American Mineralogist, Volume 80, pages2I-26, 1995 Crystal chemistry of the monazite and xenotime structures YuxxnNc Nr, JonN M. Hucnns Department of Geology, Miami University, Oxford, Ohio 45056' U.S.A. ANrrrotvv N. M.q'nr,lNo 48 PageBrook Road, Carlisle, Massachusetts01741' U.S.A. Arsrnlcr Monazite and xenotime, the RE(PO,) dimorphs, are the most ubiquitous rare earth (RE) minerals, yet accuratestructure studiesof the natural phaseshave not been reported. Here we report the results of high-precision structure studies of both the natural phasesand the synthetic RE(PO4)phases for all individual stable rare earth elements. Monazite is monoclinic, P2r/n, and xenotime is isostructural with zircon (spacegroup 14r/amd)- Both atomic arrangementsare basedon [001] chains of intervening phosphate tetrahedra and RE polyhedra, with a REO, polyhedron in xenotime that accommodates the heavy lanthanides(Tb-Lu in the synthetic phases)and a REO, polyhedron in monazite that preferentially incorporatesthe larger light rare earth elements(Ia-Gd). As the struc- ture "transforms" from xenotime to monazite, the crystallographic properties are com- parable along the [001] chains, with structural adjustments to the different sizes of RE atoms occurring principally in (001). There are distinct similarities betweenthe structuresthat are evident when their atomic arrangementsare projected down [001]. In that projection, the chains exist i! (100) planes, with two planes per unit cell. In monazite the planes are offset by 2.2 A along [010], relative to those in xenotime, in order to accommodate the larger light RE atoms. The shift of the planes in monazite allows the RE atom in that phaseto bond to an additional 02' atom to complete the REO' polyhedron. INrnooucrrox AND PREvrous woRK common problem of metamictization and demonstrated changesare induced upon annealingof the The rare earth (RE; here consideredas lanthanides * that structural phase. of his film study are incompatible with Y) phosphates, RE(PO"), exist in nature as the phases The results (1967) study and yield tetrahedral P-O bond monazite and xenotime; monazite preferentially incor- the Ueda long as 1.69 A, certainly unreasonable.The poratesthe larger, light rare earth elements(LREEs, here lengths as natural xenotime is poorly character- La-Gd) whereasxenotime tendsto incorporate the small- crystal structure of well. The most recent study reported an R value er, heavy rare earth elements (HREEs, here Tb-Lu, + ized as (Krstanovic, 1965), on the basis of 38 film-de- Y). The phasesare important accessoryminerals in gran- of l0% P-O bond distances that are unrea- itoids and rhyolites, and, becauseoftheir incorporation rived intensities, and of rare earth elements(REEs) and high distribution coef- sonably short. crystal-chemicalsimilarity between the ficients of REEs, they can effectively control REE distri- Becauseof the and actinide elements,monazite has bution in igneousrocks despite the fact that they usually lanthanide elements for use as a solid-state repository for occur only as an accessoryphase. A review ofthe com- been investigated waste(see, e.g., Boatner and Sales,1988). A position and geologicaloccurrence ofmonazite in the ac- radioactive portion evaluative work has involved refining the cessory mineral assemblageof granitoids was given by of this of all synthetic RE(PO.) phases save Rapp and Watson (1986). crystal structures (Mullica et al., 1985a,1985b, 1984; Milligan et Despite the fact that monazite is the most common Pm(PO.) 1983b;Beall et al., l98l). However,the REE-bearing mineral and both RE phosphatesare ubiq- al., 1982,1983a, of several parametersderived from the crystal uitous ore minerals of the REEs,the atomic arrangements variation refinements is discontinuous when plotted vs. of the natural phasesare not well characterized.The most structure number, and electron density residua of the re- recent structure studiesofnatural monazite were reported atomic >4 for severalof the structures.There by Ueda (1967) and Ghouse(1968). The former study, finementswere e/At high-precision undertaken using film data for h}l (55) and hk} (58) re- is a clear need to obtain a complete set of flections, refined to R values of I 6 and I 9ol0,respectively, structure refinements for the complete seriesof RE(PO.) for the two sets of planes, and yielded impossibly large phases (save Pm). The following reports the results of P-O distances.The work by Ghouse (1968) detailed the those studies and the first high-precision structure refine- 0003-o04x/95/0I 02{02 l $02.00 2l 22 NI ET AL.: CRYSTAL CHEMISTRY OF MONAZITE AND XENOTIME TAELE1. Crystal data and results of structure refinementsfor monazite structure phases Phases Monazite' La(PO.) Ce(PO4) P(PO.) Nd(PO4) Sm(PO.) Eu(POr) cd(Po.) Unitcells by least squares(unconst.ained) i (4) 6.7902(10) 6.8313(10) 6.7880(10) 6.7596(8) 6.70s200) 6.6818(12) 6.661q10) 6.64ss(9) o(ll 7.0203(6) 7.0705(9) 7.016q8) 6.9812(10) 6.s500(e) 6.8877(9) 6.861qs) 6.8414(10) 6.4674(7) 6.5034(9) 6.4650(7) 6.4344(9) 6.4049(8) 6.3653{9) 6.3491(8) 6.3281(6) a"(n) (') 90.007(8) 89.993(11) 89.997(9) 89.997(11) 90.004(10) 89.9s2(11) 9o.oo5(11) 9O.O1q9) p f) 103.38(1) 103.27(11 1m.4q1) lm.sql) 103.68(1) 103.S6(1) 103.96(1) 103.97q9) r (") 89.989(9) 89.981(11) 90.011(10) 89.979(10) 89.967(11) s9.979(12) 89.975(12) s9.9s3(11) No.collected 3541 1945 1910 1884 1860 't912 1797 1778 No.unique 945 891 943 861 851 g2Z 819 811 No.> 3o, 774 790 717 753 763 792 712 710 R*,e 0.012 0.014 0.011 0.012 0.010 0.013 0.012 0.011 R 0.015 0.017 0.014 0.016 0.015 0.016 0.016 0.016 R. 0.023 0.026 0.019 0.021 0.017 0.019 0.019 0.019 Largestpeaks on difterencemaps (e/A!) (+) 0.741 0.897 0.653 0.971 0.841 0.885 1.039 1.073 () 0.704 1.023 0.632 1.276 0.995 1.236 0.884 1.265 Atomicpositions REx o.28152(41 0.28't54(2) 0.28182(2) 0.28177(2) 0.28178(3) 0.281sq3) 0.28152(3) 0.281sq3) v 0.15929(4) 0.16033(2) 0.15914(2) 0.15862(3) 0.1s806(3) 0.15638(3) 0.15595(3) 0.1ss29(3) z 0.10006(4) 0.10068(2) 0.10008(2) 0.09988(2) 0.099s0(3) 0.09813(3) 0.097s7(3) 0.09695(3) B(41 0.329(4) 0.294(3) 0.431(3) 0.351(3) 0.280(3) 0.228(3) 0.1sq3) 0.306(3) Px 0.3048(2) 0.3047(1) 0.3047(1) 0.3040(1) 0.3037(1) 0.3034(2) 0.302q1) 0.3031(2) v 0.1$q2) 0.1639(1 ) 0.1635(1) 0.1630(1) 0.1626(1) 0.1618(2) 0.1615(1) 0.1612(2) z 0.6121(2) 0.6121(1) 0.6124(1) 0.6127(1) 0.6127(1 ) 0.6130(2) 0.6130(1) 0.6131(2) B (A1 0 31(2) 0.33(1) 0.51(1) 0.36(1) 0.300) 0.2s(2) 0-24(11 0.38(2) 01 x 0.2501(5) 0.250q4) 0.250q4) 0.2498(4) 0.2502(41 0.2499(s) 0.2513(4) 0.2s39(s) v 0.0068(5) o.0077(4) 0.00s5(4) 0.00s1(4) 0.0046(4) 0.0020(5) 0.0012(4) 0.0013(5) z 0.4450(5) o.4477(41 0.4458(4) 0.4441(4) 0.4430(4) 0.4405{5) 0.4409(4) 0.,1i185(5) I (41 0.63(6) 0.78(4) 0.93(4) 0.69(4) 0.67(s) 0.57(6) 0.68(5) 0.80(6) 02x 0 3814(5) 0.3799(4) 0-3811(4) 0-381q4) 0.3815(4) 0.3822(5) 0.3833(4) 0.38it7(5) v 0.3307(6) 0.3315(3) 0.3320(3) 0.3327(4) 0.3331(4) 0.3341(s) 0.3350(4) 0.$4q5) z 0.4975(6) 0.4964(4) 0.4982(4) 0.4990(4) 0.4987(4) 0.5008(s) 0.5017(4) 0.5024(5) B (A') 0.69(6) 0.61(4) 0.80(4) 0.67(4) 0.61(4) 0.57(5) 0.s2(5) 0.66(s) 03x 0.4742(61 o.4748(4) 0.474s(4) 0.4744(4) 0.4747(4) 0.4748(5) 0.4743(4) 0.4729(5) 0.1070(6) 0.1071(4) 0.1054(4) 0.1046(4) 0.1040(4) 0.1025(5) O.1O2214) 0.101 6(s) z 0.8037(6) 0.8018(4) 0.8042(4) 0.8057(4) 0.8073(4) 0.8102(5) 0.8116(4) 0.812q5) B (41 0.73(6) 0.63(4) 0.84(4) 0.63(4) 0.62(5) 0.s6(s) o.sqs) 0.62(5) o'4x 0.1274(5) 0.1277(31 0.1268{3) 0.1260(4) 0.1249(4) 0.1217(5) O12O4(4) 0.11 87(5) v 0.2153(5) 0.2168(3) 0.2164(4) 0.2150(4) 0.215q4) 0.2125(s) 0.2135(4) 0.2138(s) z 0.7104(6) 0.7101(4) 0.7108(4) 0.7120(41 o.712714) 0.7113(5) 0.7119(s) 0.7131(5) I (41 0.63(6) 0.66(4) 0.77(41 0.62(4) 0.s2(4) 0.57(5) 0.48(4) 0.67(5) .