4t5T he C anadian M inera Io gis tVot. 36, pp. 415-431 (1998)

COIVIPOSITIONAL VARIATION OF TOURMALINE IN THE GRANITIC PEGMAf,ITEDYKES OF THE CRUZEIRO MINE, MINAS GERAIS. BRAZIL

MARCELLAFEDERICO, GIOVANNI B. ANDREOZZI, SERGIO LUCCHESII AND GIORGIO GRAZIANIDipanimento di Scienze della Tema, IJniversitd di Roma "kt SapienTa", P.le A. Moro, 5, I-00185 Romn, Italia

rtir,ro cEsan-MENDES2DEGEO, Escola de Minas, UFOP Campus do Morro do Cruzeiro, 35400-000, Ouro Preto, Minas Gerais, Brasil

ABSTRAcT

Schorl - dravite and schorl - elbaite from different zones of the rare-element-enriched complex granitic pegmatite dykesof the Cruzeiro mine, in Minas Gerais, Brazil, were analyzed by electron and ion (H, Li, B) microprobi s. Geniril mechanismsofsubstitution (common to schorl and to elbaite independently oftheir position in the pegmatites) drive compositions to"proton-deficient schor1", "alkali-free schorl", and "alkali-free elbaite" end-members, each involving Al enrichrnent. Amongthe specific mechanisms (1e., those characterizing tourmaline within each separate zone), "octahedral-defect" and "alkalifreeproton-rich" types are particularly important in OH-rich, pocket elbaite. The substitution x! + OH- <-> xNau + Oz , drivingcompositions to an "alkali-free proton-rich elbaite", is proposed to explain both the lack of Na and the excess of (OH + F).Bond-valence calculations based on results ofunpublished structure-refinement data allow the OH excess to be assigned tothe O(2) site. The composition of the tourmaline in the Cruzeiro suite (high Al content; Mg, Fe, Zn, Mn, and Li covariation;antithetic behavior of oH and F ) records the evolution of the pegmatite-forming melt.

Keywords: tourmaline, crystal chemistry, mechanisms of substitution, hydroxyl excess, alkali-free proton-rich elbaite, granitrcpegmatites, Cruzeiro mine, Minas Gerais. Brazil.

Sonav.qrnn

Nous avons analys6, af moyen de microsondes 6lectronique et ionique (H, Li, B), la tourmaline des sdries schorl - draviteet.schorl - elbaite provenant des diverses zones des pegmatites granitiques i 6l6ments rares exploitdes pour la tourmaline d lamine Cruzeiro, au Minas Gerais, Brdsil. Les mdcanismes de substitution gdndraux (ceux qui sont commun. au schorl et dI'elbaite inddpendamment de leur position dans les pegmatites) conduisent aux p6les "schorl d6ficitaire en protons", "schorl d6-pourvu d'alcalins", et "elbaite d6pourvue d'alcalins", chacun de ceux-ci contribuant A un enrichissembnt en Al. Parmi lesmdcanismes splcffiques, ceux qui caract6risent la variation de la tourmaline au sein d'une seule zone, les sch6mas impliquantles lacunes dans les sites octa6driques et les d6ficits en alcalins d la faveur d'un enrichissement en protons sont particulibmentimportants dans I'elbaite provenant des cavitds miarolitiques, milieux riches en OH. La substitutio; x! + OH <-> xNa+ + OL,qui mdne d I'elbaile sans alcalins et riche en protons, serait i I'origine d la fois du manque en Na et de I'excbs en (OH + F). I-esvaleurs des valences de Iiaison, calcul6es tr partir des r6sultats non publi6s d'analyses de la structure cristalline de ces6chantillons, nous permettent d'attribuer I'exc6dent en OH au site O(2). La composition de Ia tourmaline dans la suite deCruzeiro (teneur 6lev6e en Al; covariation en Mg, Fe, Zn, Mn, et Li, variation antithetique des proportions de OH et de F)tSmoignent de l'6volution du magma qui a cristallis6 sous forme de pegmatites granitiques.

(Traduit par la R6daction)

Mots-clds: tourmaline, chimie cristalline, m6canismes de substitution, exc6dent en hydroxyle, elbai'te riche en protons et sansalcalins, pegmatites granitiques, mine de Cruzeiro, Minas Gerais, Br6sil.

INrnooucnoN

Minerals of the tourmaline group have been studiedextensively because of their complex composition,structure and importance as petrological indicators.Their systematic compositional variations ire related to

the P-T-X-/(O2) conditions of the environment ofcrystallization and to interactions with the host rocks(Henry & Guidotti 1985, Shearer er al. 1986). This isespecially the case for granitic pegmatites in which thesensitivity of tourmaline to the activity of Fe, Mg, Al,and Mn makes them recorders of internal evolution

I E-mnil address: graziani@axrma.uniromal.it2 E-mail add,ress: julio @ degeo.ufop.br

416 THE CANADIAN MINERALOGIST

(Staaz et al. 1955,Foord 19'17, Walker et al. 1986,Jolliff et al.1986, Foord et al. 1989).

In this paper, the first part of a general study oftourmaline from deposits in the Cruzeiro graniticpegmatites, our objective is to clarify some aspects ofthe mechanisms of chemical variation in tourmalineduring crystallization of the pegmatite. The second partwill focus on the crystal chemistry of tJre tourmalinegroup.

The Cruzeiro dykes are rare-element-enrichedcomplex pegmatites with distinct horizontal andvertical zoning, and strong enrichment in Li, F and Rbin the upper parts (C6sar-Mendes 1995, Cdsar-Mendes& Svisero 1992,1994, C6sar-Mendes et al. l993a,b).The characteristics that led to the choice of theCruzeiro pegmatites as the source of tourmaline for thisstudy are: (1) their well-defined zoning, (2) the abun-dance of tourmaline, and (3) the presence of quartziteas the host rock. The latter suggests limited interactionwith the pegmatite-forming melt. Thus the chemicalcomposition and mechanisms of substitution in thesesamples of tourmaline can be expected to be exclusivelydependent on the composition of the melt and the localconditions of its crystallization.

GeoLocIcaL Serrnrc eNo REctoNeI- Pstnocnapnv

The Cruzeiro mine is located 13.5 km northwestof the small village of Sdo Jos6 da Safira, nearGovernador Valadares, in the eastern center of theBrazilian state of Minas Gerais (C6sar-Mendes 1995).The pegmatite dykes @ig. l) are emplaced in a quartziteof the Serra da Safira sequence. This sequence may bedescribed, from base to top, as gently west-dippinglayers of meta-ultramafic schists, of metapelitic schiss,and of a coarse-grained quartzite several hundredmeters thick (Cassedanne et al. l98O). This sequenceoverlies a regional lithostructural unit, the GneissPiedade, composed of gneiss, quartzite and schist(Silva er al. 1987). The mineral assemblages in therocks of the 56o Jos6 da Safira region are indicative ofthe middle amphibolite facies. Although granite wasnot found in the surrounding area" the pegmatite bodiesof the region may be derived from a granitic pluton.The emplacement of the pegmatites seems to have beencontrolled by regional structure, as indicated by theconcentration of many bodies along lineaments orientedNl0-20'W (C6sar-Mendes & Jordt-Evangelista 1994).

lNrBRNar Stnucrunn oR THE PEGMATIE DyKES

The three subvertical, parallel dykes (Fig. 1),running N20"W and intruding 80-86"SW have sharplydefined contacts with the host quartzite. The first (dyke1) is 1300 m long and up to 60 m wide, the second,now inaccessible, is 900 m long and about 20 m wide,and the third (dyke 3) is 700 m long with a maximumwidth of 8 m in outcrop. Their internal characteristics

Frc. l. Sketch of the Cruzeiro area. Dl: Dyke I , D2: Dyke 2,

D3: Dyke 3 (from Cassedanne et al. 1980).

are essentially similar: according to the differentassemblages of minerals (Table 1), the dykes showsymmetrical development around a quartz core and ahorizontal zoning. This zoning consists of a borderzone, a wall zone, an intermediate zone and a core(C6sar-Mendes 1995).

The border zone (hereafter referred to as Bz) isnormally of very limited thickness, ranging from 3 to5 cm and rarely exceeding 10 cm. T\e wall zone (Wz)gradually evolves from the Bz, and its thickness variesin direct proportion to the thickness ofthe pegmatitebody. In dyke 1, it reaches about 1 m, whereas in dyke3 it is thinner. The intermediate zone is economicallythe most important, as it produces colored tourmaline.In dyke 1, this zone can be further subdivided intoexternal, medium and internali in the smaller dyke 3,the zone can be divided into only two parts, externaland internal. In dyke l,the exterruzl intermediate zone(EIz) is characterized by an increase in grain size. Themedium intermediate zone (MId is a transitional layerthat was locally subject to late metasomatic processes.The internal intermediate zone (IIz) grades into thecore. Pockets (P), or miarolitic cavities, are found inthe IIz of all dykes near their cores. The core (C) isdiscontinuous and is composed mainly of milky quartz;no tourmaline was found.

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COMPOSITION OF TOURMALINE, CRUZEIRO MINE, BRAZIL

TABLE I. SAMPLE DISTRIBIMON OF TIIE TOURMALINES FROM THE CRUZIRO PECMATITE DYKES

417

Zm No. ofqygals No. ofEMPmd Elais aDbtssmsly4d petfomed

PangeBis TouEafire abudare,textm.l chaactsistiG and

bulk @lor

Bodq (Bz)

Wall (tlz)

Extqnal intmrcdiae(EIz)

Medim insm€diaE

Qrtz)

Lteml i$mediate(rI4

P@kets (P)

Corc (C)

Firc-Emircd Eisre of feldEpar,quehmdt|lll6ryite;nircrallsli-por beryl,'ln'dherichgaret ed dobiotmtalqts

Mediu€nined nb(m offeldspar, ldge n|ls8ite sh€€'tsed iilqsitisl $ErE

Clwerod nilore of qutz,feldspr atrd oimr l|lllwite

Mwileb@Ls, quaEadfeldspqr

FeldsTar, "cl@elrdile", largetabula pitrk Mitq qu{tzmd $odm@; $bodildemgaotmdite and {oradictight pirt n|lswite

Qwtz, "clfflsndite", pint@vile, lepidolite, spodtre;sbordiEte spwrtiHic.hg@et ed pink beryl

Qdaru, feldspd 6d Ei€

Mircr; erbhedEl qysrls

intoely ftac-urd blacl

SEbordiDle; subhedEt qydalsinrFsly trac-lred; black

Spomdic; Eedim€Eised$bhedDl qyrcals; bladqblack-gr@ rc!€d

Medim-gaircd; ilbh€dnlcrygalq blaclq gH

,lbmdaq sthednl qysafs

of big !o ellmm size;blrckwfthgm+lE rie, geq,pink

Euhednl qystrls; tra5lded,g!€r!, pink md @lq arcd

ExpentunNrar

A large number of small black grains of fracturedtourmaline from the outer zones of the pegmatites andseveral prismatic, colored crystals from their innerzones were hand-picked, representative of the core-rimzoning, where present.

X-ray dffiaction

Unit-cell parameters were determined with aSiemens P4 single-crystal diffractometer, wirh MoKa,radiation, using 26 independent reflections and theirFriedel pairs, from 85 to 95"20, after a preliminaryorientation, matrix calculation and indexins.

Analytical procedures

Ele ctron- mic roprob e analy s is (EM PA): Tourmalinecompositions were determined with a Cameca SX-50operated with five wavelength-dispersion (WDS)spectrometers and a Link eXL energy-dispersionsystem (EDS), both controlled by Specta software, atthe "Centro di Studio per il Quaternario e I'EvoluzioneAmbientale", C.N.R., Roma, Italy. Data were reducedwith the ZAF4|FLS software. Operating conditionswere as follows: accelerating voltage 15 kV, samplecurrent 15 nA, 100 and 20 s counting time for EDSand WDS determinations, respectively. The natural andsynthetic standards were: wollastonite (Si, Ca), corundum

(Al), jadeite (Na), periclase (Me), fluorphlogopite (F),magnetite (Fe), orthoclase (K), rutile (Ti), metallicMn and Zn. Average experimental errors (wt.7o) were:SiOz 0.18; TiOz 0.03; AlzOr 0.18; FeO 0.20 (schorl7,0.10 (elbaite), MnO 0.09, MgO 0.10, ZnO 0.09,CaO0.05, Na2O 0.05, K2O 0.02, F 0.04 (schorl),0.10 (elbaite).

Ion-microprobe analysis (SIME: The SIMS(Secondary-Ion Mass Spectrometry) analysis was donewith a Cameca IMS-3f instrument, for measurementof H, Li, B, Be, C and N contents, at the "Centre deRecherches P6trographiques et G6ochimiques", C.N.R.S.,Nancy, France. Experimental conditions were: primarycurrent of oxygen negative, with an intensity of 5 nA,focused on l0 pm; secondary current ofpositive ions;voltage offset of -60 V energy window of 10 V.Measurements were carried out with an electronmultiplier in counting mode and with automaticcentering of the magnetic field (Deloule et al. 1992).Average experimental errors (wt.7o) were: B2O3 0.56,LirO 0.10 (elbaite), H2O 0.16.

The H and Li values, obtained using the calibrationcurves with some glasses and one schorl-dravitetourmaline as standards, did not match the datapreviously obtained through replicate analysis usingTG (H) and AAS (Li) techniques. The latterdeterminations were done on unzoned crystals of thesuite for which sufficient material was available.Checks subsequently made on the above mentionedstandards provided evidence that the SMS data were

418

affected by matrix effects, caused by the difference insilica content between glass and tourmaline, as reportedfor Li by Wilson & Long (1983). This f indingsuggested that the compositional difference betweenthe schorl-dravite standard and the elbaitic samplescould cause systematic errors, and implied that it wouldbe necessary to use an external procedure of standardi-zation. For this reason, the SIMS measurements werecalibrated with the TG and AAS data.

For B measurement, no matrix effect due to silicawas observed using a calibration curve with glasses andtourmaline slandards. However, two clusters were shownby measurements of Fe-rich and Fe-free tourmalines,respectively. This suggested a matrix effect due to ironcontent (L. Ottolini, pers. commun.). For this reason,two calibration curves were used, based on glasses forFe-free samples and on the schorl-dravite standardfor Fe-rich samples.

The presence of Be, C and N was tested to establishwhether they are significant for possible substitution atthe boron site. However, the results obtained did notsupport this conjecfure, as Be content reached only amaximum of 15 ppm, and C and N were practically absent.

Atomic absorption spectroscopy (A4S): Analysis forLi was done also by AAS. Samples were dissolvedusing a solution of HF and HrSOo in an autoclaveoperating at 150"C for 4 h. One reference sample ofhomogeneous elbaite from Espiritu Santo, Brazil, wasanalyzed several times to quantify the experimentalreproducibility: its mean LirO content was found to be2.08(12) wt.Vo.

Thermnl analysis: Thennal analysis (TG and DTA)was done to measure H2O contents using a SetaramTAG 24 instrument operating at the following condi-tions: heating rate 10"C/mn, nitrogen atmosphere,sample weight about 10 mg, alumina crucible, calcinedalumina as inert material.

Ferrous iron determination: The determination wasdone by titration method using KMnOo. Samples weredissolved with a solution of HF and HrSOo in aMilestone MLS 1200 Mega microwave-digestionsystem, operating for I h; a nitrogen atmosphere wasused to minimize oxidation.

DtsrntsL'rroN, OccunnrNce, Texnrnn on ToURMALTN'E

The Cruzeiro pegmatites contain euhedral prisms ofschorl to variously colored elbaite (Table 1), a sequencethat is not frequently found in the pegmatites of Brazil.Crystal dimensions range from some centimeters to onemeter in length, but unfortunately the large prismaticcrystals observed in situ could not be sampled becauseof their intense fracturing, particularly the schorl, sothat only small fragments or occasional small euhedralcrvstals could be extracted.

Samples from dyke I were collected from the JoZode Matos and the Descarga adits; the fonner completelycrosses the dyke and allows examination of the almostperfect symmetry of the zones in the western and easternsides. Samples from dyke 3 were collected from theLadim adit that ends at the pegmatite core. Tourmalinecrystals are ubiquitous, and a-re particularly abundant inthe IIzand the pockets. Tourmaline composition variesfrom outer to inner zones, in accord with the change incolor from black to green and, locally, to pink. On theother hand, there is no tourmaline in the quarz core.

The Bz and Wz contain centimetric black tourmalinecrystals. The prisms commonly appear with the c axisorthogonal to the contact with the country rock. Theyare intensely ftactured, with the fracture plane subnormalto the c axis.

The EIz and M[z contain black tourmaline crysta]sthat vary considerably in size, mostly subnormal tothe contact between the zones. In dyke I, color-zonedcrystals, such as sample 84 with a black core and greenrim, are occasionally found in areas subject to latefluid-rock interaction.

Tourmaline crystals from the 112 of dyke I arecentimetric to decimetric, transparent, and deep pinkand green, whereas those from pockets are smaller,euhedral, transparent, homogeneous in color, green,light green and deep pink (samples 65, 60" 6l R,respectively), or core-mantle-rim zoned, characterizedby different hues of green (samples 62 and 64).

Tourmaline crystals from the intermediate zone ofdyke 3 generally occur as enorlnous, black prisms up toone meter long and 30 centimeters wide, locally with anarrow blue-green rim, and attain up to 30 vol.Votoward the core. Moreover, some small color-zonedprisms also were found, such as sample 93 with a blackcore and a deep-blue rim. Prisms from the pockets aregreen and blue, euhedral and inclusion-free, such assample 95 (whose core is labeled 95V).

Uivrr-Celr- DTTvENSIoNS

Unit-cell dimensions of samples ftom dyke I @ig. 2)plot close to the elbaite-schorl join (Deer et al. 1986).Tourmaline from Bz and, MIz plots close to the schorlend-member, whereas IIz and pocket tourmaline isclose to, or exactly at, the elbaite end of the join. Inparticular, tourmaline from the Wz and the E/z showsevidence of a dravitic trend.

ConaposlttoNaI- RELATIoNS

The chemical composition of the Cruzeiro tourmalinewas investigated in detail on more than 150 5ampleswith over 1500 electron-microprobe analyses (Fig. 3,Table 1). These samples consist of crystalline fragments(from I to 3 mm) and {0001} sections; they weremalyzed at random spots and along traverses (no lessthan 100 pm steps), respectively. The data provide

COMPOSITION OF TOIIRMALINE. CRUZEIRO MINE. BRAZIL

15.90 15.95

4(A)

419

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7.24

7.18

7.t2

7.06

o -

I 5.80 16.0015.85 16.05

FIc. 2. Unit-cell dimensions of tourmaline crystals from the Cruzeiro granitic pegmatitesuite. Reference lines according to Deer et aI. (1986). Open triangles: tourmalinesfrom Bz and MIz. Open squares: tourmalines from Wz and EIz. Open circles:tourmalines from IIz and pockets. The dimensions of the symbols used correspond toabout four times the average experimental error.

Fe, Mn, Zn

Frc. 3. Compositions of tourmaline from the Cruzeiro granitic pegmatite suite, in terms of(Al + Li) - Se, Mn, Zn) - (Mg, Ti), i.e., atomic proportions of the l-site cations (Jolliffet aL 1986). Dotted area: tourmalines from Be to M/2. Striped area: tourmalines from112 and pockets.

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THE CANADTAN MINERALOCIST

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l 9 ! , 9 9 9 9 P i 9 r P P - P 9 -F q u 6 O 6 O € N O O O o F U9 N { U O N O 9 S r O O O 6 @q 9 6 + o O O F O € o H O { O

5 O o O O O o N F O O O O o F' o i r ' a { b i ^ b i o b b L ' o b ' o ' o

- € o H o € N o u o u u H o 69 6 F O l N o U H O O U q F P

A O o O o O O N F O O O o o Hb i ^ b \ b l ' o i o ' o ' o i . J b b i ; -t s a A 9 o 6 N A A - - 5 N 6 4A U E 9 u 6 9 A { O H O O S H

O O o O O O O N O O O O O o Tb i ^ i , !

'@ b l . r 'o

'6 '6 'o ; - . b 'o l r b

6 O U F O 6 p 6 F O { O A 6 gN A E u 6 6 - A { O 6 { - O -

o o a o o o o o o o o o o N oi ^ b t r i ^ b ' o b L L ' o L b ' o ; - i ^6 H { N H O O 6 A { F 6 p O Oa N N 5 F { u u e o o 6 @ { 9

: r 9 : r 9 9 9 9 P 9 9 9 9 P r - 99 N O O O U O O O O N O O 9 69 o O O t s 6 F u 6 O O 6 O t s 96 o b 6 o o @ 9 { o u N F 6 {

! - r 9 t , P P 9 9 j ' P 9 p 9 9 i - 9u N a { o { o - o o 9 o o u uS P + U F F N O o F O F A P No H u o O u @ N 5 9 { { u { 5

o O O O O O O O o O O O O F O' 6 i J r \ b i ^ b b b b i D b b i L

W N t s N F 6 N 9 6 N 9 N U { Ua q N N o u N q 6 H o S e e 6

! r 9 l i P 9 P P : r 9 9 P P P i 99 N P { o O 0 - - o 9 o o A uE N q F F { O O U N N O U 6 He q o 9 5 5 F o a q € o u 6 N

J ' I : a I I 9 I : ' I I I 9 I r - 99 - 5 9 9 q e - C Q o O O 9 {g 3 G s R i S 5 \ 9 6 8 8 9 N

g . g F ^ F F ; r $ F F I F L E3 ^ F F * - 3 S

s

{

{r

{

{

{

F

{

6

6

6

N

e

N

ro N '

I

j{r

{I

Ie s

420

s

l.

?a

I

a

;r

i.F

ot

a

a

x

tid

@

l'q

qa

COMPOSITION OF TOIJ'RMALINE. CRUZEIRO MINE. BRAZIL

TABLE 3. REPRESENTATIVE ELECTRON AND ION MICROPROBEDATAOFTOURMALINES FROM T}IEZONES OFTHE CRUZEIRO PEGMATITE DYKE3

421

Zones

Sample

Bz

8 8 A 88 9 0 A 90

Ltz

9 l 92F 92D 92E 95 V

PIIzll/z

sio2 35.2m 34.71TiO2 0.27 0.32Brq 10.38 10.23AlrQ 33.50 33.46FeO l3.q 11.70MnO 0.30 0.12Mso 1.04 3.r4ZnO 0.27 0.30CaO 0.19 0.13NazO l.9l 2.16KrO 0.04 0.04uzo 0.15 o.o2F 0.43 0.40HzO 2.96 3.lt

SumO=F

Total

si 5.9t2 5.8s9Ar(4 0.018 0.141

B 3.027 2.980

34.48 34.25 36.160.11 0.25 0.1I

l0. lE 10. t2 10.5535.28 34.99 35.E912.21 11.52 8.450.97 0.32 l.3l0.08 1.48 0.250.67 0.23 0.33o. t2 0.53 0.091.99 1.95 2.380.0s 0.04 0.020.45 0.41 0.880.49 0.62 0.993.03 2.88 2.94

0.861 0.7E11.726 1.5310.014 0.0310.139 0.0460.020 0.3730.0E4 0.0290.303 0.2883.146 3.178

0.022 0.0950.652 0.6rm0.01I 0.0090.684 0.745

3.4t7 3.2520.259 0.3323.676 3.584

36.50 37.55 37.48

10.53 10.86 10.8437.63 ,$0.15 37.81

5.23 l .16 3.39l.4l 1.98 2.090. 140.25 0.10 0.270.15 0.28 0.142.51 2.16 2.5r0.04 0.03 0.061 .31 1 .86 1 .58r.26 1.33 1.492.99 3.07 3.34

34.970.20

10.2833.E9t4.21o.40.430.34o.o71.760.020.130.453.08

100.23 99.85 100.28 100.10 99.59 100.35 100.17 100.53 101.20-0.18 4.17 4.19 4.20 4.26 4.42 {.53 4.56 {.63

100.05 99.6E 100.09 99.90 99.33 99.93

Number of iom on the basis of 3l (O,OII,F)

5.924 5.829 5.798 5.9750.076 0.171 0.202 0.025

3.006 2.970 2.956 3.008

99.63 99.97 100.57

5.94s 5.956 5.9840.0s5 0.044 0.016

N(4 6.000 6.000 6.000 6.000

0.655 0.517 0.6931.894 1.651 2.0130.034 0.041 0.0250.043 0.017 0.0660.262 0.79t 0.t0E0.033 0.038 0.0430.100 0.014 0.0873.02|:1 3.069 3.035

0.035 0.023 0.0140.626 0.708 0.5790.00t 0.009 0.0050.569 0.7q 0.598

3.336 3.502 3.4780.227 0.215 0.2413.563 3.7t7 3.719

0.966 Ll691.158 0.7120.013 0.0000.183 0.1940.060 0.0350.040 0.0310.586 0.8503.018 3.000

0.015 0.0260.764 0.8240.005 0.0090.784 0.8s9

3.241 3.2470.518 0.65 t3.758 3.898

2.973 2.986

6.000 6.000

1.464 t.0990.154 0.4520.000 0.0000.266 0.2830.000 0.0000.012 0.032Ll88 1.0143.083 2.879

0.048 0.0580.664 0.7770.005 0.01I0.718 0.847

3.252 3.5570.666 0.7513.917 4.308

6.0006.000

2987

6.000

A(r)FeTiMnMg7^UtstotCaNaKXtoL

OHFOH+F

B2O3, Li2O aad H2O fim ion microprobe. Total iron as FeO. - : Below detrction limit See te)d fofavefage experimefltal €rror&

evidence of almost homogeneous compositions withineach single fragment of tourmaline. Locally, however,chemical variations were found among samplesbelonging to the same pegmatite zone owing to thepresence of large zoned crystals, which subsequentlyfractured. This is indirectly confirmed by a clearlyvisible color zoning in euhedral tourmaline found inthe zones and in the pockets. Inspection of Figure 3allows two areas to be identified. schorl-dravite. related

to crystals from Bz to MIz, and schorl-elbaite for IIzand pockets tourmaline. This is in accord with thedi stribution of unit-cell dimensi ons (Fig. 2).

Thirty-five samples of tourmaline were chosenas representative ofthe chemical variation in dykes Iand 3, and their compositions are presented in Tables 2,3 and 4. The cation distribution was accomplishedaccording to the general formula lDeer et al. (1986),Foit (1989), and references therein; MacDonald et al.

422 THE CANADIAN MINERALOGIST

TABIJ, 4. REPRESENTATIVE EI..ECTRON AND ION MICROPROBE DATA OF ZONEDTOIJRMALINES FROM TIIE CRUZEIRO PECMATITE DYKES

Sampl6 64

m m maril.le m ffi madle

sio2 35.28 36.36 34.85TiO2 0.04 0.06 0.12Brq 10.35 t0.59 to.26AlrG 36.37 37.74 34.04FeO 8.t6 5.56 l3.rzMno 1.44 1.42 0.80MgO 0.40 0.20Z^O 0.33 0.27 0.64CsO O.O2 0.09 0.05NszO 2.29 2.57 1.94KzO 0.03 0.02 0.05Li2o 0.95 t.46 O.52F 1.03 t.4l 0.76HzO 3.13 3.t2 3.0O

Sm 100.53 100.86 100.t6o=F {.43 {.59 4.32

Tolal 100.10 t00.27 99.a4

si 5.83s 5.886 5.905Al(?) 0.165 0.114 0.095

B 2.954 2.959 3.000

N(A 6.000 6.000 6.00{)

A(y) 0.927 1.089 0.703Fe 1.226 0.752 1.859Ti 0.005 0.007 0.015I!!n 0.202 0.195 0.115Mg 0.09t 0.049 0.000Zn 0.040 0.033 0.081Li 0.632 0.951 0.354).tot 3.130 3.076 3.128

C€ 0.004 0.015 0.009Na 0.735 0.808 0.638K 0.006 0.004 0.010Xtot O.7M 0.827 0.658

oH 3.449 3.366 3.390F 0.537 0.722 0.67oH+F 3.9E6 4.088 3.797

36.65 37.q 37.62 3E.340.05 0.08 0.t4 0.09

t0.64 10.83 t0.t7 n.0436.73 38.06 39.15 40.106.03 1.45 2.13 1.15l.l2 3.55 1.83 0.A70.16 - 0.36 0.420.25 0.160.22 0.43 0.17 0.t92.67 2.37 2.29 2.080.04 0.04 0.03 0.031.24 l.6t t.64 l.to1.33 1.49 1.26 0.993.00 3.16 3.22 3.43

100.12 100.63 100.90 100.53{.56 -0.63 -0.53 4.42

99.56 100.00 100.37 100.1 I

1.0630.8230.0070.1550.0400.0300.8152.932

0.03t0.8470.007o.8y2

r.179 1.297 r.4210.194 0.309 0.1510.010 0.017 0.0t0o.4t2 0.246 0.1150.(x)0 0.0E4 0.0990.0tt 0.000 0.0001.039 1.047 l.l3E2.122 3.001 2.935

0.074 0.029 0.0310.735 0.705 0.6330.00t 0.006 0.0060.817 0.7q 0.670

37.80 37.76 38.80- 0.22 0.07

10.91 10.9t I l . l438.57 3t.65 39.650.97 2.56 0.933.3t 1.35 0.92- 0.41 0.16

o.+s i.zt2.33 2.3t0.041.82 1.84t .26 l . l l3.2E 3.33

0.302.010.03t.c71.053.52

Nua$s of im m the basis of 3I (O,OH,F)

5.989 s.992 s.nl 6.0180.0u 0.008 0.029 0.000

3.000 2.994 2.978 2.991

6.000 6.000 6.000 6.000

100.82 100.80 100.45{.53 4.47 4.44

100.29 100.33 t00.01

6.006 5.9s7 6.0TI0.000 0.013 0.000

2.992 2.9a5 3.01t

6.000 6.000 6.000

t.226 1.213 1.3220.129 0.339 0.1220.000 0.026 0.0080.456 0.182 0.1220.000 0.096 0.0370.000 0.000 0.0001.164 1.174 1.1792.975 3.031 2.790

o.oTt 0.0470.71t 0.7320.008 0.0000.803 0.779

3.477 3.s220.634 0.s554.110 4.077

0.0500.6100.0060.666

3.6E3o.5204.202

3.270 3.377 3.4t0 3.5920.686 0.754 0.632 0.4943.955 4.131 4.OA 4.085

BzOg, UzO md Hzo fim im miscprcbe. Total im n FeO. - : Below det€ctim limil S@ l€xt fc avtrageeMimsddm.

(1993)l of the tourmaline group: XY{,&376O27(O,OH)3(O,OH,D, whereX= tst(Na, K Ca, tl), 1- I0r(Mg, pez+,

Mn,Zn, Al + Li, Fek, Ti), Z=t6)(Al,A1+ Mg, Fe*) andz= r41(si, Al).

In Tables 2,3 and 4, iron content is referred to asFeO,o,, because the results obtained show that, for themost part, Fe is in the ferrous state. The few analysesdone indicate that FerO3 varies in schorl from zero(sample 92) to 2.0 wt.7o (sample 9l), whereas in theFe-rich elbaite, FezO.is undetectable. The shortage ofmaterial available for analysis and, above all, thedifficulties in preventing oxidation during the longdissolution times required for tourmaline, did not allowFeO to be determined for the whole sample set.

Composition of tourmalines from dykes I and 3

The most significant chemical variations oftourmaline from both sides of dyke I (Fig. 4) areshown by the l/-site cations (Fe, Mg, Al, Li, Zn, Mn)and the anions (OH, F). The same element variationsoccur for the tourmaline crystals of the western sideof dyke 3 (Fig. 5). Inspection of elemental trends ofFigures 4 and 5 shows common similarities anddifferences between dykes 1 and 3. In particular:

(i) the atuminum content is high in schorl compositionfor dykes I and 3, whereas in pockets of dyke 3 it doesnot reach the values observed in dyke 1;

(ii) the content of iron is particularly high in the inner

I I: l

i l! !h i: iI I

: : r: :! i

- i

COMPOSITION OF TOURMALINE, CRUZEIRO MINE, BRAZIL 423

A 7 A I M I P C IT M BTVR B V i l M N P C M I N v E

l0o 2fi 3(n 440 5&) 6@ 7(X) &)0 9@Zm6(@)

0.&)

.s

0.,10

L50

L6

a 1.50

k l .m

050

0.@l@ 200 3(m 4t0 t(x) 6(n 7@ 8@ 900

ZoB(u)

M i l V D

lm 200 3q) {n rd) 6& 7@ &nZoc(m)

0 l@ 2(}0 3(n ,t00 500 6{t0 700 800 90oZm6(@)

R 7 f l M

M] BTTB D 7 il MI

rm 2m 300 41X) ,@ 6@ 7@ &p roZoE(@)

0 I(x) 2m 300 4{X) Jm 6{n 7@ 8@Zons (6)

a 7 i l M

A 7 H U T

lf

M B ' V B

M g v a

D fl il M N P c N MI EIWD D 7 H M I N P C N M g N D

0.50

0.4)

6O30

3N o.2o

0.10

0.@

0J0

0.()

a030

.9

E o.ro

0.1 0

0.@

l.m

0.80

^ 0.60

-9* o-ao

0.20

0.00

4.m

3f,o

a3.60

.9

O 3.,$

3.m

3.000 l@ 2@ 3@ ,t0O 5@ @ 7d Clo 9@

Zon6 (@)0 100 200 300 400 5@ @ 700 g)0 s

ZoG(m)

Frc. 4. Compositional variation trends of tourmaline-group minerals at Cruzeiro with respect to their position from wesrem (left)to eastern zones (right) of dyke 1. Data from the representative compositions of Table 2.

i i rI I

i

F

I I

t

F i

: i tI

: :: I

: :: :: ii i r

. ! . ! t

: :: :

i i .

:

I

F :

424 THE CANADIAN MINERALOGIST

zone of dyke 3, where large amounts of huge crystals ofschorl are found. In addition, the elbaitic rim of thesecrystals (as in sample 93) does not show the high Licontent and Zn enrichment of the elbaite crystals fromdyke 1;(iii) the levels of magnesium and titanium (Tables 2and 3) reach their highest values in !I/e ofboth dykes;

(iv) the levels of lithium, as of aluminum, do not reachin dyke 3 the values shown in dyke 1;(v) zinc enrichment is characteristic of the strictlyelbaitic composition of the western side of dyke 1;

(vi) manganese enrichment is common to inner zonesof both dykes;(vii) fluorine reaches its highest values in 112 of dyke Iand in the pockets of dyke 3;(viii) hydroxyl reaches its highest values in pockets ofboth dykes.

Finally the Na content of the X site does not show adefinite trend, its values ranging from 0.54 to 0.87 apfu(atoms per formula unit). Since the Ca and K contentsare negligible and no other cations are of importance atthis site (Lucchesi et al., in prep.), the proportion ofvacancies becomes significant in Na-depleted samples.

Figure 4 also allows comparison between tourmalinecompositions from western and eastern sides of dyke 1.In particular, in spite of the symmetry shown by mostof elements, compositions of western-side tourmalinefrom the //z have higher Li,Zn,Mn and F than those ofthe opposite side (Table 2). Zn reaches values up to2.02 wt.Vo ZnO (0.25 apfu), among the highest valuesrecorded in the literature. Because of their Fe contentand lack of Zn, eastern-side elbaite, therefore, showssignificant similarities with the pocket elbaite of dyke3 (Table 3, Fig. 5).

Composition of color-loned crystak

Euhedral centimetric crystals with marked color-zoning occur in dykes 1 and 3. Sample 84,from EIz,and samples 62 and 64, from a pocket, were collectedfrom dyke 1, and sample 93 was collected from the //zof dyke 3. Both samples 93 and 84 (Table 4) show apredominant schorl component in the black core and asignificant amount of elbaitic component in the rim.The Y-cation trends show a sharp drop in Fe from coreto rim, with simultaneous increase in rAl (Fig. 6a) andLi. Moreover, in sample 93,2n decreases sharply fromcore to rim, whereas Mn increases more subtly(Fig. 6b). No well-defined trends for Zn and Mn areobserved in sample 84. Inspection of the data inTable 4 shows that fluorine and hydroxyl behave in aninverse manner: F increases and OH decreases fromcore to rim.

Core-mantle-rim trends of samples 62 and 64 (Table4, Fig. 6c) show enrichment in elbaitic componenttoward the rim, whereas their core and mantle, respec-

tively, are characteized by high Mn and Fe values: upto 3.55 wt.Vo MnO (0.48 apfu Mn) and up to 2.56 wt.VoFeO (0.34 apfuFe). Regarding F and OH, the oppositebehavior with respect to samples 93 and 84 is observed:F is high in the core, whereas OH is typically enrichedin the rim (Table 4).

The composition of zoned crystals of tourmalineshows characteristics that allow comparison betweentheir core-rim trends and the elemental trends along thepegmatite dykes. In particular, core-to-rim trends ofschorl samples 84 and 93 (Table 4, Fig. 6a) reflect theelemental variation from the Bz b ttre IIz (Figs. 4, 5),whereas those of elbaite samples 62 and 64 (Table 4,Fig. 6c) reflect the chemical evolution from the //z tothe pocket in dyke I (Fig. 4), especially in terms ofFand OH.

DISCUSSIoN

(OH + F) excess and bond-valence sum calculations

In Tables 2,3 and 4, the (OH + F) contents in somecases exceed the ideal value of4.00 apfu(Fig.7). TheH2O excess was already pointed out by Foit &Rosenberg (1977) in a systematic study of tourmalinechemistry based on literature data.

For tourmaline, bond-valence sum (BVS)calculations on anion sites (Donnay & Allmann 1970)yield values close to 2.0 vu (valence units) where thesites host oxygen atoms, and values of less than2.O vuin the presence ofother anions such as OH and F.

For the Cruzeiro suite of tourmaline crystals,bond-valence calculations were accomplished usingthe parameters from Brese & O'Keeffe (1991) andthe interatomic distances obtained from l8 structurerefinements with R values from 1.9 to 3.07o (Lucchesiet al.,inprep.). Calculations gave values close to2.Ovufor anion sites from O(4) to O(8), and values lowerthan2.O va not only for O(1) and O(3), as would beexpected from the literature, but also for O(2). In thetourmaline structure, the O(3) site hosts only OH(except in buergerite, olenite, and povondraite), andO(1) can host O, OH and F (Grice & Ercit 1993, Griceet al.1993, Gorskaya et aL.1982, Barton 1969). TheBVS calculations for the Cruzeiro tourmaline crystalsofdyke 1 (Fig. 8a) confirm the occupancy ofthe O(3)site by OH, as the mean value obtained is 1.10 va, andthe occupancy of the O(1) site mainly by F and OH,with values ranging from 0.95 to 1.33 va for elbaite andschorl samples, respectively.

The O(2) site should be occupied by an oxygenatom, so its BVS should approach2.Uvu. The calculatedBVS values decrease from schorl and Fe-rich elbaite toelbaite, with less than2.0 vz for almost pure elbaite.For tourmaline crystals with (OH + F) higher than 4.00apfu,the values reach a minimum of 1.88 vu and areinversely correlated with (OH + F) contents (Fig. 8b).This suggests a partial occupancy of the O(2) site

a

COMPOSMON OF TOIJ'RMALINE. CRUZEIRO MINE. BRAZIL 425

L10

2(X)

1.60

9rro,9

0J0

0.40

om

0J0

0.40

a030

S o:o

0.10

0.00

Zotr6 (@)

B'

R N P C

I

a

II

a

0 20 ,to 60 e re) lm 140 l@ l$ 200

0.s

0.,(}

0 20 ,10 60 &) l(X) 120 t,t{) 16{) 180 200ZoB(@)

20 N 60 $ 100 lz) l,t{) l@ l$ 2fl)ZoB(@)

20 0 & $ lm lD I40 lo lto 200zdE(@)

0 z) ,O 6() 80 lu) lz) l$ 160 180 ?lX)

0J0

0,,t0

a030

5 0..

0.10

0.q)

ZoB(r)

N

ZorB (@)

&l.m

(l80

^ 0.60

* o.lo

0.20

cq)

4.@

3.9)

a 3.0

O 3-d

3.24

3.000 2 0 0 6 0 8 0 1 @ n n l s l & I s m

+

o 2x m s $ r(o 120 l,l0 l@ l$ 2ll0706(@) zon8(@)

Ftc. 5. Compositional variation trends oftourmaline-group minerals at Cruzeiro with respect to thek position from westem (left)zones to core (right) of dyke 3. Data from the representative compositions of Table 3.

a

t

I

t

i

a

a

0 2t 40 60 &) lm lm 140 160 l$ 200

Fo

t r o o o o o

YAIo

o o o o o o

426

1.60

Y r.zo

o

0.80

. l r ' -

by OH, a hypothesis already proposed by Hawthorne(1996) on the basis of stereochemical considerations.As OH-rich tourmaline crystals come mainly frompockets formed in the late stages of crystallization,the presence of OH at O(2) could be a response to thegrowth environment.

M e c hanis ms of subs titution

Compositional variations in tourmaline from theCruzeiro granitic pegmatites are shown in Figure 3.Coupled substitutions that refer either to the schorl orto the elbaite across the pegmatite dykes, are defined as"general", whereas those that chatacteize the featuresof tourmaline from a single pegmatite zone are definedas "specific". They are written as suggested by Henry& Dutrow (1990, Table tr) and Burt (1989). The varia-tion of monovalent cations and anions (R* + OH + F)with trivalent cations (R3.) for all samples allowsidentification of two clusters aloneside end-membertrends (Fig. 9a).

General substitutions: For schorl compositions (inaddition to rMg e rFe), the general substitutions are

rAl3+ + 02- <+ vFe2* + OH AIO(FeOH)_, (1)

xE+ vAl3+<r xNa++ rFe2+ x!Al(NaFe)_, (2).

The first substitution produces "proton-deficientschorl", whereas the second substitution produces foitite,n[Fez*r(Al,Fe:*)]Al6Si6O1t(BOr):(OH)o (MacDonald eral. 1993). The following substitution (Foit et al. 1989)is also present, although to a lesser extent:

vAl3+ + rAl3+<-) rFe2+ + ISi4+ Al2(FeSi)_, (3).

Starting from the elbaite end-member, the maingeneral substitution is 2vP* <+ vAl + rl-i (with trP+ = Fe,Mn, Zn). Another general mechanism to be emphasizedis

zxZ+ vA13+<)2xNa*+ {Lt x[12AlNa_2Li_, (4),

which implies the presence of the "alkali-free elbaite"component. It accounts for the Al excess with respectto Li at the I site and the lack of Na at the X site. Asmentioned by Foit & Rosenberg (1977), this substitutionis not commonly found in elbaite.

Specific substitutions: In the Bz, EIz, and MIz thetourmaline substitutions can be fully described withthe general mechanisms indicated above for schorl.

In the Wz, the dravite substitution reaches itshighest extent (Mg up to 0.99 apfu), and the presenceof Ti (even if in a small quantity) requires the specificmechanism

r|j+, + 2IA13+ <+ YFe2+ + zrsia+ TiAlrFe_,Si_, (5)

which could be considered simply a variation ofmechanism (3) (Foit et al. 1989).

E o.l5

N

.d 0. lo

0.,t0

0.00

0.00 0.,10 0.60 0.80 Lm 120 1.,10 1.60Tmverse (mm)

1.80 2.00 220

2.@ 2.200.00 +

0.00 0.60 0.80 1.00 1.20 1.40 1.60Travene (mm)

a0.40

E o.ro

0.00 1.00 2.w 3.m 4.00 5.00 6.00TraveNe (mm)

FIG. 6. Elecffon-microprobe traverses showing trends in thezoned crystals: (a) sample 93, Fe and rAl; (b) sample 93,Mn and Zn; (c) sample 64, Mn and Fe.

b

Ml

o Lo

7a

t a '

COMPOSITION OF TOURMALINE. CRUZEIRO MINE. BRAZIL

M I E I W B

E Io : 1

E

1

Fi

i

i

Il s

II

I

0 100 200 300 400 500 600 700 E00 900

Zones (cm)Ftc. 7. Variation in (OH + F) in tourmaline crystals from dyke 1 (open squares) and dyke

3 (full triangles) with respect to their position in the pegmatite zones. Tourmalinesamples from the IIz and pockets show the highest (OH + F) values, locally exceedingthe stoichiometric value of 4.O0 apfu. Owing to differences in relative dimensionsbetween the zones ofthe two dykes, the position ofdata from tourmalines ofdyke 3are indicative onlv.

427

4.60

E c.zoR'

I

fr r.ao

3.40

3.00

In the IIz, the general substitution 2vP* <-> v Al + rLi

reaches its highest extent (Fe + Mg + Mn + Zn = 1.45apfu) and, in addition" the specific mechanism

YAll++2O2-eLi*+2OH* AlOrLi_rOH_2 (6)

generates a variable content of the "proton-deficientelbaite" end-member.

In pocket elbaite, several mechanisms are operative:

(i) in crystals 62 and 64;

vE+oH-<_+y l i++o2, r loH(L io ) r Q)

(ii) in sample 65, the proton-deficient substitution (6)was found, as was the case for elbaite from the IIz;

(iii) for samples 60, 6l R, 62mantle, and 64 mantle, inaddition to known end-members, the OH excess couldbe related to X-site vacancy by the following mechanism:

xn + OH- e> xNa* + 02- xnOH(NaO)_1 (8)

trending toward an "alkali-free proton-rich elbaite"end-member (Fie. 9b);

(iv) sample 95, in particular, exhibits (OH + F) excess(0.31 apfu) corresponding to its y- and X- site vacancies:therefore, it is reasonable to assume that the octahedral-cation-deficient elbaite (7) and alkali-free proton-richelbaite (8) end-members act together to produce theOH excess.

Tourmaline composition related to internnl evolutionofthe Cruzeiro dykes

The composition of touma"line in granitic pegmatitesis related to melt composition (London 1986) and inter-actions between pegmatite-forming melt and countryrock (Henry & Guidotti 1985, Shearer et al. 1986). AtCruzeiro, country-rock interactions can be considerednegligible, as the bodies of pegmatite are emplaced inquartzite. The chemical variation of tourmaline is thusdue to the internal evolution of the melt-fluid system.

The ubiquitous assemblage of muscovite andtourmaline indicates the peraluminous character ofthe early pegmatite-forming melt. The high Li contentin tlre elbaite of the IIz and pockets, and the presence ofabundant spodumene in dyke I, suggest that the parentmelt of dyke I was slightly more evolved than that ofdyke 3. A disequilibrium fractional-crystallizationprocess through liquidus undercooling (London 1992)is considered responsible for internal zoning of thepegmatite. The compositional variations of tourmalineobserved in the Cruzeiro suite from the Bz inwardreflect the thermal history of the pegmatite system. Inparticular, the behavior of the l-site cations (includingZn and Mn) shows strong similarities with the idealcovariation of these elements in response to bothdecreasing temperature and increasing fractionationof the melt (Jolliff et al. 1986). The "compositional

428 THE CANADIAN MINERALOGIST

t30

!r.zo

o2 r . loa

L000.90 l.oJ 1.20

BVs to ql) (w)

230

22.n

5 2.r0

N

O 2.oo

6

2 r.goq

I .80

t.70

J0 3.60 3.70 3.t0 3.9t) 4.00 4.10 4.20 430oH+F (aptu)

Ftc. 8. Bond-va'lence sums (BVS), expressed as valence units(ra), to anion sites of tourmaline in dyke 1. (a) Plot ofBVS to O(3) versas BVS to O(1). Horizontal line outlinesthe mean value (1.10 u&) of BVS to O(3). (b) Plor of BVSto O(2) versus (OH + D contents. BVS values higher than2.O vu are from schorl, those values on or around 2.0 vzare from Fe-rich elbaite. and values lower than 2.0 arefrom pure elbaite.

asymmefy" in tourmaline from the western and easternsides ofdyke I can be explained by upward movementof an evolved melt rich in H2O, Li, B, R Mn andZnduring the late stages of the solidification process.This is in accord with the enrichment of Li-, F- andOH-minerals in the upper part of the pegmatite dyke(C6sar-Mendes et al. 7993a), and could have producedthe concentration of incompatible elements toward thewestern side of the solidifying dyke because of itssouthwesterly dip.

The relation between the stability of Li-mineralsand F activiry implies the crystallization of spodumenein F-poor environments (fernf & Burt 1984) and thedeposition of lepidolite in the case of high activity of F(Heinrich 196'1).ln dyke I at Cruzeiro, the high Li andlow F contents in the melt are recorded by ubiquitousF-poor tourmaline and large amounts of spodumenenear the core.

' ' - o E @o : t r "

Coxct-uontc REMARKS

With continuous crystallization of pegmatite,general substitutions produce proton-deficient schorl(eq. 1) and alkali-free schorl (eq. 2) in the outer zones,and alkali-free elbaite (eq. 4) in the inner zones.

Specific substitutions charucterize the Wz schorl,in which rAl balances vTi (eq. 5), and the //z andpocket elbaite, in which compositions trend towardproton-deficient elbaite (eq. 6). Moreover, in pocketelbaite, distinct crystals exhibit particular substitutionssuch as the octahedral-defect type (eq. 7), whichexplains OH excess and vacancies at the Y-site, and theproposed alkali-free proton-rich type (eq. 8), whichexplains OH excess and vacancy at the X-site. Theselatter mechanisms characterize tourmaline fromparticular environments, in which a supercriticalaqueous fluid was plentiful (e.9., pegmatite pockets).This OH excess in tourmaline structure occurs at theO(2) site. It should also be noted that,via the proposedmechanism (8), the deficit of Na at the X site seems tobe related to the partial occupancy of O(2) by excessOH. Finally, the compositional trends of zoned crystalsfrom both intermediate zones and pockets record thechemical variations in tourmaline across the wholepegmatite.

Assuming the "disequilibrium fr actional-crystalliza-tion model" (London 1992) to account for the internalzoning, the evolutionary features shown by thecomposition of tourmaline from the Bz to pocketssuggest the following features of melt evolution:

(1) Tourrnaline crystals display Al enrichment,reflecting the abundance of this element in the melt.

(2) Systematic changes in Z-site cations from the Bz tothe pockets indicate that Mg, Fe, Zn, Mn and Lireached their maximum in activity at different stages ofpegmatite evolution.(3) Crystallization of large quantities of tourmaline inthe /12 removed large amounts of boron from the meltand lowered the solubility of HrO. This accountsfor the exsolution of an H2O-rich fluid, responsible forpocket formation and late crystallization of OH-richtourmaline.(4) The "compositional asymmetry" displayed by thetourmaline crystals from the western and the easternsides of dyke I enabled the late stages of melt consoli-dation of the pegmatite to be reconstructed.

(5) The low F content of tourmaline in this suite reflectslimited F activity in the volatile phase and in the melt,further confirmed by the stability of spodumene both inthe IIz and in the pockets.

AcKNOWLEDGEMENTS

The manuscript benefitted from a review by P.dernj, to whom the authors are indebted. J. MacManusrevised the English text. M. Serracino is thanked for

L35

COMPOSITION OFTOURMALINE. CRUZEIRO MINE. BRAZIL

alkali-frae elbaite orproton-def. elbaite

alkali-free schorl orprotondef. schorl

alkali-free, proton-deficem schorl

alkali-fiee, proton-

defic€nt elbaite

5.00 6.00 7.00 E.00 9.00 10.00Al (aptu)

429

E5.00

4AT

* 4.00

1.00

0.80

40.60

Cd

Z o.qo

8 0 B

84 riE7 s A -

a

.tglo;* '7s

ur# I

62rc

64m

64 rimI

6 l RI

9 5 V

elbaite

l\

I ----f*,i-*

i *m*dksli-fr€

elbaite

0.20

4.00 4.10 4.20 4.30 4.40 4.50OH+F (aptu)

Ftc. 9. Schemes of coupled substitution in tourmaline crystals at Cruzeiro. (a) Plot of(OH + F) + Na yersas AI. The diagram shows how the Cruzeiro samples compare toideal trends for schorl and elbaite. (b) Plot ofNa yerszs (OH + F). The diagram showsthat in OH-rich elbaite, the Na deficiency is balanced by OH excess.

430 THE CANADIAN MINERALOGIST

his assistance during the collection of the electron-microprobe data. This work was supported by aMURST grant. Final reviews by F.C. Hawthorne andF.F. Foit Jr. are gratefully acknowledged.

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Received July 31,1997, revised manuscript acceptedAp r i l T ,1998 .