Beusite-Triphylite Intergrowths from The
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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 zona- tion 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<solutiond partir d'un pr&urseur sem- perthite (graphic) + quartz + muscovite + blable i la graftonite, riche en Li et Ca, et stable i temp€- cleavelandite + beryl zone that hosts most of the rature 6levde. accessoryminslals. Th€s'eminerals include, in addi- tion to the beusite-triphylite intergrowths, biotite' (Traduit par la Rddaction) yellow-gteen beryl, ferrocolumbite-ferrotantalite, ferrotapiolite, almandine and pyrite. The core is dis- Mots-cl6: bdusite,triphylile, exsolution, Yellowknife, Ter- quartz -r ritoires du Nord-Ouest. continuous and consistsof coarse-gtained microcline + accessoryberyl. Replacementmusco- *Present addrssi Department of Mineral Sciences,Smith- vite units are present locally. sonian Institution, Washington, D.C. 20560,U.S.A. The beusite-triphylite intergrowths occur as a 6 133 t34 THE CANADIAN MINERALOGIST ('\S\ I[,\A\..{ ii\\.1\$\r Ftc. l. Location map of the regionally zoned Peg swarm (modified after Meintzer 1987from Hutchinson 1955), show- ing 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.