Beusite-Triphylite Intergrowths from The

Canodian Mineralogist Vol. 28, pp. 133-139(1990)

BEUSITE-TRIPHYLITEINTERGROWTHS FROM THE YELLOWKNIFEPEGMATITE FIELD, NORTHWESTTERRITORIES

MICHAEL A. WISE,'.AND PETRCERNf Department ol Geological Sciences, University of Manitoba, Winnipeg, Msnitoba R3T 2N2

ABSTRAC"T INTRoDUcTIoN

Calcium-rich beusite occurs in a pegmatite from the Beusite, (Mn,Fe,Ca)3@Oa)2,is the rare man- regionally zoned Peg swarm of the Archean Yellowknife ganeseend-member of the gaftonite-bepsite series, pegmatite field, Northwest Territories. Beusite occurs as which ocsursexclusively as a primary acc€ssoryphase pale brown tabular lamellae, up to 5 x 2.5 cm x I mm in granitic pegmatites. Beus (1950) was the first to in size,interlaminated with greenishgrey triphylite lamellae. report the existenceof a graftonite-like mineral in Beusiteis monoclinic,ospacegroup P21/c, a 8.818(l)s D Brooks & 11.750(l),c 6.175(t) A, p sg.ls(t)" zort.:(t) er. which Mn exceedsFe. A decade later, ana (1960) presented chemical data on a Optical and single'crystalX-ray-diffraction studiesconfirm Shipway also the presenceof three crystallographic orientation relation- Mn-dominant graftonite-like phase. Beusite was ships between beusite and triphylite, the most cornmon given speciesstatus following thediscovery of a Mn- being [010] of bzusite parallel to nl0l of triphylite. Beusite dominant graftonite from the San Luis Province, is chemicallyhomogeneous, the Mn/(Mn + Fe) ratio vary- Argentina (Hurlbut & Aristarain 1968). Sincethen, ing slightly from 0.54 to 0.57. Calcium contents are higher beusitehas beenfound from southernFinland (Lahti than in other reported specimens,the maximum CaO con- 1981),the Cross Lake pegmatite field, Manitoba tent Yellowknife reaching 16.l wt.9o. The of the beusite CI.S. Ercit, pers. comm. 1984), the Tsaobismund elevated Ca content results in a larger D cell dimension. pegmatite, Namibia (Fransolet et al, l98A' and Beusite-triphylite intergrowths represent exsolution from pegmatite field. a high-temperature,Li,Ca-rich graftonite-like parent phase. recently in the Yellowknife

Keywords: beusite, triphylite, exsolution, Yellowknife, OCCURRENCE Northwe$t Territories. Beusite occurs in a small pegmatite dyke of the regionally zoned Peg swarm of the Archean Yel- Souuenn lowknife pegmatite field, located 75 km northeast of Yellowknife, between Upper Ross Lake and riche Nous avons trouv€ de la bdusite en calcium dans Redout Lake (Fig. 1). The pegmatitelies 3.5 km to une despegmatites de I'essaimde Peg, d'6ge arch6en,dans is lenticular, strikes la r6gion de Yellowknife (Ierritoires du Nord-Ouest). EIle the east of the Redout Granite, forme des interlaminations tabulaires brun ptle atteigtrant N-S, dips 45-70o E and cuts an interlayeredsequence 5 x 2.5 cm x I mm avec la triphylite gris verd6tre. La ef amFhibolite and granodiorite. b€usiteest monoclinique, €troupespatial P21lc, a 8.818(1), The pegmatite is of the beryl-columbite- b 11.750(l),c 6.175(l)A, B 99.35(l)", et I/631.3(l) A3. phosphate sub{ype of rare-element pegmatites' as Les donn€esoptiques et de diffraction X obtenueszur cristal classified by Cernf (1989)' and exhibits well- unique confirment la prdsencede trois relations d'orienta- developed,although not continuous, internal zonation entre b6usite et triphylite, la plus r6pandue€tant [010] tion. Much of the primary zonation is obscured by parallde de la b6usite n [l0l de la triphylite. Les cristaux units. The border zone consists of a fine- sont homogbnes;le rapport Mn(Mn+Fe) ne varie que peu, albitic plus grained muscovite t quartz assemblage;it is fol- entre 0.54 et 0.57. Les tenews en calcium sont €lev6es quarfz (usqu'd 16.190de CaO en poids) que dans les exemples lowed by a fine-grainedmisrocline perthite + signal€sauparavant, ce qui a pour effet un agrandissement + muscovite + cleavelanditewall zone. Most of the de la dimension D. L'intercroissancede b6usitea de triphy- pegnatite is composedof a coarse-grainedmicrocline Iite r&ulterait d'une er

('\S\ I[,\A\..{ ii\\.1\$\r

Ftc. l. Location map of the regionally zoned Peg swarm (modified after Meintzer 1987from Hutchinson 1955), showing westward regi6a6l 2ening (l-5) from tle Redout Granite. I Pegmatites hosted by interlayered granodiorite and amphibolite, 2 Redout ganite, 3 Burwash Formation and 4 Cameron River mafic volcanic suite. Star denotesloca- tion of beusite-bearing pegmatite.

x 5 x 3 cm nodule in blocky, pink microcline that partly altered to a fine-grained, greenishmuscovite appears to have undergone a moderate degree of that gradesinto a dark greenband, 0.3-0.5 cm wide, metasomatic alteration. The microcline has been of microscopically intermixed biotite and a berthierine-like mineral (R.E. Meintzer' pers. cornm.; Fig. 2). This aggregateh in sharp contact with the beusite-triphylite intergrowths and hosts tiny veinlets of calcite. Incipient Na-metasomatism is evident by the presence of a small patch of alluaudite-group minelsls near the beusite-triphylite intergrowth. Minor ferrisicklerite is present as a weathering product of triphylite.

PHYSIcALAND OPTIcAL PnoPrnrrss

Beusite is pale brown, displays well-developed cleavageon {010} and {100}, vitreous luster and a pale brown streak. The density, measuredon a Ber- man balance, is 3.@0(5) e/c#. The indices of refraction for beusite were measwed by the Becke line method on crushedfragments in white lieht. The 2V angle was determined from extinction curves refined from spindle-stagedata using ttre program EXCALIBR (Bloss l98l). The optical propertis and o2 density of the Yellowknife bzusiteare comparedwith cm those of beusite from three other localities in Table Frc. 2. Sketch of beusite-triphylite paragensis. Mc 1. Using the Gladstone-Dale constants of Mandar- microcline, Ms fine-grained muscovite, Bt-b biotite- ino (1981), a compatibility index of 0.0M was berthierine intergrowths, Be-tp bzusite-triphylite inter- obtained, indicating superior agreement between growths, Cal calcite, All alluaudite-group minsml. optical and chemical data and density. BEUSITE-TRJPFTYLITE INTBRCROWTIIS 135

IABLB ].. OPrICAI. PROPFRSTEA FOR BEUSIIE rROU CAI,IOMqTXFE AND TBE 3. rcM&.DTTMOIOtr DATA rcR BWSIE qIEgR ISCAIiITIS

B€uslts1 &usl.!62 lr Yollorknlf,€, N.r.T., haala Ioa AleroB, Sn Lula, Algentlna

d 1.685 1.695 t.70a hJ.L at d I al d (@Lc) (obs) (@lc) (obs) I 1.688 1.695 1.711 1,703 llx 1.700 4.293 4.29 20 1.715 L.723 L.722 3.607 130 2V(') 3.510 3.49 100 obe 45 25 25 25 220 ?.642 T31 3.130 3.13 40 2V(') etc 53 26 53 26 r02 3.O33 3.031 100 3.O21 3.01 35 131 2.995 3.000 44 2.950 Di6tarEioD t>g l>Y E>g t>g 012 2.934 2.94 25 300 2.447 D@/d' 3.600 3.698 3.202 040 2.934 2,942 2.aao 2.49 30 230 2.911 2.90a 69 2.473 2.453 Loo 22r 2.475 2.A76 70 2.450 2.446 20 ysll@lrnif€, 1 tbls studyt 2 handa, 3 au galvador, 4 los 23L 2.745 2.747 L2 ALeeE. saEp16 2-4 fr@ Su trl6 prwl'@, Argonha (Erlbur 3XX 2.7L2 q Ar1ataEln 196s) . z02 2.70L 2,70a 60 -Inall@s 2r2 of Ffhotlon Edsu!€d ln ebtte ttEht. Sa4rts 2-4 2,642 2.642 a@gued ln ua 1lgbt. 721 2.511 23r 2.494 2.502 5 311 2.415 2.4L2 2.402 240 2.395 2.402 30 132 2.375 2.472 330 2.331 2.330 11 2.303 2.30r t02 2.295 2,297 1a 2.246 2L2 2.240 2.242 45 2.264 2.273 150 2.227 432 2.224 2.222 222 2. L47 {1X 2.L27 2.130 39 410 2.L25 2.137 2.097 2.099 14 2.063 2.064 J40 2.064 2.064 1a 340 2. O39 250 2.034 2.038 2.006 2.006 t3 242 1.970 1.980 1.979 23 1.965 1.967 aa3 1.932 Y2a x.938 1.937 24 L.927 L.926 40 050 L.920 023 L.920 1.919 9 4L2 r.isr r.igr 5 r13 1.879 131 1.893 r.894 1.475 L.474 5 061 1.864 1.464 IA2 !.832 1.432 10 350 1. AO1 242 1.€03 1.802 133 1.805 260 1.746 1.785 t4 252 L.776 r.775 \4 1.753 \.744 431 L.746 4.147 31 1.733 500 L.732 L.7t2 5 440 x.731 133 L.1!7 1.716 510 L.1X3 t.712 3s1 1.640 1.640 5 0{3 1.671 1.671 43 223 1.654 1.654 T70 1.64a 1.64S 1l 402 1.640 L.632 1.635 20 !33 1.530 170 1.6x7 1.515 l0 360 1.599 T42 1.596 1.594 4L2 L.632 1.632 24 171 1.604 1.603 13 1.591 1.591 11 2?O 1.556 1.565 13 351 1.534 1.534 22 352 1.500 1.500 35 PIua 12 6ddLtional L.442 1.443 7 1ln€8. 504 4.449 1.449 L9 ra1 1.41A 1.41a 12 z3a 1.414 1.414 15 Fto. 3. Intergrowth of beusite (dark) (iebt) 461 l3a2 1.382 1A and triphylite zax L.372 1.371 31 viewed down their c axes. 533 1.354 1.35t 12 !a2 1.322 L.322 23 64X 1.3XX X.311 T15 14

tsADLB 2. mM$ll Or Unre WA FOR mAm i*lioitT"L*".!aln (1e6€)r F& la.uatron h f,trtd. sarqrl6 a(A) b(A) o(A, pl.l v(A3l lo. Optical studies of the Yellowknife beusite have 1 8.818(1) 11.75O(1) 5.175(1) 99.35(1' 631.3(1) revealed crystallographic characteristics similar to 6.?97(3) 11.75A(4) 5.17O(2) 99.31(2' 529.5(3t tlose observed by Hurlbut & Aristarain (1968). A 3 s.7a(1) 11.52(1) 5,15(1) 99.42(35) 613.7(1) thin-sestion examination shows uniform and son- 8.784(3) 11./r34(2, 6.255(1) 99.98(21 620.8(4) tinuous extinction for all beusitelamellae (0.1-1.5 1 IM itata (thla ahaly), 2 atuql€-cryahr data, yetl@knLf,o mm thick), thus indicating a sinele common crystal- (nls st' al. 1989), 3 bs Af€es, gd ble, A4qtle (tuIbut E etgtetn 1968r, 4 qbtbetlo bdalt6, (nord & MeEtd 19a7) . lographic orientation of this phase within its tabu- t36 THE CANADIAN MINERALOCIST

beusite. The contact between the two minerals is strongly undulatory. The secondorientation is much r less common: < 0.1- to 0.5-mm-widelamellae of triphylite, elongatealong [001], which alsoparallels "^ ".7^ :;.i " : "l.^. ^ the [010] of beusite,have straight planar contacts i.: with beusite. ;.:.o 11r$,r The banded intergowth grades into a medium- grained, granular intergrowth of the same phases : :i :.1':'-';*:' :lti 1>*'toward the end of the lamellae. The optical orientations within the granular intergrowth are the same ;;.;': ;.iT:;.f:: : as describedfor the lamellar intergowths (i.e., a sin- -o------o-t--- ..-to--.-"- -a? gle, uniform orientation for beusite and several "^,,-ti..I:---l orientations for triphylite). Unlike the triphylite found in the layeredintergrowth, much of the granu- : :i.i${'.;j;..#il.|,lar triphylite has undergone extensivealteration to secondaryphosphates. o...E' X-Rav Cnvsrer-locnaPrrY '-o l.ft o::j.:T'"' Beusite is monoclinic with space gtoup P2r/c. Table 2 comparesthe unit-cell dimensionsof the Yel- lowknife beusite with those of other samples.Unit- ftc. 4. Sketch of precessionphotograph showing crystal- cell dimensions were determined for both a singls lographic relationship of the third type of the beusite- crystal and powder. X-ray powder-diffraction data triphylite intergrowths. Open circles: triphylite, closed were obtained on a Philips PW l7l0 automatedpow- (Note extinction criterion in triphylite: circles: beusite. Operating conditions were as fol- for hlcO,k=2n + I are extinct). der diffractometer. lows: grap$te-monochrom atrzed CuKal radiation O 1.5z()60A); operatingvoltage, z[0kV; operating zl0 th' TTE 4. Mro (:@SITId rcR BEOSITE M MWUE current, mA; scanningspeed ?j/mln. Powder- diffraction data wer^e calibrated usiug SiOz [a 4.9145(l), c 5.4e41(3)Al as an internal standard,aud cell dimensions were refined by least-squaresusing P:OS 41.40 41.6 42.1 41.4 41.0 46.2 46.L 45.7 45.6 45.6 a modified version of the CELREF program of trgo 2.64 O.0 0.O 0.0 0.0 0.o 0.0 0.0 o.o 0.0 Appleman & Evans (1973). Table 3 comparesthe GO 4.78 16.1 15.9 14.3 15.5 0,2 0.0 0.1 0.2 0.0 diffraction data of the Yellowknife beusite and the ho 36.56 23.1 23.r 25.r 23.2 LA.7 19.0 14.3 17.7 14.4 type beusite from Argentina. FS 14.62 19.8 X9.8 19.1 L9.O 27.6 26.4 26.4 27.L 26.5 The cell dimensions of the single crystal were rour li6ii6 i66;6 i65; I6b:3 9€-- 6ri 6* 5m- ru ru- refined from the setting anglesof thirteen automat- &usik f,om1a baB€d on 8 o*yg€n 6tl@ aal tliptryu.b f,omla ically aligned intensereflestions collected on a Nico- baaed on 2(P) !F f,omla ult. let R3m automated four-circle diffractolneter. p5+ 2.005 1.99 2.oo 2.01 2.oo 2.oo 2.00 2.oo 2.oo 2.oo Graphite-monocbromatizedMoKa O 0.7107A) radi- f,g2+ 0.229 o.oo o.oo o.oo o.oo o,oo o.0o o.oo o.oo o.oo ation was used at an operating voltage and current GZi 0.293 o.9g 0.96 o.a7 0.96 o.01 o.oo o.01 o.or 0.oo of 40 kV and 40 mA, respectively. h2t 1.771 1.10 1.10 1.21 1.13 0.79 o.a2 o.80 0.78 o.al The unit-cell dimensions of the Yellowknife Fe2+ o.g99 o.94 0.93 0.90 o.9x 1.ra 1.15 1.10 1.17 r.15 beusite,particularly the D dimension,are quite differ- rour i:da m 5:nT # is 5o- M 5s w w ent from tlose of the Argentinian and synthetic sam- &cl-b: I los Al€ros, Atgdtlna (Eufbut & bigbaln 1968), ples. The anomalously large b dimension appearsto (2-5) Y€Irdkltf€, thla atudy. rlphyllte: (5-10) Y€lloslolf€, tbla atudy. -Wat-ch@L@l elyats by h. Ju Ito (Eutbut & be related to the elevated Ca content (YViseet al. &Iabraih 1968) , @posttion @ldlatd b loot dlaregErding O.L1 LtZo, 1.5 siOZ, O.A7 EAO. ilI o$€r dalys r€r€ Mde by 1e90). Ue auuoB ob Se eldrcn Dicroprob. Precession photographs of a randomly selected fragment of the beusite-triphylite intergrowth, taken lar morphological units (Fig. 3). In contrast, the with the c axis of triphylite as the precessionaxis, lamellaeof grey to greeniih grey triphylite axepresent show a third orientation relationship of the two phos- in several optical orientations, two of which could phates, in addition to those established optically. be identified. Most of the triphylite lamellae (0.5-2 Beusitereflections are oriented along preferred crys- mm thick) are elongatealong I l0], which is parallel tallographic directions of triphylite, which indicates to [010] of beusite.Morphologically, the platy lamel- that the n00l of beusite nearly parallels [110] of lae of triphy[te are parallel to the { 100} cleavageof triphylite (Fie. ). BEUSITE-TRIPHYLITE INTERGROWTHS t37

Crmurcal CoMPosrrroN tite (P,Ca), MnF, (Mn), and chromite (Fe,Mg). Specimencurrent was 0.010mA at 15 kV. Acquisi- Chemical analyseswere perfor.med on a MAC 5 tion time for each spectrum was 200 seconds.Data electron microprobe in the energy-dispersionmode. reduction was made using a modified version of The following standardswere usedfor analysis:apa- MAGIC V (Colby 1980).Electron-microprobe data

@ = o o o E ?- I J I =o d '= o F o ll.+ c c

Ftc.5.compositionsofcoexistingbeusiteandtriphylitepairsfromltheYellowknifepeenatitefield'2theLosAleros peematite (Hurlbut & Aristariin 1968), 3 tne isaotishund pegnatite (Fransolet et al. l98Q and 4 the Jussinuvori Ca, irelmatite (l,anti tSAt). a) Comparison of Mn/(Mn + Fe) values, b) comparison of compositions in terms of (Fe+Mn+Ms) and Li (at.Vo.)

..i.1.': Itrtl

FIG. 6. Graftonile-beusite compositions in terms of Ca, Fe and Mn. Dashed line denoteshypothetical limit of Ca solubility based onthe Yellowknife data. Dots: data of Hurlbut & Aristarain (1968)and Fransolet (197), open circles: tlis study' Open squares: synthetic data of Nord (1982) and Nord & Ericsson (1982). 138 THE CANADIAN MINERALOGIST

of Yellowknife beusite and triphylite are presented beusite are a direst reflection of the composition of in Table 4. Becauselithium is not detectableon the the parent mineral before exsolution. microprobe, the results listed for triphylite reflect A comparison of the Mn/(Mn + Fe) values in partial analysesonly. triphylite and beusite from four different localities The Yellowknife examples of beusite are shows that the graftonite-structured mineral tends homogeneousin composition. The range of Mn/(Mn to incorporate Mn more readily than triphylite (Fig. + Fe) valuesis very narrow, from 0.54to 0.52, and 5a). Similarly, the graftonite structure is befter suited somewhat lower than for the Argentinian examples to incorporating Ca than tlat of triphylite (Fig. 5b). (0.ffi4.72; Hurlbut & Aristarain 1968).By contrast, The compositions of the Yellowknife beusite the Ca content found in beusite from Yellowknife extend the previously experimentally determined is high. In all casesof natural beusite and terrestrial boundaries of Fe3(POn)r-Ca3(PO)2 and graftonite, someCa is present,but the beusitefrom Mnr(PO/2-Cas(POa)z solid solutions. Nord & Yellowknife contains the highest amount of Ca Ericsson(1982) and Nord (1982)determined tle max- recordedto date (16.1m.qo CaO). Unlike all other imum solubility of Ca to be about 35 atom 9o in known elamples of beusite, no Mg was detectedin Fe3@Oa)2and 15 atom aloin Mn @O)2. The low the Yellowknile sample. solubility of Ca in Mn3(PO)2 is surprising in light of tle flexible nature of the graftonite structue. D$cussroN Ideally, one third of the cation sitesare six- or seven- coordinated, depending on the presence of large In nature, graftonite-beusite commonly occurs cationssuch asMn or Ca. Calvo (1968)showed thar intergrown with a member of the triphylite- Ca2* occupies and prefers the unique seven- lithiophilite seriesor with sarcopside.Crystattization coordinated M(1) site over rhe M(2) and M(3) sites. ofgraftonite-beusite is regarded as primary, gener- However, Nord & Ericsson (1982)showed that in ally occurring during the stagesof inner intermedi- spite of the larger size of the M(l) site, the substitu- ate zone and core consolidation. The interlamina- tion of Ca for Fe in the graftonite structure takes tion has been examined by previous workers and is placeat both theM(l) andM(3) sites,with virtually considered to represent exsolution from a high- no preference. Since one third oftle cation sitesare temperature, (Li,Ca)-rich graftonite-like parent sufficiently large to accomodate the available Ca, phase. Subsequentdecrease in temperature relulted there is no obvious reason why the maximum solu- in the miexation and concentration of Li into triphy- bility of Ca should be less in beusite than in lite and Ca into beusfte. Hurlbut (1965) suggested efaftonite. In most graftonite-beusitespecimens exa- that the role of Li in the parent phase determines mined to date, the amount of Ca presentis not suffi- ttre type of intergrowth that will form. If Li is absent, cient to completely fill the M(l) site; the difference sarcopside-graftoniteintergrowths will occur; where is madeup by the incorporation of Mn or Fe. Based Li is present, graftonite will occur with triphylite- on cation abundances, the M(l) site of the lithiophilite. Hurlbut & Aristarain (1968)interpreted Yellowknife beusitemay be nearly filled with Ca, as the granular intergrowth of beusite-lithiophitirc .t Ca closely approaches 33% of the total cations. being causedby rapid 6eeling from crystal bound- Based ou our present understanding of the crystal aries, with reduced ionic mobility. chemisty of the graftonite-beusite seriesand on tfie According to published compositions of Yellowknife data, we suggestthat Ca could sonsti- gxaftonite-beusite,all terrestrial examplesof the ser- tute one third of all cations in both graftonite and ies contain someCa, tlpically between3 and 13wt.go beusite (Fig. O. The substitution may, however, be CaO. Only graftonite found in fine octa.hedrirc different in Ca-disordered phasesand in those with meteorites is known to be Ca-deficient (Olsen & Ca restrictedto the M(l) site. Fredriksson 196Q. The most prominent feature of the Yellowknife beusite is its high Ca content. A metasomatic, cation-exchange introduction of Ca AcKNowLEDGBIVGNTS into beusite might be envisaged at hydrothermal stages. However, late fluids that have locally This study was supported by a NSERC operating deposited calcite with apatite inclusions in the par- grant to P. Cern9. The sampleswere collected dur- ent pegmatite would have affected triphylite first; ing a regional study of the Yellowknife pegmatite triphylite-lithiophilite minslah are sensitive to field that was financially supported by the Tanta- flloride-, carbonate- and calcium-bearing fluids, lum Mining Corporation of Canada, Ltd. and the yielding (manganoan)apatite and calciteas alteration Geology Division, Indian and Northern Affairs of products (unpublished observationsof p. tlernf and Canada,Yellowknife. The authors thank Dr. F.C. A.-M. Fransolet). The triphylite lamellae in the Hawthorne for his helpful comments. The authors Yellowknife beusitehave not suffered this alteration, alsothank R.F. Martin, E.E. Foord and two anony- which suggeststhat the levels of Ca observedin the mous refereesfor critical reviewsof the manuscript. BEUSITE-TRIPFTYLITE INTERGROW'THS 139

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