaging techniques. This technique increases the intensity of only the zircon composition, allowing for ease in analysis.[10] North ofMount Funabuse inGifu Prefecture,Japan, a notablebasalticrock 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 aschrysanthemum stone (translated from theJapanese 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 Malaysiantin 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 usingsulfuric acid (H2SO4) orsodium hydroxide (NaOH), leaving behind the mixed RREs. Various techniques can be applied next to further separate the individual elements. One is the use ofion 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 asAliquat 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.[11]
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