NORSK GEOLOGISK TIDSSKRIFT 47 DISTRIBUTION PATTERNS OF RARE EARTH ELEMENTS IN CERIUM RICH MINERALS BY BRENDA B. JENSEN

Transcription

1 NORSK GEOLOGISK TIDSSKRIFT 47 DISTRIBUTION PATTERNS OF RARE EARTH ELEMENTS IN CERIUM RICH MINERALS BY BRENDA B. JENSEN (Mineralgical-Gelgical Museum, University f Osl, Sarsgt. l, Osl 5) Abstract. Evidence is presented which suggests that certain cerium-rich minerals (in particular allanite and mnazite) cmmnly take up the larger rare earth elements with little (if any) fractinatin, and that apatite shws a fixed degree f selectivity. Intrductin Gechemical fractinatin f the larger rare earth elements (La t Gd) tends t fllw simple trends. As a result, the relative abundance f these elements in almst any rck r mineral can be pltted as a smth curve {JENSEN & BRUNFELT 1965). Analysis f the curves btained by pltting much f the available data f rare earth abundances in gelgical samples t this system {JENSEN & BRUNFELT 1965, NEUMANN et al. 1966) has led t the fllwing cnclusins: l) The take up f elements in the range La t Gd is strngly selective in many rck-frming minerals, and fllws a pattem which is predictable in terms f the inic radii f the elements invlved. It is this crystallchemical fractinatin which is prbably respnsible fr the steady enrichment f a melt in the larger rare earths - a trend frequently bserved in the early stages f magmatic differentiatin. 2) A reverse trend, als invlving strng fractinatin f the rare earths, but with prgressive relative depletin f the larger ins, takes place when an envirnment is enriched in vlatiles, alkalis, and rare earth elements. It is nt knwn whether this is anther case f crystallchemical cntrl (selective remval f the larger rare earths when

2 10 BRENDA B. JENSEN 'cerium' rich1 minerals such as allanite and mnazite begin t crystallize), r whether it is sme ther frm f chemical fractinatin, e.g. due t differing degrees f rare earth cmplexing in a melt, r differing degrees f inic migratin acrss a gechemical gradient. It is cmmnly assumed that a mineral which is 'cerium' rich is als 'cerium' selective, but this is a nn sequitur. When rare earth rich minerals begin t frm, the melt is cmmnly enriched in the larger rare earths (as a result f the strng fractinatin by earlier frmed rck-frming minerals). Crystallizatin withut fractinatin wuld thus equally well prduce minerals which are strngly enriched in the larger rare earth elements. The bject f this paper is t examine sme f the evidence available frm lcalities where the degree f selectivity may be directly assessed in rder t test the hypthesis, based primarily n theretical cnsideratins, that a number f the rare earth rich minerals are nt strngly selective in their take up f the larger rare earth elements. Such lcalities are nt, hwever, easy t find. Rare earth rich minerals cmmnly crystallize when the melt is rich in vlatiles (ften in a pegmatitic envirnment). Therefre, it is nt pssible t assume with a reasnable degree f certainty that the rck in which they ccur has crystallized as a 'clsed system'. Much f the supprting evidence in this paper is drawn frm the smewhat specialized cases f in situ replacement f ne rare earth rich mineral by anther. Theretical cnsideratins The questin f selective take up f rare earth elements during crystallizatin has already been examined in sme detail in the case f rck frming minerals (NEUMANN et al. 1966). Many f the bserved patterns in rck frming minerals can be readily explained, if it is assumed that the inic radius f the element which mst cmmnly ccupies a given lattice psitin is dse t the ptimum value fr that psitin. It is clear, fr instance, that if such is the case, it will be 1 Minerals cntaining a high prprtin f the larger rare earths are cmmnly referred t as 'cerium' rich, since, in terms f ttal abundance, cerium is the element which predminates. Use f the term des nt imply anmalus fractinatin f cerium. The relative abundance curves fr these minerals still shw maxima at lanthanum.

3 DISTRIBUTION PATTERNS OF RARE EARTH ELEMENTS 11 prgressively mre d.ifficult fr the larger rare earth ins t ccupy the relatively small sites cmmnly filled by Fe, Mg, and Ca, and their incrpratin will place the lattice under strain. Hence the ferrmagnesian and calcian rck frming minerals cmmnly fractinate the larger rare earths strngly and take up nly a small prprtin f the rare earth ins available in the melt. Certain rare earth rich minerals incrprate rare earths by substitutin in the same way as rck frming minerals. Allanite, fr example, is essentially an epidte structure in which certain f the Ca ins are replaced by rare earths, but it d.iffers frm the majrity f rck frming minerals: a) in being capable f taking up relative ly large quantities f rare earths, and b) in frequently cntaining a high percentage f the larger rare earth ins. This can be explained if it is assumed that the allanite structure is nt s 'rigid' as structures in the majrity f rck fnning minerals, i.e. there is nt ne ptimum inic radius fr ins entering the Ca lattice psitin, but a range f acceptable values. The lattice is thus capable f taking up larger ins in the Ca psitins withut strain, and nt nly accepts a relatively high percentage f the larger rare earths, but des s withut appreciable fractinatin. If this is true, then the relative abundance f larger rare earths in allanite will be determined entirely by their relative abundance in the envirnment at the time f allanite crystallizatin. In the case f minerals where cerium earths are an essential part f the structure (e.g. mnazite (Ce, La)P04), it can be pstulated that the ptimum inic radius fr the rare earth psitin in the lattice is dse t that f the mst abundant cerium earths, i.e. La and Ce. In which case it can be expected that fractinatin will be minimal amng the larger rare earths and will increase steadily twards the Lu end f the series. Since this is a cac;e f substitutin f ins which are smaller than the ptimum fr the lattice, fractinatin, even in the case f a relatively 'rigid' lattice, may well be negligible ver the first half f the series (La t Gd). Since rare earth elements smaller than Dy are virtually absent frm mnazite, it may be that in this mineral the smallest ins which can fill the La, Ce psitins withut strain t the lattice are f the size f Gd r Tb and that beynd this limit selectivity is very strng.

4 12 BRENDA B. JENSEN Plttin Methd The pltting methd used in this paper was first described by JENSEN & BRUNFELT Primary differences in the ttal abundance f the individual rare earth elements are eliminated by ratiing each analysis element by element with a reference distributin, while differences in the cncentratin f rare earths as a grup are eliminated by nrmalizing each analysis t Gd = Since the bject in this paper is t examine the degree f rare earth selectivity exhibited by certain minerals, plts have been ratied, nt t a cmmn reference distributin, but t the distributin which, in each case, is thught t have been that f the envirnment at the time f crystallizatin f the mineral. Distributin patterns in selected minerals Allanite MuRATA et al. (1957) recrded an example f allanite (in a pegmatite at Little Switzerland, N. Carlina) replacing mnazite withut fractinating the larger rare earth elements. Since a big reductin in ttal percentage f rare earths accmpanied the replacement (a reductin f tw-thirds if the replacement is assumed t have been cnstant vlume), it is clear that rare earth inic migratin ccurred relatively freely in this envirnment. The lack f fractinatin must be regarded as very strng evidence in favur f lack f selectivity in the replacing mineral (Fig. 1). Data in a paper by YEs'KOVA & GANZEYEV (1964) als supprt the suggestin that allanite frequently incrprates the larger rare earths withut appreciable fractinatin. They reprt examples frm the alkalic cmplex f the Vishnevyye muntains, in which a reactin rim f allanite arund a) bastnasite and brithlite and b) mnazite has, in each case, the same rare earth distributin pattern as the riginal mineral. This is particularly striking in the secnd case, where an inner zne f apatite, between the mnazite and allanite, has a very different pattern (Fig. 2). The suggestin that the lack f selectivity f allanite is nt a questin f cnditin f frmatin, but due primarily t structural

5 DISTRIBUTION PATTERNS OF RARE EARTH ELEMENTS ! c: ::s:.. " - c: 1.0.5! et: l l l l l l l Fig. l. Allanit rplacing mnazite in a pegmatit frm Litt! Switzrland, Nrth Carlina. l l l l l l l l La c Pr Nd Pm Sm Eu Gd l Tb 2.0! c: ::s: Gl c: ø Fig. 2. <D Apati! and allanite surunding mnazite the 'vi ds' f the Vishnevyye al kalic cmplex. in et: <D La Pr Pm Sm Gd Tb Fig.3. (!) Rati apatit/mnazite, Vis hnvyye mtns.; Ø Rati apatit/whle rck, Kirvgrad; {ID Rati apatite/whle rck (Dy: 0.83), Bnsall. Dy

6 14 BRENDA B. JENSEN prperties, is supprted by the instance just cited f apatite and allanite crystallizing tgether. These minerals have apparently frmed under identical cnditins. In bth cases, rare earths ccupy Ca lattice psitins, and yet their rare earth distributin pattems are very different (Fig. 2). The nly factr which can cause this difference is mineral structure. Since there is little difference in the c-rdinatin number f Ca in these tw minerals, the variatin cannt be explained by unequal change in the effective inic radii f calcium and rare earth ins with change in c-rdinatin number (KHOMYAKOV 1963). It seems much mre likely that it is related t the 'rigidity' r 'flexibility' f the structure, i.e. its ability fr ismrphus substitutin. In a previus paper, it was shwn that the range f rare earth distributins fund in allanites is as great, r greater, than that shwn by the cmplete range f igneus rcks {NEUMANN et al. 1966). Evidence which, althugh nt in itself cnclusive, again pints t lack f selectivity in this mineral. MuRATA et al. (1957) have reprted ne case f allanite which was apparently strngly enriched in the larger rare earth elements relative t the minerals which it replaced. Mineralgical relatinships in this example, hwever, are cmplex, and the allanite is nt nly replacing several vein minerals, but als aplitic wall rck. The sum ttal f evidence seems t be strngly in favur f a nnselective incrpratin f larger rare earth elements in the allanite lattice. Cerianite It has been shwn {JENSEN 1967) that cerianite (essentially Ce02}, which ccurs as pseudmrphs after mnazite in a pegmatite at I veland, has incrprated the larger trivalent rare earths in relatively large quantities withut fractinatin. Since quadrivalent cerium has an inic radius dse t that f gadlinium, it is necessary t pstulate here als a less 'rigid' lattice capable f expansin withut undue strain. Because f the nature f the replacement, it may be that the ceriætite (nnlike the allanite) was 'under pressure' t accept the ttal rare earth cntent f the riginal mnazite, and may nt incrprate the larger rare earth elements withut fractinatin when frmed in an envirnment f relatively free inic migratin.

7 DISTRIBUTION PATTERNS OF RARE EARTH ELEMENTS 15 Apatite Since it is virtually certain that the relative abundance f available rare earths at the time f crystallizatin f the Vishnevyye apatite was that shwn by the mnazite and allanite which frmed befre and after (p. 12), it is pssible t determine, with a fairly high degree f certainty, the selectivity f the apatite structure under these particular cnditins. An attempt t determine the degree f selectivity f apatite was made previusly using data frm the Bnsall tnalite and the Kirvgrad granite (NEUMANN et al. 1966). In spite f a) the incmpleteness f the data, which necessitated nrmalizing t Dy in sme cases and Gd in thers, and b) the need fr majr assumptins cncerning the cmpsitin f the magma at the time f apatite crystallizatin, the degree f selectivity shwn by these earlier plts is very similar t that f the Vishnevyye apatite (Fig. 3). This is particularly surprising in view f the majr differences in envirnment and the very variable cmpsitin f the minerals which frm the apatite grup. If it can be established by further studies that apatite invariably fractinates the larger rare earths t the same degree, then this prvides a means f determining the rare earth distributin f an envirnment (at a given stage f crystallizatin) which is independent f any assumptins cncerning a 'clsed system' fr that envirnment. Since apatite is f such widespread ccurrence, this culd be very valuable. Mnazite The mnazite grup f minerals apparently include a cmplete slid slutin series frm (La, Ce)P04 t CaTh(P04)2 Therefre, the mnazite lattice is unlikely t have difficulty in accepting rare earth ins with inic radii between thse f Ce and Ca, i.e. Pr t Eu. V AINSHTEIN et al. (1956) analysed a mnazite frm a pegmatite vein at Tedin, Karelia, and the allanite which it was replacing. The mnazite data, ratied t the allanite, are pltted in Fig. 4. The plt is very nearly hrizntal and dse t the base line if nrmalized t any element ther than Gd (suggesting that Gd is anmalus and that virtually n fractinatin f the larger rare earths has ccurred during this replacement). GAVRILOVA & TURANSKAYA (1958) have analysed the Kirvgrad mnazite bearing granite and all f its main mineral cnstituents. The

8 16 BRENDA B. JENSEN 2.0 l l l l l l l Fig.4. Mnazite frm Tedin, Karelia (nrmalized t Sm = 1.00) Gl c c Gl 1.0 N c ::!: a: l l l l l l l l _l La Ce Pr Nd Pm Sm Eu Gd Tb early frmed felspar shws the same relative distributin pattern fr the larger rare earths as the whle rck (Fig. 5), indicating: a) that felspar crystallizatin ccurred withut fractinatin f these elements and b) that virtually n selective rem val f larger rare earths ccurred at any stage during the crystallizatin f the rck. Crystallizatin as a 'clsed system' is als indicated by the apatitefwhle rck plt in Fig. 5, which (as explained in the previus sectin) shws the 'expected' degree f apatite selectivity. Selective take up f rare earths in apatite (and garnet) will cause steady enrichment f the melt in the larger rare earth elements, and it is t be expected that a nnselective mineral, crystallizing ver the same perid, will shw sme enrichment in the larger rare earths relative t the whle rck pattern - such as is seen in the mnazitefwhle rck plt in Fig. 5. The cnclusin, therefre, is that the mnazite here is either nn-selective, r nly very slightly selective in its take up f the larger rare earths. In the Vishnevyye muntains alkalic cmplex (p. 12) the mnazite, apatite, and allanite ccur in 'vids' frmed at a pst-magmatic stage f crystallizatin f an igneus instrusin. Mnazite frms the central cre with an intermediate zne f apatite and a peripheral zne f allanite. Allanite replacing apatite shuld shw an apatite rare earth element distributin, and it therefre seems likely that this is primarily a case f vergrwth rather than replacement. It is then neces-

9 DISTRIBUTION PATTERNS OF RARE EARTH ELEMENTS 17 -".s:. c 41 c: l. CD Fig.5. Minerals f the Kirvgrad granite CD Felspar Garne Apati te Mnazite c er La Ce Pr Nd Pm Sm Eu Gd Tb sary t pstulate that: a) mnazite has crystallized, taking up the larger earth elements frm the envirnment withut fractinatin, b) apati te crystallized frm the same envirnment, fractinating the larger rare earth elements in its nrmal fashin, and c) allanite crystallized last, again withut fractinatin f the larger rare earths.1 It has been shwn previusly that analyses f a large number f mnazites shw a range f rare earth element distributins as great as that f allanite (NEUMANN et al. 1966). This is again nt cnclusive evidence, but a further indicatin f lack f selectivity in this mineral. It cannt be determined n this evidence, whether take up f the larger rare earth elements by mnazite is slightly selective r cmpletely nn-selective. The difference is an imprtant ne, since a mineral which is slightly selective can, if it captures a sufficiently high prprtin f the ttal rare earths available in a melt, have a very marked effect n the relative abundance f larger rare earths in the residuum. Therefre, it is nt pssible t rule ut crystallchemical fractinatin as a pssible cntrlling mechanism in the 'reverse trend' f fractinatin amng the larger rare earths. 1 If the same mde f frmatin is pstulated fr the bastnasite, brithlite, allanite 'vids' frmed in the metamrphic aurele f the same cmplex, then we have sme (admittedly tentative) evidence fr lack f rare earth selectivity in bastnasite and brithlite.

10 18 BRENDA B. JENSEN Summary l. On theretical grunds, a fairly cnvincing case can be made fr the suggestin that a number f rare earth rich minerals are nt strngly selective in their take up f the larger rare earth elements. 2. In the few cases where published data yield any direct evidence f the rare earth distributin in an envirnment during frmatin f allanite and mnazite, it wuld appear that these minerals have accepted the larger rare earth elements with very little, if any, fractinatin. 3. There is reasnably strng evidence t suggest that this Jack f fractinatin is ften nt due t the cnditins f frmatin, which varied widely in the cases examined, but is primarily a matter f cntrl by the mineral lattice. 4. It seems likely that, with a little mre data, it will be pssible t establish a list f indicatr minerals, which either d nt fractinate the larger rare earths (such as allanite and mnazite), r which fractinate them t a knwn fixed degree (such as apatite). Thee culd be used t deduce the rare earth distributin in the melt at varius stages during crystallizatin, which wuld clearly be f untld value fr petrgenetic studies. AcKNOWLEDGEMENTS The authr wishes t thank Prf. H. Neumann fr his cnstructive criticism f the manuscript. The wrk was carried ut while the authr held a research fellwship granted by the Ryal Nrwegian Cuncil fr Scientific and Industrial Research. REFERENCES GAVRILOVA, L. K. & TuRANSKAYA, N. V Distributin f rare earths in rck-frming and accessry minerals f certain granites. Gechemistry (English translatin) 2, JENSEN, B. B Distributin pattems f rare earth elements in cerianite. Nrsk!J*Ol. Tidsskr. 47, 1-8. jensjtn, B. B. & BRUNFELT, A. O Distributin pattems f rare earth elements in terrestrial rcks. Nrsk gel. Tidsskr. 45:

11 DISTRIBUTION PATTERNS OF RARE EARTH ELEMENTS 19 KHOMYAKOV, A. P Relatin between cntent and cmpsitin f the rare eap:hs in minerals. GechemistYy (English translatin) 2, MuRATA, K. J., RsE, H. J., JR., CARRON, M. K. & GLASS, J. J Systematic variatin f rare-earth elements in cerium-earth minerals. Gechim. et Csmchim. Acta 11, NEUMANN, H., JENSEN, B. B. & BRUNFELT, A. O Distributin patterns f rare earth elements in minerals. NOYsk gel. Tidsskt'. 46, VAINSHTEIN, E. E., TuGARINOV, A. I. & Tti'RANSKAYA, N.V Regularities in the distributin f rare earths in certain minerals. Gechemistyy (English translatin) 2, YEs'KOVA, YE. M. & GANZEYEV, A. A Rare earth elements in accessry minerals f the Vishnev muntains. Gechemistyy (English translatin) 6, Accepted fr publicatin September 1966