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Canadian Mineralogist Vol. 16, pp.325-333 (1978)

THE TANCO PEGMATITEAT BERNICLAKE, MANITOBA X. POLLUCITE*

P. dNNNf ANDF. M. SIMPSONT Department ol Earth Sciences,University ol Manitoba, l(innipeg, Manitoba RJT 2N2

ABSTRACT grosso modo du dee!6 de substitution (Cs,Na). Les amas de pollucite sont recoup6spar des filons de Pollucite forms large lenticular bodies and pods , non-perthitique(A 0.92), in the upp€r intermediate zone of the Tanco peg- ordonn6e(/ 1.10') Oord6e localementtl' matite, in association with quafiz, microcline c6sique) et lithique violac6e QMt) ave* perthite, ( * quartz) and ambly- l6pidolite 1M. Des entrelac,sde filonnets tardifs gonite. It was formed as a late constituent of contiennent de la muscovito lithique blanche (2MJ primary zonal crystallization and shows no evidence et un peu de spodumdne,suivis d'adulaire d6sor- of metasomatic oriein. The composition of pollu- donn6e.Le dernier stade d'alt6ration est repr6sent6 cite, established chemically or estimated optically par de la kaolinite verdatre et de la montmorillonite on 148 samples, is centred at 32 wt,Vo CszO (ap- (avec ou sans calcite). nroximately PollrgAnalrr). Some pollucite bodies tend to be enriched in Na along their margins. Cs Clraduit par la R6daction) and Na are the only major alka.lis; KRb and Li are subordinate. Negative Cs/Na and positive Rb/Tl correlations are the only well-defined trends among INrnoouctroN the alkalis. Refractive indices can be roughly correlated with tle (Cs, Na) substitution. pollucite Pollucite was discovered in the Tanco peg- bodies are veined by quartz, mostly non-perthitic matite in 1960 when Dr. R. W. Hutchinson microcline (A 0.92), low albite (/ 1.10.) locally noticed peculiar physical characteristicsof what rimmed by cesian analcime, and purple lithian had been previously logged as a quartz-feldspar muscovite (2M) with (lM), A late net- intergrowth. Subsequent laboratory examination work of braided veinlets carries white lithian mus- by Dr. W. Moorhouse led to its correct identifi- covita (ZMr) and some spodumene, followed by cation 1,967).A substantial pollucite sanidine-type adularia. Greenish kaolinite and @rohberg body in established shortly montmorillonite (-r calcite) formed last in the the was alteration sequenc€. afterwards. Recent underground excavations in the western part of the pegmalile have estab- lished the shape and size of several other bodies Sorrarrtarns detected earlier by drilling. The total reserve of pollucite in the deposit exceeds350,000 tons La pollucite se pr6sente en amas lenticulaires averaging 23.3 wt.Vo CszO; this representsthe dans la pegmatite zone moyenne sup6rieure de la largest accumulation of this mineral known in Tanco, accomDagnde de quartz, microcline perthi- tique, p6talite (spodumdne quartz) et ambligonite. the world today. + (1961) Elle s'est form6e tardivement lors de la cristallisa- With the exception of Nickel's de- tion zonaire primaire et ne montre nul indice d'ori- scription, there are no mineralogical data on the gine m6tasomatique. Sa composition, 6tablie par Tanco pollucite available in the literature, and analyse chimique ou d6duite des propri6t6s optiques its paragenetic position in the pegmatite is (en de 148 6chantillons, se situe verc 32Vo poids) rather poorly understood ,(Hutchinson 1959, de CsrO, i peu prds Poll6Analrr. Certains amas Wright 1963, Crouse & Cernf 1972). Also, pollucite de ont tendance i s'enrichir en , mining and exploration at Tanco offer a rrnique bordure. en K, Rb et Li sont nettement subordon- opportunity to study compositional and altera- n6s i (Cs Na); les corr6lations Cs/Na (n6gative) * patterns pollucite three dimen- et Rb/Tl (nositive) sont les seules nettes pour les tion of bodies in m6taux alcalins. ks indices de r6fraction d6pendent sions; all extant descriptions of pollucite from individual localities are based on examination of one or two specimens.These considerations *Publication No. 26, Centre for Precambrian initiated the present study; it is based partly Studies. on a thesis by Simpson (1974), partly on the tPresent address: 75 Linden Avenue, Winnipeg, investigations of the first author before and Manitoba R3K 0M7. since the completion of this thesis. 325

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Senapr.rxo ergo ExpSRIMBNTALMETHoDS The methods of optical study, density deter' mindies, chemical analyses,and- X-ray powder yielded Underground and drill-core sampling diffraction are as described bv Cernf & Sime 148 specimens of pollucite for laboratory study. son (1977), with the only exception that the An additional 30 specimens were collected to standards used 'for determination of sommon sllmins the alteration processes.The suite of constituents were heavily spiked with cesium, 148 samples was checked for refractive indices to balance the Cs content of pollucite and its (ru) and 41 specimens were then selected for effects on determination of other elements. partial chemical analyses. Three samples con- sidered representative of pollucite with low, OccunnnNcp IN THE PBoMATTTE average, and high z were chosen for a detailed parts of mineralogical study. Pollucite is restricted to the upper the pegmatite Pollucite from Tanco is generally homo' the eastern and western 1l6nks of to, the upper geneous on a small scale. Four randomly se- where it occurs within, or adjacent (5 1). This zone lected hand specimenswere checked fot n aI 3 intermediate zone in Fig. giant-size crystals to 5 spots on their surfaces. The ranges-of n consists mainly of quartz and -tn petalite to spodu- individual samples did not exceed 0.001' the of microcline perthite, altered (cl. Cerni & equivalent of about 0.6 v'rt.Vo C*O. The only mene * qaartz, and amblygoilte are the specimens veined by late Simpson L977, Table 1 for detailed mineralogy exceptions pods alteration products; these were excluded from of all zones). Pollucite forms numerous - and several the study. Fine veining by mica and spodu- and nodules I Io 2 m in diameter pollu-cite mene proved to be much more of an obstacle large lenticular bodies that constitute 180 75 x for obiaining reliable compositional data than zone (8). The largest of these, X !2y flank of the any inhomogeneity in the pollucite itself. Seven in size, is located in the eastern bodies are found of the 41 partial analyseshad to be discarded oeernatite. and three smaller positions are projected becauseof complex contamination of the sam- ioil" *"tt"tn part; their 1. ple, and 17 other analyseswere adjusted for ad- on the section plane in Figure simple smooth iri*t.rres of spodumene or muscovite, quanti- Masses of pollucite show are anhedral tatively determined by X-ray powder diffraction. boundaries against quartz, and

LAKE LEVEL

Frffi flflTtTflil| ffi @@a(a) m ffi |d@a(4, I T'Ttv @|@&6t [F,A

Flo. 1. Longitudinal sectiotrthrough tle Tanco pecmatiq'.Most of tirc easxernpollucite body is exposedin this section; some of its boundaries shown 6ere and all ihe western pollucite bodres are projected on tlis section to show ttreir full extent.

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Fro. 2, Typical yeining pattern in the eastern pollucite body, shown here separatedfrom blocky microcline perthite (upper left) by a dark selvage of quartz. Dark veinlets of quartz, feldspars and lepidolite dissect pollucite into polygonal blocks. Faintly visible system of subparallel whitish veinlets, gently sloping to the right, carries muscovite and spodumene.The hammerheadis 20 cm long.

with respect to pdmary all other constituents of TABLEl. ATKALICoNTENTS AND n 0F THETANC0 P0LIUCI'IT zone (5). Pollucite may locally cement fractured or brecciated silicate assemblagesbut it is never Zone Sample Li20 K20 Rb20 observed # Na20 Cs20 Tl* n as a replacement. 'r.88 Veining and replacement by quartz, feldspars, R-H .09 .t2 .71 32.90 1.522 R-2-3 .06 'rt. tu .aa ,94 32.65 105 1,5224 micas and spodumene described later in-this R-3-4 ,ll .65 .09 ,79 34.10 1.524 article are widespread, though minor in volume R-4-2 no taEnq .89 3t.35 1, sl 98 R-2-6 .It t.od .J/ .86 3l .50 1.522 (Fig. 2). to These constitute the only major con- r- t-c '|I.64 .lt .97 33.04 1.5224 taminants of the bulk of the pollucite bodies; P-2-4 .ll 'I.63.87 . t5 .88 33.20 1.5225 P-4-6 .25 .t9 .74 32.00 1.5205 these were essentially meneminslslic before P-3-2 .08 1.72 .07 .72 3?.20 I.5198 P-4-4 .85 31.56 1.5i92 the onset of secondary processes. tq 834-r47. 5 r.84 .08 'I..80 31.52 1.5185 .20 ! 834-157.5 i.76 .t0 10 32.00 1.5205 CotrposltroNer, Vanresrlrry o 834-l87.5 .14 1.62'r .25 .59 3l.60 1.522 834-206.6 no .68 . 10 .73 32.40 1.5202 The variation of refractive index of pollucite M5-205.5 .23 I .46 .19 .82 32,26 1.5202 with the Cs/(Na*H:O) substitution has been M4-'165.5 'I.341.43.20 .80 3l.67 1.5206 M4-163.5 .l9 .82 32.03 I.5218 well established(e.g., Richmond & Gonyer 1938, **u39-95.8 'I .28 '1.96. 98 .17 .77 30.60 r00 1.517 Nel 1944, denf L974. Thus an optical survey UJY-OZ. ! ?l .18 .44 29.67 1.518 u39-75.5 1.73 .23 .86 31.27 r.5204 of 148 pollucite specimenswas undertaken as fB12-207.2 'I .29 .31 . l0 .68 32.57 80 '1.52041.5232 a first step in determining their compositional nB12-226.2.43 1.74 .16 .74 32.20 range. M20-179.5 .53 1.45 .24 .77 33.10 1.5238 The mean value of n is 1.5206, with a stand- M?0-181.2 .41 'IlEa?qe? 30.70 1.5202 -+- u-aul.5 .50 'I.45 . 15 .67 31.75 'r69 i.520 ard deviation 0.0013 that encompassesmost TL-19 .il .51 .22 1,22 3r.50 I.5185 plimary pollucites from other TL-20 .15 t.6t .aI .J3 30.20 47 1.5176 (Cernf TL-21 .21 r.48 .20 .85 33.60 107 1.5228 1974, Fis, L). Extreme values are 1.517 TL-84 .16 1.48 .16 .89 33.60 120 1.s22 anld t.524. Frequency distributions of z values TL-85 .12 1.3r .25 .94 32.80 108 1.5212 E TL-86 .14 1.48 .l9 I.16 32.20 169 1.5208 in o eastern and western bodies are very similar CsA-5 .12 1.60 .22 .20 31.40 1.5194 gaussian .'17 and close to when combined. 7 CsA-8 1.70 .?6 .19 30.90 I .5198 Chemical analyses of 37 specimens yield a CsA-3 .18 1.70 .20 .51 3l.50 1.5214 csA-7 .12 l.!! .tf, .16 32.30 1.5234 mean of 31.96 wI.Vo CsrO, with standard devia- llEx-c . t 3 1.48 .21 .81 31.20 I .5196 tion :L .96 and extreme values of 29.67 alid l,lEx-D .18 1.42 .20 1.06 32.30 I .5214 34.10 (fable 1). This mean is also close to the Analysts: R.M. Hi'11and R. Chapman(Dept. Earth Sciences" Univ. of ltlanitoba) CssO content most frequently quoted for pri- 'lhallium mary pollucites. However, frequency distribu- * detenninationsby C. thllaire (Laboratoirede GeochimieAnalytique, Ecole Polytechnique,Montreal) ; tions of CsrO in the eastern and western bodies data glven in ppn. and in all the bodies combined are asvmmetric. * Full analysesof these specinEnsarc given in Table 2.

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MTNSRAI-ooy on TrnBn SPEcIMENS

Tbree specimens representative of pollucites low, intermediate, and high n were selected AA with for a detailed study. The data obtained are 33 .t,l ^ .o listed in Table 2. a t The crystal chemistry of these pollucites wt% aa a'a CsaO b. correspondsto the general characteristicsof the - a- tat ta' as derived by Beger (1969) and con- a species 3t ^ firmed bv eernf (1974). The Si/Al ratio is typically higher than 2.00, and the charge balanceAl3+/( ++2R2+) deviatesfrom the ideal value of 1.00 only within the limits of analytical error. The sum of Cs * HrO should equal 1.00 but is somewhat lower, possibly due to incom- n plete dehydration of pollucite on heating FIc. 3. Plot of wt.VoCszO against refractive indices (Fleischer& Ksanda 1940, Barrer & McCallum n of the Tanco pollucites; dots, easternbody; 1951).The unit-cell dimensionsshow no appre- triangles,w€stern bodies. Distinct positive corre- ciable variation with the changing cationic con- lation with considerablelateral .The tent as they are significantly influenced only cross indicatesstandard errors of compositional by the Si/Al ratio. and optical data. Generally, the relationships between different crystallochemicaland physical propertiescorre- skewed towards high values. Correlation of n spond to those establishedearlier (Cenf 1974, with Cs"O shows that the refractive indices can Figs. 3,4,6 and 7). The only noteworthy devia- be used as an approximate measure of the Cs tion within individual sets of otherwise well- content of Tanco pollucites, despite consider- balanced properties is a difference between the able transversal scatter (Fig. 3). measured and calculated densities. This varies Optical data were used in assessingpossible between 0.03 and 0.04. with the measured regularities in spatial distribution of composi- values invariably higher than the calculated data; tional variations. No meaningful patterns were a reverse relationship is much more common. found in horizontal sections. Several vertical This puzzling discrepancy will be investigated profiles through the eastern body reveal Cs/Na in a crystallochemical study of pollucite in ratios decreasingalong, or close to, its margins' progress. However, this pattern is not persistent,and some Figure 4 shows relationships between pairs profiles show a rather erratic variation. of alkali elements.based on data from Table 1.

TABU 2. CHgitCAr @mosntd Am ft|tStCA! PRoPERTIIS0F nruE m[ltrllt sA]fLES 812-n7.2 [39-95,8 S12-46.2 812-207.2 sr02 47.@ 46.09 47.42 sl 2.161 2.112 '15.70 Ala% I 6.02 AI .836 .884 FuZo: .01 .01 .01 }!0 .(tr3 .@3 .01 X etr, 2.997 2.W 3.007 C!0 .04 .0t .06 sr/Ar 2.58 2.t9 2.51 Lt20 .8 .43 .a Ca .589 .6A .530 &zo 1.98 1.74 I .30 M .173 .156 .l 14 t0 .1? .16 .10 LI .05t .079 .053 sao .77 .74 s ,02. ,022 .0m Cs20 30.60 32.m 32.57 X .ol0 .009 .006 H2s Ca .@e - .@3 Hao- .(E .04 .04 ilg .003

99.54 9.53 99.87 X (n"rnzt) .g47 .89g .a29 D @6. (e/@:)r 2.s 2.8 2.W o @lc. (s/@r) 2.81 2.85 2.83 H2o .33 .27 .21 .92 .90 .84 f r.5r7 r.5204 1.5232 Ca+H20 A^(l) 13.571(r) 13.67211l 13.672(l) Poll+ 69.6 70.2 75.7 ' vil3t z5so.76(43) zs5s.57(43) 2555.57(43) cnk 73.2 73.7 n.7 A@l$t3. ( nelll, R.lt. Hlll, and i. Ch!@n (DoPt. [!rth scls@s, Unlv. of ilanltoba). A&lc @!on!9 ba6odon r 3ubcsll ,tti 7 orygoa ln lnhydrcB fo@18. . AEAgs of 3 to 4 9nln9 of !6lyz€d dtoial. F Aveago of.nllyzod @terl!I, ,lth f1!.t!a!to6 e ! 0.001. a poll . (cs r'100)/(ca { Ll i t|! r Rb+ K). * cRr ! [(ca + nb+ () r loo]/(ca + Rb+ ( ] Ll ] tu).

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351-(b) I 34f I 33 CsaO cgo "f -l rzl- ^ wf.70 wf.% -.

r3o 1401.50 160 t.701.80 r90 200 Noa0,wt%

.T(c)

K2o *f wr.% t "'" '1...

20 .40 .60 .80 1.00 t.20 40 60 80 t@ t20 r40 l@ Rb2O,wt. % Tl, ppm Frc. 4. Major and minor alkalis in the Tanco pollucite. The CseO/Na2O and CszO/Rb2O correlations are poorly defined, and the KrO/RbrO plot seems to be inconclusive, whereas the Rb,/Tl diagram shows good positive alignment of data. Symbols as in Figure 3.

The plot of CsrO vr. Na2O displays the expected Coarse veins negative correlation but also a wide scatter of Coarse veins, 5 to 40 mm across, fill in points, attributable to either variable contents polygonal patterns as shown in Figures of other alkalis, to slight variation in the Si/Al 1 and 5a. Microcline is usually confined to the ratio which regulates the alkali or to total, marginal parts of pollucite bodies, whereas al- relatively high standard deviation in the cesium bite and quartz penetrate farther. Telescoping determination. RbzO shows a poor positive of these three minerals along a single fracture correlation with CszO,in accordancewith other is common. Columnar spodumeneis a rare con- geochemicaland crystallochemicaldata on these $titutent of some of the quartz veins. Albite is alkalis (e.9., Stavrov 1963). Surprisingly, the I(zO v^r.RbzO diagram shows a random scatter TABLE3. PROPERTIESOF MICROCLINE predominant of data, unless we accept the hori- NaZo KZO Rbr0 Csro Cao Pbo zontal belt of points as indicative of variable 't Rb at a constant K content. In contrast, the PtlI-l 7 .77 13.002.19 .24 .07 .002 Rb/Tl ratio shows a well-defined correlation, PMI-I9 1.08 13.632.42 .28 ,17 .00r with individual values ranging from 82 to 63. PMI-20 .2s I3.00 3.6I .21 .004 .001 These are the lowest found in the silicates of the Tanco pegmatite (dernf, unpubl. data) but obliquity* (l l ): .92 (.86 - .e5) still lie within the range of 300-65 given by (AbrAn)r.** (ll ): n (7-0) de Albuquerque & Shaw (1972) for terrestrial igneous rocks. U.c.d'sof PMI-19: dmeas. 8.594(4)R o( 90033.7(r.S) ALTERATIoN PRocEssEs (upr"d.*t*) s.4sff p I 1so56.6('r.6) As in most other localities, the pollucite b l 2.e5r (3)A 87053.'l(1.7) bodies in the Tanco deposit underwent several r (2)fi V stages of low-temperature hydrothermal altera- c 7.211 721.3(3)83 tion. Three consecutive alteration stages may be n1nr.mb"" distinguished: (i) coarse polygonal veining by of samples): arithrnetic mean(range). microcline, albite, quartz and lepidolite; (i1) **by fine braided veining which carries spodumene, x-r"y diffraction (0rv'llle ]967); othenrlseas under*. (iii) lithian muscovite and adularia, and replace- *n*f"* ment by kaolinite and montmorillonite (:L ste$art t Wright(1974, Fig. 1). quattz, calcite).

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Frc. 5. Late veining in the Tanco pollucite: (a) dark nonperthitic microcline in whitish pollucile, locally with a black halo of adularia aird clay minerals (polished hand specimen); (b) dark veinlets of mus- covite and spodumene penetrating clear pollucite 6hin section, polarized light); (c) white microcline corroded by- grey aduliria along contacis with isotropic pollucite (thin section, crossed polars); (d) grey fine-giaiol"d-adol-iu on whlte coarsely granular microcline and along white veinlets of muscovite in isotropic pollucite (1hin section, crossed polars).

occasionally accompanied by manganotantalite 1.13o, indicating a well-ordered structure (Smith or wodginite. Lepidolite penetrates those parts 1974, Fig. 7-43). of pollucite bodies located close to lepidolite Pollucite in the immediate vicinity of the massesof unit (9). Where associatedwith the albite veinlets occasionally shows changes in feldspars, lepidolite is always of later origin, X-ray powder diffraction intensities, and re- filling central parts or margins of the feldspar fractive indices as lolY as 1.500. Thase proper- veins. ties indicate cation exchangeor recrystallization Compositional and physical characteristicsof into cesian analcime eernf 1974, Figs. l,- 8), the microcline are given in Table 3; represent- due to the action of albite-producing solution$ ative veinlets are shown in Figures 5a, c. The from the adjacent veins. eiratic variation in the Na:O content found by Lepidolite consists of a mixture of 2Mt and chemical analyses seems to be caused by vari- l,M polytypes;-riassive, it is deep reddish purple, fine able contamination of the analyzed material by flaked to with r-efractiveindices in the granular albite. The nonperthitic nature of this cleavageplane between 1:570 and 1.580.These microcline, and the rather low and uniform properties match those of massive lithian mus- (9) (Rinaldi er at' (Ab + An) content determined by X-ray dif- covite + lepidolite of zone fraction support this explanation. 1.972, samplesR7, R9, REN5). constant - I.529 Albite shows an almost d F in e-braid.ed. v eining on (001) fragments; this corresponds to Ano-r (Morse 1968). Its average obliquity A network of subparallel, locally braided value is 1.10'. with variations from 1.05o to veinlets of white muscovite with spodumene

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penetrates most bodies of pollucite (Fig. 5b). matites has been discussed in literature since Its orientation shows no relation to tle bound- the recognition of pollucite as a mineral species. aries of polygonal blocks framed by the pre- Amongst modern authors Dymkov (1953), ceding vein type, and it often holds for several Quensel (1956), Ginzburg (e.e., 1960) and Solo- metres s/ithout significant change. The mica- dov (1960) advosate a late metasomatic origin ceous veinlets occasionally penetrate and cross- whereas Beus (1960), Melentyev (1961), Sy- cut the feldspar veins. mons (1961), Cooper (1964) and Heinrich (1976) Lath-shaped crystals of spodumene are irre- consider pollucite as a primary constituent of gularly dispersed or clustered in the mica vein- central parts of pegmatite bodies. lets, and are partly replaced by them. Spodu- In the Tanco pegmatite, Hutchinson (1959) mene may repre$ent early inclusions in pollu- and Wright (1963) interpreted pollucite bodies cite, connected later (as centres of mechanical as late metasomatic features, in a close spatial weakness) by the braided fracture system in- relationship with the obviously metasomatic vaded by mica-generatingsolutions. This spodu- lepidolite units (9), developed at the expense mene, however, has nevet been found outside of microcline-rich zone (6). Wright also observed the micaceous veinlets, and most of it is pollucite replacing spodumeneon a microscopic oriented subparallel to their course. Thus it scale. As previously indicated '(Crouse & most probably represents an early replacement eern.i 1,972),the evidenceis questicnable; more product that preceded muscovitization. recent work supports the primary nature of Muscovite yields X-ray powder diffraction pollucite. patterns of. thie 2Mt polytype, and the average The distribution of lepidolite units generally z on its cleavage flakes varies between about follows, though not closely, that of the pollucite 1.600 and 1.590. This indicates that the second- bodies. Large segmentsof pollucite segregations ary mica yaries between typical and somewhat are not underlain by lepidolite, and some lepi- lithian muscovite in composition. dolite units lack ttre overlying pollucite. Lepi- The muscovite veining was locally followed dolite and pollucite bodies rarely come into by a second generation of K-feldspar. Fine- direct contact; they are usually separated by a grained adularia replaces microcline of the quartz selvage, and lepidolite veins commonly coarse vein system and pollucite along their criss-crosspollucite. Lepidolite bodies are found contacts, and it also grows from the margins mainly withiu the K-feldspar-rich central inter- of nearby muscovite veinlets into the pollucite mediate zone (6) and along the lower margins (Figs. 5c, d). Gandolfi X-ray diffraction photo- of the upper intermediate zone (5), whereas grapbs show the adularia to be of a high sani- pollucite bodies are confined to the latter. Thus dine structural state. the differences in distribution and relative age rule against cornmon origin. Argillic replacement The pollucite-like mineral observed by Clay minerals occur in two major forms. Wright (1963) as a replacement of spodumene Most abundant are white spheresup to 3 mm in is most probably cesian analcime, a late hydro- size, randomly disseminated through the mus- thermal alteration product (eernf 1972), It was covite-veined pollucite. These consists of dusty recently found to vein and replace "clouds" producing faint 14A and 7A reflections and . Primary pollucite has never in Gandolfi photographs. Restricted to the mar- been observedto replace petalite or spodumene gins of pollucite bodies are apple-greenpatches * quartz aggregates formed by its retrograde and veinlets of illite, montmorillonite and kao- thermal breakdown. linite, determined by optical, X-ray powder dif- The evidence pointing to the primary nature fraction and differential thermal methods of the Tanco pollucite may be summarized as (dernf 1978). Larger pods of the green clay follows: (i) Pollucite occurs in lenticular, loaf- minerals usually contain some quartz and cal- shaped or ellipsoidal bodies, within or in direct cite. Partial chemical analysis of the green clay, contact with the upper intermediate zone (5). in part contaminated by residual pollucite, gave This zone consistsof giant-size petalite, ambly- csso 5.00, cao 4.95, Kro 4.94, NazO 1.92, gonite and microcline perthite imbedded in MgO 1.76, Rb,O 0.18 and LizO 0.09 wt,Vo. abundant quartz. Smdl rounded blebs of pollu- cite n'float" in quartz among crystals of the GBNnrrc CoNspsRATroNs above-mentioned minerals without any textural indication of a late origin. (ii) Simple outlines position pollucite The ol in the crystallization and rather straight boundaries of even the larg- sequence est pollucite bodies show no indication of cross- The timing of pollucite crystallization in peg- cutting or metasomatis relationships with other

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minerals or assemblages. Pollucite is anhedrai physical properties of the natural members of against other minerals but does not replace the analcime-pollucite series is in progress, un- them. Petalite adjacent to ttre pollucite bodies der the guidance of the first author. New finds locally contains round grains of pollucite, regu- of pollucite-like minerals should be checked for larly distributed, and somewhat graphic in out- physical properties and thoroughly examine{ line. This may suggest a simultaneous crystal- when sodium-rich compositions are indicated. Iization of both minerals. (iii) In close vicinity Laboratory synthesis of pollucite has been to the albitized zone (6), pollucite is veined by successful over a wide range in temperature albite containing manganotantalite and wd- (Barrer & McCallum 1953, Barrer et al. L953, ginite. Fine-flaked lithian mussovite and lepi- Plyushev 1959, Kume & Koizumi 1965, Kopp dolite filling fractures in pollucite show the & Clark 1966, Richerson & Hummel 1972, Suito same physical properties as the micas in lepi et al. 1974, Martin & Lagache 1975). The dolite units (9). Thus, both well-established stability field, however, has never been estab- metasomatis events in the pegmatitq the lished for either pure Cs-pollucite or Na-bear- (Ta,Be,Sn,Zr,HD enricbment associated with ing phases comparable to natural minerals. albitization and the lepidolite units, postdate Similarly, pollucite crystallization in simplified the crystallization of pollucite. lithium pegmatite systems has never been simu- lated experimentally. Research projects into C ompositional v ariations these aspects are highly desirable. Most of ttre Tanco pollucite is rather uniform ia ssmposition but a considerablepercentage of samples covers a range of about PollegAnalrr - AcrNowr,epcEMENTS PollzoAnalgo Na-rich compositions occasionally This study was supported by National Re- pol- tend to be located close to the margins of search Council of Canada Operating Grants to lucite bodies; otherwise tleir distribution is the first author and to Dr. R. B. Ferguson, by rather erratic. Available evidence suggests that the Department of Earth Sciences,Univenity of at least some of the variation in Cs,/Na comes Manitoba, and by the D. E. M. R. Research from alkali exchange along feldspar-bearing Agreements awarded to the first author h 1973 presence primary veins; the and extent of varia- and 1974. The authors wish to thank the man- present. tions cannot be evaluated at agement of the Tanco mine for their coopera- Late alterations tion and hospitality, in particular Messrs. C. T. Williams and J. C. Hayward. Mr. R. A. Crouse yeining parts High density sf in the marginal provided the section of the pegmatite. Thanks pollucite se- of bodies, and a roughly zoned are also due to K. Ramlal, R. M. Hill and R. quence products of alteratio,n from the margins Chaprnan of Earth Sciences,University inward of @ept. suggestan external source alteration of Manitoba) and C. Dallaire (Laboratoire de fluids. principal veining The and replacemenl G6ochimie Analytique, Ecole Polytechnique, Tanco pollucite are assemblagesfound in the Montr6al) for carrying out the chemical those from remarkably similar to described analysesand to Dr. J. Ito (JamesFranck Insti- other ogcurrences (Ginzburg 1946, Quensel tute, University of Chicago) fol helpful advice. 1956, Neuvonen & Vesasalo 1960, Melentyev 1961). Albite, lepidolite and late muscovite (oogilbertite","oncosine") veining is most wide- REF'ERENcES spread. K-feldspar veins seem to be typical of Tanco, whereas veinlets of fine spodumene and BARRER,R. M., BevNneu, J. W. & McCu-r-uu, N. (1953): chemistry of silicates V. spodumene-bearing symplectites with other Hydrothermal Compounds structurally related to analcite. J. phases seem to be more characteristic of other Chem. Soc. Lond., 40354047. localities. somewhat varied but recurrent The & McCer,r,urvr, N. (1951): Intraoystalline pattern of pollucite alteration at many localities water in pollucite. Nature L67, L077. position occupied by this stresses the fixed & - (1953): Hydrothermal chemistry mineral in the crystallization sequenceof com- of silicates. IV. and alumino- plex granitic pegmatites and the qualitatively silicates. l. Chem. Soc. Lond, 4029-4035. uniform character of low-temperature hydro- Brcnn, R. M. (1969): The and thermal processes. chemical composition of pollucite. Z. Krist..Lz9, 280-302. SuccnsrtoNs non Futuns RESEARcH Bsus, A. A. (1960): Geochemistry ol Beryllium and Genetic Types of Beryllium Deposits. Tzdalel. A review of the compositional variations and Akad. Nauk S.S.S.R., Moscow (in Russ.).

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dERNi,, P. (1972): The Tanco pegmatite at Bernic NeL, H. J. (1944): Pollucite from Karibib, South Lake, Manitoba. VItr. Secondary minerals from West Africa. Anter. Mineral. 29, 443-452. the spodumene-rich zones. Can. Mineral. tl, NEwoNE\ K. J. & VEsAsALo,A. (1960): Pollucite 7t4-726. from Luolamiki, Somero, Finland. Comm. G6ol. (1974): The present status of the analcime- Finlande Bull. 188, 133-148. pollucite series. Can. Mineral. L2, 334-341. NIcKEL, E. H. (1961): The mineralogy of the (1978): Alteration of pollucite in some Bernic Lake pegmatite, southeastern Manitoba. pegmatites of southeastern Manitoba. Can. Mines Branch Tech. Bull. 20. Mineral. 16, 89-95. Onvu.rr, P. M. (1967): Unit-ceU parameters of the & SrrursoN, F. M. (1977): The Tanco peg- microcline-low albite and the sanidine-high al- matite at Bernic Lake, Manitoba, D(. Bervl. bite solid solution serbs. Arner. Mineral. 52, Can, Mineral. t5, 489-499, 55-86. Cooren, D. G. (1964): The geology of the Bikita Plvusurv, V. E. (1959): High-temperature syn- Fgmatite. In Tbe Geology of Some Ore Deposits- thesis of pollucite. Zap, Vses. Mineral. Obshchest. i! S,outhern Africa, 2 (S. H. Haughton, ed.), 88, 152-156 (in Russ.). Geol. Soc, S. Africa, Johannesburg. QUENSEL,P. (1956): The paragenesis of the Varu- CRousE, R. & eERNi, p. (1972): The Tanco peg- triisk pegmatite, including a review of its mineral matite at Bernic Lake, Manitoba. I. Geology and assemblage.Ark. Mineral. Geol. 2, 9-125. paragenesis. Can. MineraL ll, 591-608. Rrcrcnsox, D. W. & HUMMEL, F. A. (1972): Syn- on Ar,ruqunnqus, C. A. R. & Sruw, D. M. (1972): thesis and thermal expansion of polycrystalline Thallium. /n Handbook of Geochemistry IIl8, cesium minerals, J. Amer, Ceram. Soc. 55,269- Springer-Verlag, Berlin. 273. DyMKov. Yu. M. (1953): Morphology of pollucite RIcHrv{oND,W. E. & GoNven, F. A. (1938): On deposits and their genesis. Trudy Mineral. Mu- pollucite. Amer. Mineral, 23, 783-789. seya Akad. Naa& ^!.J.J.R. 5, (in 132-L45 Russ.). RrNar,or, R., dpnNt', P. & FsncusoN, R. B. (19721: FLETScHER,M. & Ksaxte, C. J. (1940): Dehydra- The Tanco pegmatite at Bernic Lake, Manitoba. tion of pollucite. Amer. Mineral. 25, 666-6i2. VI. Lithium-rubidium-cesium micas. Can. Mineral Fnounrno, M. H. (1967): Presidential address. Can. 11,690-707. Mineral 9, 278-281, SrnarsoN, F. M. (1974): The Mineralogy ol Pollu- Grxzoung A. I. (1946): Pollucite in pegmatites of cite and Beryl lrom the Tanco Pegmatite at the Kalbin Range (eastern Kazakhstan). Dokt. Bernic Lake, Manitoba. M.Sc. thesis, Univ. Mani- Akad. Nauk S.S.S.R. 52, 335-377 (in Russ.). toba, Winnipeg, Man. (1960): Specific geochemical features of Surnr, J. Y. (1974): Feldspar Minerals. I. Crystal tlre pegmatitic process.Rep. 21st Int. Geol. Cong, Structure and Physical Properties. Springer-Ver- t7, ItL-tzl. Iag, Berlin. HntNnrcn, E. W. (1976). A comparison of three Sorooov, N. A. (1960): Dstribution of alkali metals major lithium pegmatites: Varutriisk, Bikita, and and beryllium in minerals of a zoned pegmatite Bernic Lake. U.S. Geol. Surv. prof, pap. t008, of the Mongolian Altai. Geochem. 874-885. 50-54. Sravnov, O. D. (1963): Principal features of geo- HurcruNsorrr, R. W. (1959): Geology of the Mont_ chemistry of Li, Rb, and Cs in the process of gary pegmatite. Econ. Geol. 84, ISZS-L542. consolidation of granites and associated p"g- Korr, O. C. & Cum, G. W. (1966): matites. Geol, Mestorozhd. Red. Elementov 21. Synthetic (in pollucites in the system Cs2O.AlrOs.4SiO, _ 1-141 Russ.). CsrO.Fe2O..4SiOe - H2O - their phase relation- SrEwARr, D. B. & Wnrcrrr, T. L. (1974): AllSi ships and physical properties: disCussion.,4.rzer. order and symmetry of natural feldspars, and Mineral. 51, 1243-1245. the relationship of strained cell parameters to composition. ,loc. Mindral. Crist. BuIl. Kuvre, S. & Korzurur, M. (1965): Synthetic pollu- bulk frang. cites in the system CsrO.AlrOs.4SiO, 97,356-377. Cs2O.psrQr.{SiO, - HrO - their phase relation- Surro, K., Lacevr, A. & Irvaue, J. T. (1974): Sta- ship and physical properties. Amer. Mineral. 80. bilit6 des solutions solides de la s6rie pollucite- 587-592. leucite sous une pression d'eau de 30 kb, C. R. Acad. Sci. Paris 278(D), 2397-2400. MARrnr, R. F. & Lacacua, M. (1975): Cell edges and infrared spectra of synthetic leucites and SvuoNs, R. (1961): Operation of Bikita Minerals pollucites in the system KAlSirO. - RbAlSirO6 _ (Private), Ltd., Southern Rhodesia. Bull. Inst. CsAlSizOu. Can. Mineral, 18, 275-28L. Mining MeL 661, 1.29-172. Mnr.nNrvnv, G. B. (1961): The first finding of WRIcHT, C. M. (1963): Geology and origin of the pollucite in the granitic pegmatites of Sayan. pollucite-bearing Montgary pegmatite, Manitoba. Dokl, Acad. ^lci. UJ.,S.R. Earth Sci. Sect. i4L. Geol. Soc. Amer. BuIl. 74. 919-946. 1287-t289. M9ns1, S. A. (1968): Revised dispersion method Received January 1978, revised manuscript accepted for low plagioclase. Arner. Mineral. 5A, 105-115. May, 197g.

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