639

Canadian MinerologistVol. 30, pp.639-651 (1992)

THE CRYSTAL CHEMISTRY OF SPODUMENE IN SOME GRANITICAPLITE_PEGMATITE OF NORTHERN PORTUGAL.

BERNARD CHAROY AND FRANQOIS LHOTEEcole Nationole Supdrieure de Gdologie et Centre de recherches pdtographiques et g1ochimiques,

Botte postale 20, 54501 Vandoeuvre-Nancy Cedex, France

YVES DUSAUSOYLaboratoire de Mindralogie et Cristollographie, UA CNRS 809, Univenitd de Nancy I,

Botte postale 239, 54506 Vandoeuvre-Nancy Cedex, France

Centro de Geologia, Faculdode de Ci€ncias da Universidade do Porto,Instituto Nacional de Investigagdo Cientifico,4000 Porto, Portugol

AssrRAcr

Lithium-rich aplite-pegmatite bodies intruding the metamorphic basement of northeastern Portugal are describedfor the first time. They constitute only a small percentage of an important granitic aplite-pegmatite populationassociated with Hercynian synorogenic two-mica granites. Aplites and pegmatites are deformed by the last event ofdeformation, which postdates the peak of metamorphism. This paper focuses on the mineralogy and crystal chemistryol spodumene that is present mainly in the pegmatites but also in some associated aplites and, locally, in late ductileshear zones. Comparison with pegmatitic spodumene from Galicia (northwestern Spain) and with spodumene of gemquality ("kunzite") from pocket pegmatites from elsewhere in the world confirms the nearly constant compositionof the pyroxene. Compositions of all spodumene examples are close to the theoretical formula, regardless of thelocation and paragenesis (early to late) in the aplite-pegmatite sequence of crystallization. Cell parameters determinedby single-crystal or powder techniques present only slight variations, whatever the origin of the spodumene samples.

Keywords; granitic aplite-pegmatites, Hercynian, spodumene, crystal chemistry, Portugal.

Souuarns

Des corps aplitiques-pegmatitiques riches en Li recoupant le socle m6tamorphique de la province nord orientaledu Portugal sont decrits pour la premiEre fois. Ils constituent une faible fraction d'une importante populaticn decorps aplitiques-pegmatitiques associ6s d un ensemble de granites syntectoniques iL deux micas, Aplites et pegmatitessont deformees et sont e refier g6ndtiquement i la dernibre phase de d6formation, postdrieure au pic du m€tamorphismerdgional. La min€ralogie et la cristallochimie du spodumdne, essentiellement pr€sent dans les pegmatites, mais 6galementdans certaines des aplites associ6es et dans des zones de fracturation ductile tardive, sont paniculiCrement €tudides.La comparaison avec le spodumbne de pegmatites de Galice (nord ouest de I'Espagne) et la "kunzite" hydrothermalede cavit€s miarolitiques de diverses pegmatites dans le monde confirme la quasi-constance de la composition dupyroxlne. Celle-ci est fiCs proche de la composition th6orique, quelque soit la localisation et I'association paragdn6tique(prdcoce ou tardive) du spodumbne dans la sdquence de cristallisation des corps aplitiques-pegmatitiques. LesparamAtres de la maille d6termin6s sur monocristaux ou sur poudres sont 6galement trCs constants.

Mots-cVs: aplites, pegmatites granitiques, cristallochimie, spodumine, Hercynien, Portugal.

INTRoDUCTIoN

In the context of a program of regional mappingand a study of granite petrology, two of the authors

studied an apparently representative collection ofaplite-pegmatite granitic bodies very numerous inthe Covas de Barroso district, nofthern Portugal(Figs. l, 2). They discovered a previously unknownexample of lithium mineralization expressd in afew of the pegmatites as spodumene, phosphatesof the amblygonite series and, locally, lepidolite.The aim of this paper is to present paragenetic"CRPC contribution number 930.

@0 THE CANADIAN MINERALOGIST

A A A A A A AA A N A A A A A A

A A A A A A A A A N AA A A A A A A A A A

. / t ' \ A A A A A A A A AA A A A A A A AA A A A A A A A A A AA A A A A A A A A A A

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GELFA

PORTO

Frc. 1. Geological sketch-map of the northwest part of the Iberian Peninsula. l) Mesozoic and Tertiary,2)undifferentiated Paleozoic and Precambrian metamorphic rocks, 3) catazonal complexes from Galicia and Ponugal,4 to ?) Hercynian granites: 4) post-tectonic biotite granites, 5) late tectonic biotite granites, 6) syn- to late tectonictwo-mica granites, 7) syntectonic biotite granites. Inset: major geotectonic units of the lberian Peninsula: I)Cantabrian Zone, II) Asturian-Leonese Zone, III) Middle Galicia - Tras-os-Montes Zone, IV) Centro-IberianZone, V) Ossa-Morena Zone, VI) South Portuguese Zone.

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details and data concerning the spodumene occur-rences in these Li-rich aplite-pegmatite bodies. TheIepidolite-rich occurrences will only be brieflydiscussed.

GEoLocrcAL SsrrrNc

The Covas de Barroso district (Figs. l, 2) islocated close to the boundary between two largetectonic units of the Iberian Hercynian belt (Ribeiroet al, 1979): the Middle Galicia - Tras-os-MontesZone to the north and the Centro-Iberian Zone tothe south (Fig. 1). Numerous granitic aplite-peg-matite dykes and veins, locally in dense swarms of

XN

more than l0 bodies, sharply cross-cut themetasediments (Fig. 2), which form the larger partof the Middle Galicia - Tras-os-Montes structuralzone. Metasediments of Silurian age are mostlycomposed of quartziferous schists and micaceousschists with minor interbedded black schists. A fewcalc-silicate horizons contain some tungsten(scheelite) mineralization. At least three episodes ofdeformation affected the area, but only twoschistosities are recognizable in the field (Noronha& Ribeiro l98l). The peak of metamorphismpreceded the main Wesphalian episode of compres-sion (F). Maximum conditions of metamorphismcorrespond to the andalusite zone: the temperature

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attained 500oC, and the confining pressure wasfrom 300 to 350 MPa (Holdaway l97l). Differenttypes of granites, all of Hercynian age, aredistributed in the area. Three types have beenidentified, according to their structural and tem-poral relations with the main F3 phase ofdeformation (Ferreira et al. 1987): i) synorogenicbiotite granites (syn-F3), ii) synorogenic two-micagranites (syn-to late-F3), and iii) post-tectonicbiotite granites (post-Fr). The synorogenic two-mica granites are dominant in the Covas de Barrosodistrict, where they define two large northwest-trending structures that broadly correspond to thecore of F3 antiforms (Fig. 2). Aplite-pegmatite

641

bodies also are commonly distributed throughoutthe two-mica granites, but are very sparse in thebiotite granites.

GEOLOGY OF THE APIITE_PECVTATITE BODIES

The relations of the aplite-pegmatite bodies,with regard to the main metamorphic fabric, arevariable even in a single swarm of dykes. Theyrange from concordant to highly discordant, butall are more or less deformed. Dykes are vertical,flat or have a variable dip; they are elongate (morethan 100 m along strike) or lenticular in shape.Largely because of the poor quality of outcrop

THE CRYSTAL CHEMISTRY OF SPODUMENE

Frc. 2. The Covas de Barroso pegmatite field (outlined in Fig. l): 1) Silurianmetasediments, 2) post-tectonic biotite granites, 3) syn-to late-tectonic two-micagranites, 4) syntectonic biotite granites, 5) granitic aplite-pegmatite dykes' 6)dam reservoir, Single circle: spodumene occurrences; double circle: lepidoliteoccurrences.

@2 THE CANADIAN MINERALOCIST

(weathering and superficial Mn staining), theinternal structure of the dykes commonly is difficultto determine. Some common characteristics ofthese pegmatite dykes include a heterogeneoustexture, ranging from fine-grained aplitic tocoarse-grained pegmatitic. Textural layering islocally a prominent morphological feature. Layersrange from a few cm to one m thick. Aplite-peg-matite couplets alternate rhythmically or arerandomly and intimately mixed, with aplite mainlyin the footwall as a substrate for the pegmatite unit,which consists of more or less radial clusters oflarge alkali feldspar crystals. Slivers of theenclosing schists can be present close to the wallsof the dykes. However, pegmatitic units aredominant in the larger dykes and bodies, whereasaplitic units (with only a few scattered coarsecrystals of feldspar) make up most of the thinnerveins (less than I m wide). By far, the majority ofthe aplite-pegmatite dykes exhibit a fracturecontrol (linear or en dchelon). Deformation,manifested by brittle deformation, "boudinage",lineation and even folding, is statistically cor-relatable with that of the enclosing metasediments,and is considered to have been generated duringthe F3 phase of compression (Borges et al. 1979),Many of the aplite veins host low-grade tinmineralization (0.1 to 0.3 vol.9o cassiterite: Maijer1965, Noronha 1983) and were mined on a smallscale after the Second World War, wherever argillicalteration made digging by hand possible.

As stated previously, large bodies (up to 4 macross) of pegmatite with a minor aplitic fractionare commonly encountered throughout thesynorogenic two-mica granites; they also aredeformed. In these bodies, muscovite is thedominant mica; biotite, together with tourmaline(schorl) and spessartine-rich garnet, occur as minorphases and may outline a crude layering; beryl isa rare accessory mineral. Lithium mineralization isapparently absent, suggesting a less extreme degreeof chemical fractionation; however, a more sys-tematic investigation would be necessary to confirmthis hypothesis. These intragranitic aplite-peg-matite bodi€s have not yet been studied in detailand will not be discussed further.

PsTIocRApHIc AND CoMPOSITIoNAL FEATURESOF THE LTTHTuv.RTcrr ApLTrs-PecN4ATITE UNITS

The spodumene- or lepidolite-bearing aplite-peg-matite population of the Covas de Barroso districtrepresents a very small fraclion (a few percent) ofall the aplite-pegmatite occurrences encountered inthe belt. However, the survey is still in progress,and other Li-rich occurrences will certainly berecognized in the future.

Bulk compositions of minerals, phase relation-

ships and the primary ve,"rr{s metasomatic natureof mineral assemblages in the crystallizationsequence of Li-rich pegmatites are generallycomplex (Jahns 1953, 1982, Stewart 1978, London& Burt 1982, Norton 1983). Relative to some ofthe most complex pegmatite deposits discussed bythese authors, the occurrences studied in the Covasde Barroso district appear to have a relativelysimple paragenesis. Albite is commonly the onlyfeldspathic phase in the aplite units. Alkali feldsparmay be present as late-stage, interstitial, weaklyperthitic crystals. Phengite (characterized usingelectron-microprobe data) is largely dominant overbiotite, minor tourmaline (schorl) and spessartine-rich garnet. In contrast, the pegmatite units aredominated by strongly perthitic megacrysts ofalkali feldspar, which locally display a branchingor brush-like habit. They are present either on aplitefloors or on fine-grained tourmaline-garnet-richlayers. Phengite (identified from results of wet-chemical analyses on mineral concentrates) isubiquitous, locally in radiating aggregates, andobviously of several generations. A fluorine-poorphosphate of the amblygonite series (characterizedusing electron-microprobe data) occurs in bothaplites and pegmatites, but is generally rare andhighly altered. Spodumene occurs mainly in thepegmatitic part of some dykes (CHN 3, CHN 37,CHN 42). Only one dyke (CHN 5), at this stage ofthe survey, shows spodumene in both aplitic andpegmatitic units.

Bulk compositions of some aplite-pegmatitecouplets of the Covas de Barroso district are listedin Table l, together with data on spodumene-bear-ing pegmatites from the literature. Pegmatitesamples were obtained by channel-sampling byDGGM (SFM, Pofiugal), and are considered to berepresentative. Concentrations of elements weredetermined by inductively coupled plasma (ICP)spectrometry (K. Govindaraju, CRPG); concentra-tions of F, Li Sn and W were obtained by classicalwet-chemical techniques (M. Vernet, CRPG).Associated aplite and pegmatite units are roughlyidentical in composition except for alkalis, with ahigher Na content in the aplites reflecting thepresence of albile as the only feldspar present.Within the aplites, K is mainly contained in themica. This is in accordance with the model of Jahns& Burnham (1969) of alkali partitioning inaplite-pegmatite pairs. Lithium content is low inthe aplites, except for sample CHN 5, wherespodumene is present. Fluorine content invariablyis low. Comparatively, the lepidolite-bearing aplite-pegmatite body (CHN 26) is strongly enriched inRb and F. Both aplite and pegmatite units arestrongly peraluminous: the aluminum saturationindex [A/CNK : mole AlrOr/(CaO a Na2O -rKrOl is significantly greater than l.l.

THE CRYSTAL CHEMISTRY OF SPODUMENE 643

TAaLEl. ErrLKcoMposrnoNoFApLrrE-TEcMATTmpATRsFRoMNoRTIIERNIBERTANPEIIINSULAANDTIIEIJTERAT-LRE

(lIN} CIIN5

8h. zm apl. pe8n.

Sp Sp Sp

74.71 13.47 73.t5tr . t r . t r .

r6.tt 16.26 16.140.13 0.54 0. t1tr. 0.03 0.04u. t r . 0.15

0.05 0.07 0.193.37 5.13 2.452.59 2.U 3.540.97 g.2r 0.400.r1 0.03 0.030. t7 r . r9 2. t20.05 0.01 0.01

99.63 99.26 99.01

77062700.84 5a d

r .95

spl. p€SlD. apl.

Sp

14.t9 76.06 ?2.6tIr. tr. tt.

13.25 16.56 15.7t0,24 !r. O.12rr . 0.02 0.03tr. tr. 0.02

0.26 rr. 0.4E5.30 2. l l 5. t31,52 2.07 r .790.40 0.39 0.590.06 0.04 0.051 .16 1 .53 1 .53

0.03 0.02 0.02

99.55 9t.96 99.4E

521 t70 305l to t900 505t .4 1.9 2.O?8 36 29104 nd nd

1.33 2.74 1.28

pegn. apl.

Sp

7t.34 74.9tr . 0.05

r6.60 14.10!r . 0.?0

0.04 0.20tr . l r .t r . o. lE

0.56 5.2tl .ot 2.360.16 0.240.02 0.0El . t9 0.7E

0.01 0.03

9t .68 98.92

l9l 22012t00 203

r.2 0.71 1 1 2 0r d t d

CASTI CAST2 CIIN26

8pl. apl.

Lep

72.7r ?3.90 70.131r . t r . t l .

16 .43 16 .13 l? .3 t0 .25 0 .30 0 . t60 .03 0 .0? 0 .0 tt t . t r . t l .

0 .51 0 .22 0 .1?7,53 6.63 4.t91 . l l l . 4 l 2 . t 50.5 r 0.22 0.?00.0? 0.05 0.990.80 0 .92 2 .15

0.03 0.02 0.42

99.92 99.t3 99.1t

269 436 2393l t l 1 2 4 3 5 6 51.4 r .5 19 .9,43 t04 7281 4 5 1 4 0 1 7 5

r .2 t 1 .25 1 .56

cItN42

v t %

StO2TiO2Al203Fo2O3tldnoMgoCaONa2OK2oPfsFLOI

O e F .

Total

RbLivS[Be

9t990501 . 92 1 0236

6032E902 .14 l1 1 1

l5

1 .47

Sp Sp Sp Sp

72.16 72.1 74.6 79.5- 0.02

17.68 tt.2 16.4 16.70.53 1.E30 .M 0 .10

- 0.020.09 0.026.56 3.20 3.3 3.40.92 2.02 2.4 2.6

- 0.07D d n d

0.t6 0.6E

100.02 9?.5t

3635600 1096? 7000 6533

21,5 33

1.50 2.44 2.04 1.98% spod (nodal)(lolnt @ed!8)A/CNK 2.O2 8 . 1 5 t .25

1. Spodunm{osftg pegEe. ktb, calds (m Klmtog & Vldal Rmd 19El). Z Cosegrohd epore&slmg eeqEe, Flrod gxecbl98l).l-Lspoau'me+eqpegEdhes:tsttsdpadalaoab€€s(Se'wBf frSI 3:Kto$li{dah,NseCgolitra'aP€8meOe$Maire. SpsPo.tonfDls Ftsed- kF tddolib ls F€sstrl nC d &Med

SPODUMENE

General features

The presence of spodumene has been reportedonly once in Portugal, as an accessory phase in adense dyke swarm of aplite-pegmatite bodies(several generations) that cross-cut the stronglyfoliated lwo-mica granite of Gelfa (Torre deAssuncao 1954; Fig. 1). This locality was sampledfor comparison. Spodumene is present as isolatedcrystals up to l0 cm long and as clusters scatteredthroughout the tourmaline- and garnet-bearingaplites of the composite bodies. It is associated withrare megacrysts of alkali feldspar. Spodumene isabsent in the spatially associated pegmatite units.

In contrast, several occurrences of spodumenein southern Galicia, Spain, have already beendescribed (Parga-Pondal & Cardoso 1948, Hensen1967, von Knorring & Vidal Romani 1981); twospodumene-bearing aplite-pegmatite dykes at onelocality (Castelino, Fig. l) were sampled forcomparison. The dykes are vertical and ap-proximately l0 m thick; they are composite, witha thin aplitic margin along the contact with theenclosing schists. Abundant elongate laths ofspodumene (locally forming inward-radiatingclusters) occur together with large crystals of alkali

feldspar; both are distributed with a well-expressedalignment normal to the fine-grained margins,which is indicative of conditions of rapid growthfrom a metastably supersaturated melt (D. London,pers. comm.). A quartz-rich core devoid ofspodumene and alkali felspar contains only mus-covite coating fractures. The composition of aplitesfrom Gelfa and Castelino also is reported in Table1. In all the Iberian pegmatite occurrences sampledduring this study, spodumene forms coarse (l tol0 cm long) euhedral lath-shaped crystals flattenedon (100). They are milky white to pale yellowish,are bent, and in many cases, broken (Fig. 3c), withhealed fractures normal to their elongation. Thesefractures are filled by quartz or, in cases, by theenclosing aplitic matrix (Gelfa). In some cases(CHN 5 and CHN 37, Fig. 3d), the large laths arecoated by an aggregate of fine-grained white orpale greenish mica. In comparison, the spodumenein aplite CHN 5 is anhedral (Fig. 3a). The positionof spodumene in the sequence of crystallization ofthe aplite-pegmatite bodies remains ambiguous;several generations of spodumene are obvious: i) itis an early phase, preceding the fust appearance ofalbite, in the aplite occurrence (CHN 5; Fig. 3a);ii) in the pegmatite occurrences, it has a euhedralhabit (in contrast to that in the aplite), and occursas isolated laths or radiating aggregates (Figs. 3b,

THE CANADIAN MINERALOGIST

THE CRYSTAL CHEMISTRY OF SPODUMENE 645

d). However, its relationship to coarse alkalifeldspar is difficult to constrain. In several cases,coarse-grained spodumene appears to be partlycoated, and even partly replaced by a thin rim ofcloudy K-feldspar. Both ends of most spodumenecrystals in pegmatite assemblages exhibit a symplec-titic intergrowth with quartz (Figs. 3c, 4a). Asimilar texture was described by Hensen (1967),who interpreted it as a product of growth at theexpense of the contiguous alkali feldspar. Such aquartz-spodumene intergrowih also was describedby Cern! & Ferguson (1972) as the breakdownproduct of petalite. In the present examples, this

association corresponds to a late generation ofspodumene. iii) In one occurrence (CHN 3), severallate F3 ductile shear-zones (3 to 5 cm across)cross-cut a large, crudely layered, vertical,spodumene-bearing aplite-pegmatite body. Theseshear-zones contain preferentially oriented,symplectitic quartz-spodumene aggregates (Fig. 4b)enclosing mechanically transported fragments ofthe host aplite-pegmatite (alkali feldspar, a fewcoarse fragments of spodumene, amblygorrite).This late generation of spodumene appears verysimilar to the sheaf-like aggregates reported byHensen (1967) or even to the dykes of aphanitic

A D

Ftc. 4. a. Symplectitic rim of spodumene (with quartz). CAST. 2. Crossed polars; scale bar: 2 mm. b. Matted aggregatesof very fine-grained needles of spodumene surrounding early coarser-grained spodumene. Shear zone cross-cuttingthe CHN 3 aplite-pegmatite body. Plane-polarized light; scale bar: 2 mm.

FIc. 3. a. Spodumene crystals (high relief) scattered in a fine-grained matrix of quartz, albite and muscovite. CHN 5aplite. Plane-polarized light; scale bar: 2 mm. b. Large laths of spodumene of variable size in aggregates of quartz,albite and muscovite. CHN 5 pegmatite. Crossed polars; scale bar:. 4.5 mm. c. Fine-grained intergrowth ofspodumene and quartz on both sides of a fracture in a large lath of spodumene, Gelfa. Plane-polarized light; scalebar:2.5 mm. d. Large subhedral lath of spodumene coated by perthitic alkali feldspar. CHN 37. Crossed polars;scale bar: 5 mm.

646 THE CANADIAN MINERALOGIST

spodumene that cross-cut spodumene-bearing peg-matites described by Rossovskiy et al. (1976). Wepostulate that this late spodumene is contem-poraneous with the symplectitic overgrowthdescribed above.

Chemistry

After crushing, spodumene was concentratedusing heavy liquids and then further purified byhand-picking. The purity ofthe final separates waschecked by X-ray diffractometry. Wet-chemicalmethods were used for analysis (M. Vernet,CRPG). Large single crystals of gem-qualityspodumene ("kunzite" variety) from differentpocket-pegmatites from a broad range ofgeographic locations and geological environmentsalso were analyzed for comparison with the Iberiansamples. The chemical composition and structuralformula of the spodumene specimens (on the basisof 24 atoms of oxygen in the unit cell) are listed inTable 2. The abnormally high P2O, content ofsample CHN 3 is due to an unavoidable contamina-tion by amblygonite. This excess of phosphorus

was artificially removed assuming P-Listoichiometry in amblygonite. Spodumene com-positions do not show any significant deviationfrom the ideal formula LiAlSitOu, as alreadypointed out by Deer et al. (1978) and London &Burt (1982). However, the Fe content is significant-ly higher in the Iberian material than in the"kunzite" specimens. Water content, reported asH2O, (only a few H2O+ determinations), is low inthe Iberian samples, but significant relative to thenearly water-free "kunzite" crystals. This anomalymight be related to small amounts of micaceousproducts of alteration; however, there is apparentlyno correlation between H2O, and K2O contents (see,for example, sample CHN 37, with a relatively highK2O content but no clear X-ray-diffraction maxi-mum observed at l0 A). tn thin fragments, theIberian samples of spodumene are heavily cloudedby fluid inclusions distributed parallel to thecrystals' elongation. In contrast, the "kunzite"fragments are perfectly clear. It does not seemnecess.uy to invoke structurally bonded OH toaccount for the presence of water, which may betotally confined to the fluid inclusions. Conse-

TABIT 2. COMP TIION OF S'FODUMENE'vtt% Seaz@ Gpl) (Fg ) (pesn) (prsn) (pept Ge6t crnels cotipcitim!p2 61.q 61.!l q.2s u.ge s.n 63.E3 64n O4A3 s.45 64.4 64s2 &.s71q iq.S! <0.0t {.q,1 4.0r <0.01 o.v2

-oti2 <ftoi io.or 0.v2 <0.01

4hlq_ 28.q1 nr3 4q 2s:ts 26ss 2s.n ?ss3 ti:4i ,iise n.6 n.rs n.e&p3r A.e 9.40 S.1! 0.y 0.4e d.a

-d:41 -b:i6 -0.a4 o.m o.tz

!,!Q 0.m 0.(ts _0.93 o.ut 0.ll o.os 0.d ooj ir.o 0.0? 0.10!g-o q.S? g.q2 q.ql Q.a 0.06 0.0! dn2 d.oi dr.or <o.or <o.oiqs 0.r5 0.m <q.9! d.or <0.0r 0.m 0.0i d.oi if.or <0.0r d.olryaao q.q Q.q q.t6 0.16 o.b o.ia 0:i; nzi irzr o.2z o.io!p q.q1 S.a{ Q.!q 9.60 0.2s d.n oot oot ir.m 0.07 0.0rl& q.n 0.06 q.g! 0.05 0.01 a:07 0.04 0:0i o.A 0.0i .o.oi

9r.? g.!! 9.4 q.!3 0.r0 0.ii 0.id 0.0t ir.or 0.0? n&S 9.41 o.u 0.i! 0.6i o.5l o.:j 0:30 o:it ir.u o.rs 0.06u2o 7.@ 7.63 7.69 7.a 7.j8 7.4 l.6e 7'i0 its 7.7s j.ti E.o3Total 99.86 1m.09 9.9 99.99 9.96 99.92 s'.87 lm.3 r@st l@.3r lmol rm.mlb(r) d^ _ d^, n4 nd nd d 6 5 <l <l <rq_Qr) <S.l <)5 !4 nd nd a A Oz oe 0.r osm 0.m9 0.o4 <0.m2 0.o4 0.m9 o.u8

-6.ms 0.m ir.ma d.m2 <o.ma

u2o+ 03r 0.19STRTrcTURAL rctRMrJtA (ON TBE BASIS OF 24 O)

si 6.0t2 7999 E.059 8.156 8.055 7gtrt 8.m0 79v7 7gy3 l.nr 8.016 E.mA - oOt o.ors -':--

ilit o.wt o.va ': -

A 3.959 3gtt 3a90 3Ju 3a9r 396t 3925 4.015 A.Vtt 3.y74 3gE2 4.qx)F.'r o.om 0.0a7 0.m9 0.ct0 0.045 0.040 0.045 0.010 0.mJ 0.(p5 0.020u 3J18 3.829 1{!! 3.630 3,826 3.W 3A65 3.E45 3J56 3.856 3168 4.m!g q.A1 q.A1 q@ 0.o4 0.011 0.or o.mt

-': -r,h 0.m 0.m3 0.m 0.ry2 0.Orr O.Ofl 0.m O.Ot O,Wt O.tr/ 0.010G o.@0 0.m4 - 0.04 omi

-':-'r,h 0.021 0.6e q.q9 o.qp 0.056 0:or9 0:o:9 0.60 0.049 o.0rz 0.m9K 0.@6 0.m6 g.q?t 0.q6 0.@ oGs 0.0ri 0.0i0 o.m 0.010 :-p 0.tn6 0,tn6 0.m4 0.o5 0.or 0.m/ o.mA o.mi 0.m, 0.mi1r - 0.@2 o:m2

-.:-- o.w2

CRIUCAL CHEMIEAL RAIIOSwAl Lt24 2(tr6 ?.q!2 2.!40 LOO 1m6 LM r.990 t.9El 1.991 LOr3 2Si/ri 2.6E zGe ?.W ?.216 z.ios tan tini 2060 Lvs L(f,7 Lstz zr&n 0.15 0.G 0.08 o.at 0.24 d.ii' oit oto- ia i..i 0:50

SmpletdtteYarl4vfuztr':Pala,Ceffua:enyoabohamdSdmy,f,{adagasoacAn@.i6tra8fus,Brardl ortmtd@d. H2O+mdetemiDd b t*o ra'4plog oly.

THE CRYSTAL CHEMISTRY OF SPODUMENE 647

o This study r )o Literature rlKuntite

* Th@retical valuo

o o ^v o ^

t €po * "

a

-

apl

e!.\. *

q

55 s i 60 55 L i 50

FIc. 5. Plot of spodumene compositions in terms of Si-Al-Li (atomic proportions).The data are taken from this study and from the literature. Sp: ideal structuralformula; sh-z: shear zone (CHN 3); apl.: aplite (CHN 5).

quently, structural formulae are expressed as beingOH-free.

The spodumene samples from the Iberianlocalities commonly have a small excess of Sirelative to the ideal stoichiometry, whereas most ofthe gem-quality specimens have higher Al (even intetrahedral coordination). This may represent eithera real excess of Si in the structure, or the presenceof quartz-spodumene symplectitic overgrowthsdescribed above, or some silicrl residual from thereplacement of spodumene by muscovite (belowdetection limit for diffractometry) according to thereaction:3LiAlSi2o6 + K+ + 2}t+ - KAl2AlSisOro(OH)2+ 3SiO2 + 3Li+.

There is no significant substitution of Li by Na;however, Li is always deficient relative to thetheoretical value; this is also true for the largemajogity of spodumene compositions in the litera-ture.Cerni & Ferguson (1972) found a significantlyhigher concentration of Na in secondaryspodumene after petalite, which is not the case here.Compared to ordinary spodumene, manganesecontent does not seem significantly higher in the"kunzite" variety. Claffy (1953) and Hassan &Labib (1978) suggested, from an analysis byemission spectrography, that a high Mn content ora high Mn/Fe ratio is responsible for the lavendercolor in "kunzite". Rossman (1982) confirmed theimpofiance of Mn3+ as a chromofore element.Mn/Fe atomic ratios (Table 2\ are, in fact,significantly lower in the Iberian samples; iron, asa minor component in fluid inclusions, couldexplain this variation. Concentrations of trace

elements such as Rb, Cs and F are very low, closeto or even below the limit of detection.

All the available spodumene compositions (in-cluding data from the literature) are plotted interms of Si-Al-Li in atomic 9o in Figure 5. Thescatter of the data reflects variable intensity ofalteration, with leaching of Li being accompaniedby an increase in Si and a decrease in Al. Thecomposition of pristine, unaltered spodumene fromthis study is virtually constant and close to thetheoretical formula, regardless of its parageneticplacement in the aplite-pegmatite sequence, theP-T conditions of crystallization or the geologicalenvironment.

X-ray ond physical characteristics

Unit-cell dimensions, calculated from data ob-tained by X-ray powder or single-crystal diffrac-tion, are repofied in Table 3, together with someoptical and physical properties. Determination ofcell dimensions (in the C2/c space group) wasundertaken on single crystals (approximately I mm)on an ENRAF NONIUS CAD4 automatic diffrac-tometer (graphite monochromator; CuKa X radia-tion) using the least-square program INDEX forangular data on 25 well-defined reflections between15 and 25" 0 C{. Dusausoy, analyst). A powder-diffraction pattern was recorded on four samples(* in Table 3), using a Guinier-Hdgg focusingcamera, synthetic spinel standard, Cu.l(a1 radia-tion; between 39 and M indexed peaks were treatedin a least-square refinement using the program ofAppleman & Evans (1973) (R.F. Martin, analyst).

CHN3I CHN5T CHNs CHN37 GELFA CASTT A{ST2turz@ (d) (F8n) (pegn) oesA) Oesn) (po8D)

e.4680) 9]467(1) 9.4qD 9.469(l) 9.476(3) e.6t(rt eA7Xz)

839(r) us2(r) $eql) 83ql) ssvt?) 83t(l) 63970)

5 {0.4) 5,2r(0r') 5.Ut(0.8) 52.r(r.4) sW,A 5,,](OA s lls'

ll@(l) 1102(r) lrrqD 11aP!(2) lle1(4) llfi8(l) rrf22(2)

38925 38p27 3/89x2 389.,18 39OZt 36&61 38994

1.6d) 1.66r1f64 1665t.o4 1.675

Let4 L674

6 3 6 265.0 63.6x u

Dtl rd 3.1?03.181 3.189 3.rv2

r.661 d r.658t.ffi d 1.663\6n d 1.67

t.67r d 1.674

65 58 636.4 d 64.62 3 A

d.

3.t8r

ddd

d

63d

d.32tO

ddd

d

d.d

648 THE CANADIAN MINERALOGIST

TABLE 3. I,NT}CLL DIMENSIONS AND PHYSIC,{L PROPERTIES OF SPODI'MENB VARJETIBS

Unlt e|| dlreDsbB

a(A)

b (A)

c (A)

p0

v(A')

Phy8lcsl proplrtloDEoe@dDr.rl"-ld!d

Optlcal preD€rd67FoMrEffActh6hdq (€lc)

2Vz|m{l2Vz@lc f)l t z a )

r.661 r.660 d1,67 1.665 d1.678 t.675 d

1.6t9 t.678 1.679

Agreement between results obtained by the twotechniques is reasonably good.

Physical constants are also in good agreementbetween the Iberian samples of spodumene ancl the"kunzite" varieties. However, the extinction angleQ:Z) is slightly variable and seems to be a littlelower for the "kunzite" samples. The calculateddensity D, is commonly slightly greater than thatphysically determined (D.).

Data from this study and from the literaturc arecompared in Figure 6, a plot of b against B. Brown(1971) demonstrated that cell parameters, par-ticularly b, increase as does the radius of thetrivalent cations populating the Ml sites; theincrease is most important in the cell volume, andthe parameters increase in the order b, o, c (seealso Drysdale 1975). For natural examples ofspodumene, in which Al is largely dominant, apositive correlation of D versus B was suggested byBrown (1971). However, this is not confirmed bythe scatter of data from this study or from theliterature (Fig. 6); B seems more variable than b,whereas the volume of the unit cell remains nearlvconstant.

DIscussloN

For the first time, spodumene-rich pegmatitesfrom the northern part of Portugal are described

PAIA ANJANA SAIIA'TANYT ARACUAIT

9,463{2) 9.4741rt

8390(2) &39(l)

e.46e(D

8392(l)

9.469(D

83e8(D

sm{03

xm(r)

389.83

5r18@O 5rr0r) 5219(05)

ller6g) 1rf26(1) lrere(r)

388J0 389.45 W

3.t90 3.U8 3.1763.rgt 3.182 3.186

d d d65J m.4 d

u 2 3

3.tEr3.r&7

ddd

1.678

dd

Unitcdldi.'m;oosthmied@p@dqBitrgsGuiBL-HIggchbq(1}dflriso,dffiirod@dDgleqyslpsingaCAD4N@ia..n@'U! dift@@.Dmcif@r)n@dvihap!@@dDr ekohEdEirgth6fmola-fmulrwigtlxZr 1.6@3r ltaluirenrclre(Ar)x lta.Rtuirc ndiB nsrcdby&oho*imEdod (a20ec@b@rrmp€(e) @ @I lhlEeddog6€d frrg@rcMm r€&ldirc irdics etqla&d vilh the ln 0f Gladraorc @d Drte (186t): (&f)/p - & rill K- t tp. K- spocific et&6w q€(gy of tho niml f - spcifrc Bfirrriviry ofmsftml uide. pE wigb pe@t of rlocoosltuqt qile (rlan h Jame, $m): @@d@ st$roatult gi6 highswlBoplial qial nSle qa@d @ a @ivqsl slgo ed eLohed fr@ 6e rtsli8 bo*@ tb Eftadive inda6 h rlE Eidi8| elitroif1: z (1 sirairaly t@r€d d thir €logaed ftqm&

*t This study(*for kunzite)

* Brown (1971 )o kunzite from

Finland ( Lahliand Saikkonen)( 1e86 )

r Cern! andFerguson (1972)

x Deer et al.(1978)

o Drysdale(1975)

8.370 8.380 8.390 8.400 8.410

FIc. 6. PIot of D against B for spodumene. Data from thisstudy and from the literature (error bars indicated).

THE CRYSTAL CHEMISTRY OF SPODUMENE 649

in some detail. The present mineralogical workdemonstrates the variability in the expression ofspodumene mineralization; it commonly occurs inthe pegmatite units, but also appears in a few ofthe associated aplites. Spodumene crystallizationranges from early to late with regard 1o the mainstage of feldspar crystallization (albite in aplite andalkali feldspar in pegmatite, respectively). Crystal-lization during postconsolidation events also isevident: fine-grained spodumene in healed fractures(as in CHN 3 ductile shear-zones) is obviously lateand implies that Li-Al-saturated fluids percolatedthrough largely consolidated spodumene-bearingbodies of granitic pegmatite.

In most of the aplite-pegmatite bodies studied,fluorine activity (estimated from the F content inthe whole-rock compositions in Table l) was lowthroughout their consolidation and stabilized Li-aluminosilicate phases instead of lepidolite (Burt &London 1982). This is not true for the more evolvedaplite-pegmatite bodies (CHN 26-27 in Fig. 2 andTable l), where fluorine had a more significant roleand stabilized the association of lepidolite, topazand F-rich amblygonite, together with aggregatesof albite (cleavelandite habit). However, thesephases were developed at a subsolidus stage byreplacement of an earlier assemblage (alkalifeldspar but absence of a relict primaryspodumene); cassiterite was preferentiallyprecipitated or remobilized at this stage.

A fluid-inclusion study (Doria e/ c/. 1989) hasbeen performed on some of these Portuguesespodumene-bearing pegmatites. Results onspodumene and presumably coexisting quartz helpto constrain possible interpretations about thegenesis of these bodies. Fluids contemporaneouswith spodumene precipitation (data from primaryand pseudosecondary inclusions) wereheterogeneous brines of variable density, in whichLi was found to be roughly equivalent to Na, inorder to stabilize the spodumene + albite + quartzassemblage (Lagache & Sebastian 1991). Thevolatile component of these fluids in spodumeneand quartz was found to be dominated bylow-density Hp and CO2, with minor butdetectable amounts of CHa and N2 (Ramandetermination). Homogenization to the liquid orvapor phase oqcurs over the range 320-365"C.Virtually all primary inclusions are crystal-rich,although with a variable crystal/liquid ratio. Apreliminary SEM-EDS investigation indicates thataluminosilicates and phosphates occur as daughterminerals. Most of them dissolve completely at atemperature of. 475"C. A plot of these data in theP-T petrogenetic grid for Li-aluminosilicatesproposed by London (1984) constrains the maxi-mum conditions for precipitation of spodumene:475-530"C at a confining pressure of 250 to 300

MPa, which is slightly lower than conditionsestimated for the peak of regional metamorphism.These results are broadly concordant, in terms ofP and T, with those determined by London (1985,1986) for the Tanco pegmatite even if CO2 isapparently lacking as a volatile component inspodumene. CO2 also was absent during theevolution of Li-bearing pegmatites fromsoutheastern Ireland (Whitworth & Rankin 1989).Chakoumakos & Lumpkin (1990) determinedconditions of 550'C and 330-350 MPa forspodumene precipitation in the Harding pegmatite(with CO2 present).

From geological evidence, the aplite-pegmatitepopulation of the Covas de Barroso district isspatially (andpresumably genetically) related to theadjacent synorogenic two-mica granites (Fig. 2),but the numerous, poorly fractionated aplite-peg-matite bodies hosted by these two-mica granites areappruently barren of Li. However, spodumene-bearing and other rare-element pegmatites aregenerally absent within the parental plutons; theyare mostly located in distal parts of the regionalpegmatite haloes surrounding fertile plutons, farbeyond their contacts with their metamorphic cover(tern! 1990; pers. comm.). The reduced viscosity,the increased solubility of HrO and the enrichmentin solidus-depressing constituents in these morefractionated, rare-element-enriched l,nelts wouldexplain this centrifugal segregation (Cernf 1982).It is also true that lithium-rich pegmatites usuallyconstitute a restricted percentage of the pegmatite(and aplite) population characteristically related tothe final stages of evolution of two-mica graniticmagmas (London & Burt 1982). This seemsconfirmed by the relatively rare occurrence ofLi-rich bodies amongst the aplite-pegmatites in theCovas de Barroso district (Fig. 2), where there isno apparent spatial zonation within the pegmatitepopulation. These mineralized pegmatites, even ifpoorly evolved in terms of mineralogy, chemistryand fractionation, seem to belong to the high-pres-sure, low-temperature spodumene subtype of therare-element class in the classification of Cernf(1990), whereas the mineralized aplites would bemore representative of the albite-spodumene sub-type. The two lepidolite-rich bodies (CHN 26-27)that are apparently isolated in the pegmatite belt(Fie. 2) probably belong to the lepidolite subtype,but as stated before, the key assemblage of mineralswas only stabilized at a subsolidus stage. Physicalconditions for precipitation of spodumene arevariable, although pressure must always be highenough to preclude petalite stability (London 1984).The present study confirms the nearly constantchemical composition, close to the ideal composi-tion, of all the spodumene samples, with onlyminor substitution of Al and Li. The gem varieties

650 THE CANADIAN MINERALOGIST

studied here for comparison occur in ,,pocket',pegmatites hosted by amphibolite metamorphicterranes and were precipitated from hydrothermalfluids. Small variations in Fe and H,O between thegem variety and the Iberian spodumene are mosrlikely explained by the presence of fluid inclusions,which are prominent in the Iberian samples.Structural and physical characteristics of all thesamples of spodumene are similar, and the unitcells are nearly constant.

The present study is part of a more generalprogram of research concerning relationshipsbetween different types of Hercynian granite andthe widespread Sn-W mineralization exhibited inthis nofihern paft of Portugal. A larger and moredetailed investigation throughout the huge aplite-pegmatite belt, aimed at recognizing new mineraloccurrences with possible economic potential, hasalready been started by DGGM (Servico FomentoMineiro Portugal), and preliminary results arepromising.

AcKNowLEDGEMENTS

The authors acknowledge the help ofH.J. Schubnel, from the Museum d,HistoireNaturelle de Paris, for kindly supplying most ofthe "kunzite" samples. J.P. Lorand (of the sameInstitute) is thanked for his help with thedetermination of the indices of refraction. The helpof R.F. Martin for the determination of the cellparameters of four samples is greatly acknow-ledged. The thorough criticisms and fruitfulcomments by Dr. P. Cern;i, R.F. Martin and twoanonymous reviewers considerably improved themanuscript; all of them are gratefully acknow-Iedged. Tables were carefully typed by J. Gerbaut.

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THE CRYSTAL CHEMISTRY OF SPODUMENE 651

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Received April 5, 1991, revised manuscript acceptedJanuary 17, 1992.