minerals Article A Possible Radiation-Induced Transition from Monazite-(Ce) to Xenotime-(Y) M. Mashrur Zaman and Sytle M. Antao * Department of Geoscience, University of Calgary, Calgary, AB T2N 1N4, Canada; [email protected] * Correspondence: [email protected] Abstract: This study examines two pegmatitic monazite samples (2a and 4b, these numbers are related to a previous study) to determine their crystal chemistry and effects of internal radiation damage using synchrotron high-resolution powder X-ray diffraction and electron-probe micro-analysis. Both the huttonite and cheralite substitutions are discussed. Rietveld structure refinement of sample 2a shows three different phases [2a = monazite-(Ce), 2b = monazite-(Ce), and 2c = xenotime-(Y)] with distinct structural parameters. The changes among the unit-cell parameters between the two monazite-(Ce) phases is more pronounced in the a followed by the b and c unit-cell parameters. Sample 4a is a single-phase monazite-(Sm) that contains 0.164 apfu Th. Phase 2c with space group I41/amd arises from redistribution of La, Ce, Pr, Nd, Sm, Gd, Dy, Si, and Y atoms from those in monazite (space group P21/n). A possible cause for the phase transition from monazite-(Ce) to xenotime-(Y) is α-radiation events over a long geological time. However, other chemical processes cannot be ruled out as a cause for the transition. Keywords: monazite; xenotime; radiation damage; Rietveld refinements; crystal structure; structural variations 1. Introduction Both monazite and xenotime are phosphate minerals that contain rare-earth elements Citation: Zaman, M.M.; Antao, S.M. (REE). They have the general formula APO , where A = REE. Monazite is monoclinic A Possible Radiation-Induced 4 Transition from Monazite-(Ce) to with space group P21/n and is isostructural to huttonite, the high P—high T polymorph Xenotime-(Y). Minerals 2021, 11, 16. of ThSiO4 [1–3]. Xenotime is tetragonal with space group I41/amd, and is isostructural https://dx.doi.org/10.3390/min11 with thorite (ThSiO4), which is the low P—low T polymorph of ThSiO4. Thorite is also 010016 isostructural with zircon (ZrSiO4). Structural trends in zircon and monazite samples from various localities were recently discussed [4,5]. Received: 10 December 2020 Monazite contains light rare earth elements (LREE) and has a general formula (Ce, 3+ Accepted: 23 December 2020 La, Nd, Sm, Y, Th) PO4. The lanthanide series (Ln ) is subdivided into LREE—La to Nd, Published: 25 December 2020 middle rare earth elements (MREE)—Sm to Dy, and heavy rare earth elements (HREE)—Ho to Lu. LREE are common in the Earth’s crust compared to HREE. The ionic radii of Ln3+ Publisher’s Note: MDPI stays neu- cations decrease as the atomic number increases. This change in Ln3+ ionic radii control tral with regard to jurisdictional claims the crystal structure of REE (PO4). REE in monazite samples from different sources (e.g., in published maps and institutional pegmatites, granites, etc.) contain La, Nd, Pr, Ce, Sm, Gd, and Y [6]. Most metamorphic affiliations. monazite samples have a composition close to (Ce0.43La0.20Nd0.17)PO4 [7]. Although monazite contains Ce atoms, this is not always the dominant cation. Based on the dominant cation, samples are called monazite-(Ce), monazite-(Sm), monazite-(Nd), monazite-(La), Copyright: © 2020 by the authors. Li- etc. Monazite is an accessory mineral in intermediate to high grade metamorphic rocks, censee MDPI, Basel, Switzerland. This biotite granites, syenitic and granitic pegmatites, quartz veins, and carbonatites [8,9]. It article is an open access article distributed also occurs as a detrital mineral in placer deposits, beach sands, and river sands. under the terms and conditions of the Both monazite and xenotime contain PO4 tetrahedral groups. Monazite contains AO9 Creative Commons Attribution (CC BY) polyhedra, whereas xenotime contains AO8 polyhedra [5,10,11]. Both structures contain license (https://creativecommons.org/ alternating polyhedra and tetrahedra that form chains sharing O-O edges parallel to [001]. licenses/by/4.0/). The xenotime structure has a higher symmetry than the monazite structure (Figure1). Minerals 2021, 11, 16. https://dx.doi.org/10.3390/min11010016 https://www.mdpi.com/journal/minerals Minerals 2021, 11, x FOR PEER REVIEW 2 of 21 Minerals 2021 11 , , 16 alternating polyhedra and tetrahedra that form chains sharing O-O edges parallel to [001].2 of 19 The xenotime structure has a higher symmetry than the monazite structure (Figure 1). Figure 1. The structures of (aa)) monazitemonazite (space(space groupgroupP P2211//n) and ((b)) xenotimexenotime (space(space groupgroupI I4411//amd) projected down down [001] [001] with with unit unit cells cells outlined outlined in inblack. black. The The two twostructures structures are similar are similar and the and tran- the sition from monazite to xenotime results in a more symmetrical structure. Rotation of the PO4 tetra- transition from monazite to xenotime results in a more symmetrical structure. Rotation of the PO4 hedra (purple) in (a) gives rise to more symmetrical features in (b). Polyhedra AO9 in (a) and AO8 tetrahedra (purple) in (a) gives rise to more symmetrical features in (b). Polyhedra AO in (a) and in (b) are shown in yellow. 9 AO8 in (b) are shown in yellow. Both monazite andand xenotimexenotime experience experience internal internal radiation radiation doses doses because because they they contain con- tainsmall small amounts amounts of thorium of thorium (Th) and (Th) uranium and uran (U)ium atoms, (U) butatoms, they but do notthey carry do not any carry effects any of effectsradiation of radiation damage [ 12damage–15], but [12–15], radiation but damageradiation in damage monazite in wasmonazite reported was [ 16reported]. Monazite [16]. Monazitehas the ability has the to healability its to crystal heal its structure crystal betweenstructure 373–473 between K 373–473 [15,17]. K Another [15,17]. potentialAnother potentialreason could reason be itscould structural be its structural differences differences from that from of zircon. that of Monazite zircon. Monazite has P-O distances has P-O distancesthat are shorter that are and shorter stronger and stronger than the than Si-O the distances Si-O distances in zircon in zircon that maythat may promote promote the theresistance resistance to radiationto radiation damage damage [18 [18].]. Two Two different different monazite monazite phases phases werewere foundfound inin aa crystal [15]: [15]: phase e1 is well crystalline with trapped helium atoms that cause an increase in unit-cell parameters,parameters, whereaswhereas phase phase e2 e2 represents represents a a distorted distorted lattice, lattice, which which is referredis referred to toas “oldas “old alpha alpha recoil recoil tracks” tracks” that that is generated is generated by the by recoil the recoil atoms atoms after aafter radioactive a radioactive decay decayevent. event. Two different Two different monazite monazite phases phases f1 and f2f1 wereand f2 found were infound another in another study [ 19study]. Except [19]. Exceptfor the unit-cellfor the unit-cell parameters, parameters, in neither in of neither these two of these previous two studiesprevious [12 studies,15] were [12,15] the crystal were thestructures crystal refined.structures The refined. degree The of radiation degree of damage radiation in mineralsdamage in depends minerals mainly depends on the mainly ratio onof damagethe ratio accumulation of damage accumulation and thermal annealingand thermal rates. annealing If the recovery rates. If processes the recovery dominate, pro- cessesthe crystallinity dominate, is the preserved, crystallinity even is at preserved, a low temperature even at a [low20]. temperature [20]. This study investigates the crystal-chemical properties and effects of radiation doses in two monazite samples usingusing synchrotronsynchrotron high-resolutionhigh-resolution powder X-rayX-ray diffractiondiffraction (HRPXRD) andand electron-probe electron-probe microanalysis microanalysis (EPMA). (EPMA). Our Our sample sample 2a contains 2a contains three phases: three phases:phases 2aphases and 2b 2a are and monazite-(Ce) 2b are monazite-(Ce) and phase and 2c isphase xenotime-(Y). 2c is xenotime-(Y). The presence The ofpresence xenotime- of xenotime-(Y)(Y) may indicate may a indicate radiation-induced a radiation-induce transitiond fromtransition monazite from tomonazite xenotime. to xenotime. However, However,other chemical other processeschemical processes cannot be cannot ruled outbe ruled as a causeout as for a cause the transition. for the transition. Our sample Our sample4a is a single4a is a monazite-(Sm)single monazite-(Sm) phase. phase. Bothsamples Both samples 2a and 2a 4a and were 4a were examined examined previously previ- ouslyusing using single-crystal single-crystal X-ray X-ray diffraction diffraction (SCXRD) (SCXRD) and and they they were were called called samples samples 2 and2 and 4, 4,respectively respectively [5 ,[5,21].21]. 2. Experimental Methods 2.1. Sample Description 2.1. Sample Description Two pegmatitic monazite samples 2a (Ce-dominated) and 4a (Sm-dominated) were Two pegmatitic monazite samples 2a (Ce-dominated) and 4a (Sm-dominated) were used in this study and their description and occurrence are summarized in Table1. Frag- used in this study and their description and occurrence are summarized in Table 1. Frag- ments of monazite were separated from the two samples with a knife. The crystal fragments were examined with a stereomicroscope