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t 1 1 sThe Canadian M ine alo gistVol. 36, pp . ll 15-l 137(1998)

CRYSTALLOGRAPHY,INERAL HEMISTRYANDCHEMICAL OMENCLATUREF GOLDFIELDITE,THETELLURIAN EMBER FTHETETRAHEDRITEOLID.SOLUTIONERIES

ALFONSOG. TRTJDU'Department of Eanh Sciences, Monosh University, Claytott, Victoria 3168, Aastrali.a

TJLRICH C\IffTEL,Insrttut fi)r Minzralogie und lagerstiinenlehre, Rhzinischc-Westfiilische Technische Hochschule,Wiillncrstrasse 2, D-52056 Aachzn, Germany

ABsrRAcrThis comprehensive review on goldfieldite, Cu19TeaS13, nd tellurian phases of the tetrahedr ite solid-solution series(teEahedritJ, contains a summary of crystallographic properties and compositional data. A new classification scheme for theseminerals is proposed. Furthermore, the association of these minerals to specific ore environments is discussed. The bond anglesand the optical properties of goldfieldite and tetrahedrite are very similar. The cell dimension of goldfieldite is intermediatebetween that of tennantite and tetrahedrite; thus it is strongly controlled by the ionic radius of Te. As expected, Cu vacancies ingoldfieldite reduce the size of the unit cell. The best electron-microprobe analyses are obtained when S is stendardized ontetrahedrite, bismuthinite or stibnite. The prefened procedure for calculating structural formulae of tellurian elamples of thetetrahedritess from electron-microprobe results is based on total numbers of atoms, as this distributes possible analytical errorsover all elements. Owing to the occurrence of vacancies in goldfieldite, the calculation should be based on 29 atoms per formulaunt (apfu) for minerals with 2 or less Te apfu Fe/(Te + As + Sb + Bi + Te) < 0.51. For more Te-rich compositions, formulacalculation on the basis of 29 - 4f[e{Ie + As + Sb + Bi) - 0.51apfu is r*,ommended. A review of available electron-microprobe

data on goldfieldite and tellurian members of the tetrahedrite,, supports previous observations on the substitutions. For up to 2 Teaplu, substitution of Te4 for (As, Sb, Bi)3+ s coupled with Cu+ or (Fe,Zn)z* subsdrution, whereashigher levels of Te result invacancies (reduced Cu contents). A nomenclature grid for goldfieldite and the other Cu-dominant examples of the tetrahedrite""is proposed on the basis of their semimetal content. The name "goldfieldite" is reserved for those compositions of the tetrahedrite""that contein more than3 apfu of.Te. For thoseminerals with 2 to 3 apfuof Te, modifying adjectivessuch as arsenoan, tiboan orbismuthoan should be used, according to the second most abundant element. The remaining @11urianminerals of the tetrahedrite""should be named according to the most abundant semimetal (tennantite for the As-rich member, tetrahedrite for the Sb-rich one)preceded by "tellurian", provided that at least I apfa of Te is present. Most of these tellurian minerals of the tetrahedrite"" arefound in high-sulfidation epithermal Au deposits, regardless whether they are As- or Sb-rich. A few cases have been reportedfrom porphyry Cu and volcanogenic massive sulfide deposits.Keywords: goldfieldite, tetrahedrite solid-solution series, electron-microprobe data, substitutiotrs, strucnual formula,nomenclanue, ore deposits.

SoMrr{arneCette dvaluation compr6hensive de la goldfieldite, CuroTeaSrr, et des phases riches en tellure de la solution solide ditetetraedritess, contient un sommaire des propri6t6s cristallographiques et des intervalles de composition. Nous ptoposons unnouveau sch6made classification pour ces min6raux. De plus, nous discutonsde I'asso"136i6a s sssmin6raux avecdes contextesde min6ralisation sp6cifiques. Les angles des liaisons et les propri6t6s optiques de Ia goldfieldite et de la ra6drite sont trCssemblables. Le parambre r6ticulaire de la goldfieldite est interm6diaire entre celui de la tennantite et celui de la t6traddrite; iI estdonc fortement d6pendent du rayon ionique du Te. Tel que pr6dit, les lacunes dans a position Cu r6duisent le paramCtre 6ticulaire.Les meilleures donndes obtenues par nicrosonde 6lectronique sont attendueslorsque la srandardisation pour le soufre s'effectuesur la t6tra6drite, la bismuthinite ou la stibnite. Le protocole recommand6 pour le calcul d'une formule structurale de cristau( dela s6rie t6na6drite,, riche en Te i partir des r6sultats de microsonde est fond6 sur le nombre total d'atomes, ce qui distribue les

Presentaddress:CSIRO- Minerals,P.O. Box 83, Kenmore,Queensland 1069, usralia E-matl a.ddress: lfonso.trudu@minerals.csifo.auCorresponding uthor, o whom reprint requests houldbe addressed. -mail aidress: knittel@rwth-aachen.de

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I 1 1 6erreurs amlytiques Possiblessur tous les 6l6ments.Vu la pr6sencede lacunes dans la goldfieldite, le calcul devrait supposer29 alomespar t'nit6 formulaire (apzj pour les cas oil le Te 6quivaut A2 apuf ou morns Te(Ie + As + Sb + Bi + Te) < 0.51.Dansle cas de compositions plus enrichies en Te, le calcul de la formule se fait sur une base de29 -4(lel(Te + As + Sb + Bi) - 0.51apuf. Un examendes donndes disponibles de microsonde dlectronique portant sur la goldfieldite and et les membres enrichis entellure de la sdrie t6tra6drite"" 6taye les observations ant6rieures d propos des sch6mas de substitution. Oil le Te 6quivarlitd2 apttou moins, la substitutionde Te# d I'ensemble (As, Sb, Bi)3* est coupl6eau remplacement de Cu+par I'ensemble (Feln)2+; enrevanche, les niveaux plus 61ev6sde Te mbnent d des lacunes (teneur r6duite en Cu). Nous pr6sentons une grille de nomenclaturepour la goldfieldile et les aufes min6raux d dominance de Cu de la s6rie t6tra6drite"" selon leur teneur en semim6tal. Le nom"goldfieldite" est r6serv6pour les compositions de la s6rie t6tra6drite""contenantplus de 3 apuf de Te. Pour les compositionscontenantentre 2 et3 apufdeTe,nous proposons es qualificatifs arsenical,stibi6, ou bismuthifdre, selon equel des rois 6l6mentsAs, Sb ou Bi est le plus enrichi. Les autres cas porteurs de Te de la s6rie t6tra6drite"" devraient porter le nom du min6ral selonl'ldment senfm6tellique le plus abondant (tennantite pour le membre arsenical, tdtraedrite pour le membre stibi6) pr6c6d6 par"tellureux", moyennant au moins I apuf deTe.Laplupart des exemples de la s6rie t6tra6drite"" enrichis en tellwium proviennentdes gisernentsd'or 6pithermaux h sulfidation 6lev6e,qu'ils soient riches en As ou en Sb. Quelquescas ont 6t6 signal6sdans esgisements de cuivre de type porphyrique et les gisements volcalogdniques de sulfures massifs.

(Iraduit par la Rddaction)Mots-cl6s: goldfieldite, solution solide de la t6tra6drite, donn6es de microsonde 6lectronique, subsfitutions, formule structurale,nomenclature, gftes min6raux.

IlrRopucuorlGoldfieldite, ideally Cu1sTe4S13,s shown byKalbskopf (1974) and Johnson& Jeanloz(1983), is thetellurian end-member of the tetrahedrite-tennantitesolid-solution series(henceforth tetrahedriteJ. The pureend-member has not been found as natural mineral; amaximum of 3.4 Te (out of a possible4) atomsper for-mula unit (apfu) has been reported (Trudu & Kninel,

in press). Goldfieldite, tellurian tetrahedrite andtennantite have been reported to occur in various typesof ore deposit, nclurling epithermal gold deposis, por-phyry-Cu deposits, and volcanogenic massive sulfidedeposits.Goldfieldite was fint describedby Sharwood(1907)and later identified by Ransomeet al. (1909) n oresfrom the Mohawk mine at Goldfiel4 Nevada. Thompson(1946) summarized he controversyregardingthe inter-pretation of this mineral, which was thought to be pos-sibly a mixture of different known minerals (e.g.,tetrahedrile + calaverite, or famatinite + bismuthinite +calaverite or sylvanite), even if it appearedto be homo-geneous.For a goldfieldite specimen from the Clare-mont mine at Goldfield, he obtained an X-ray pattemanalogous to that of tetrahedrite and hence, proved it tobe a member of the tetrahedrilesolid-solution series.ln his classificationsof sulfosalts,Nowacki (1969)regarded goldfieldite as a variefy of tetrahedrite andbelieved that Te substituted for S. Kato & Sakurai(1970a) were the fust to infer that Te substitutes for Asand Sb. Kalbskopf (1974) ndependently came to rhes4me conclusion and suggested he end-member for-mula Cu qTeaS .Charlar & lt'vy ( I 974) speculated hatin goldfieldite, Te can be both a cation, substituting forAs and Sb, and an anion, replacing S. Kase (1986) pro-posed that substitution of Tea+ or the trivalenrsemimetals (As, Sb and Bi) is compensatedby an in-

crease f monovalent tomsmainlyCuwithminorAg)from 10 o 12apfu at the expense f a decreasef thedivalent elementsFeandZn) from2 to Oapfu eadingto the ormulaCu12(As,Sb,Bi)zTezSrs.or highercon-centrations f Te, thechargebalance n tetrahedrite""smaintained hroughvacanciesn oneof the sitesoccu-piedby monovalentmetalatoms Kalbskopf1974,Dmitriyevaet al. 1987). hese elationshipsavebeenconfirmed or naturalgoldfieldite (Trudu & Knittell99l,in pressohimizu& Stanley1991).DeMedicis& Giassonl97la) investigatedhesys-tem Cu-S-Te at 340"C,but did not report the occur-rence of goldfieldite. In contrast,Karup-Mgller(in prep.) oundgoldfielditeat 350' and450'C in hisexperimentalnvestigation f thesame ystem.We havecollected esultsof over our hundredelec-tron-microprobeanalysesof natural tellurian tetra-hedrite,"plusa few from syntheticmaterial.Thesedataform hebasisor this eview, n whichwealsosumma-rjze anddiscusshe crystnllography,pticalpropertiesandmineral chemistryof naturalandsyntheticelluriantetrahedrite"".We proposea new schemeof classi-ficationbasedon mineral chemistryand discuss heocclurenceof theseminerals n different types of oredeposits.

Cnysranocnaprilc PARATmTERSGoldfieldite,as all mineralsof the tetrahedrite"",scubic, a derivativeof the sphalerite tructure, pacegroup43m(tt/lensch1964)withtwo ormulaunits ercell. The formulaof a half unit cell of tetrahedrite""is generallygiven as: M*rcfuP*zF*+S13,hereM+ =

Cu, Ag; lE+ = Fe, Zn, M4 Cd, and Hg, andX = Sb,As, Bi, Te.Half of the twelve M sites are etrahedrallycoordi-nated Ml), and he remainderare n a triangularplanar

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1 l 8TABLE 2. MEAIIURED AI{D CAI.CUI.ATED BONIIITNGIIilIIN SYNTIIETIC GOLDFIBDTT4 ARSENOA}IGOLDFIEIDII4

B@d IR@ uid CN /,.100 IRe EIR'i80d9

neaercd A(elc.-@.)

Ml-Y At:O.63s I M.61 S:1.70 2.335 2.37(2)ffi -O.O35(2)2.313(9) cfl 0.02

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GOLDFIELDME, TIIE TELLURIAN TETRAHEDRITES 1 t1 9

-vloC'EI

(J.Eu

synth.gfd.C! r Gamble1974)t t t r M 2 - Yo M 2 - Zsynth.gfd. r

Iars.gfd.O

tenn. o

0 0.10 0.20 0.30fi

Frc. 3. Plot of fractional ionic character (J) versu.s he differ-ence beflreen calculated and measured bond-lengths (in A).The line represents the linear regression through thedata ofGamble (1974), which is represented y filled squares.Seetext for further explanation. synth. gfd.: syntheticgoldfieldite (Kalbskopf 1974), ars. gfd.: arsenoangoldfieldite @mitriyeva et aI. 7987), tenn.: tennantite(Wuensch et aI. 1966), ter.: Etrahedrite (Wlensch 1964).

tennantite o arsenoan oldfieldite s indicated. n con-trasl thesyntheticgoldfielditehasa significantlyshorterM2-Y bond-length2.16 t 0.05 A versus .240 0.0 0A); however,Kalbskopf 1974)proposedhat a rela-tively largeerror might be attachedo thatvalue owingto an ncreasedemperafure-factor.All measuredM2-Z bond lengths areshorter hanthe calculatedvalues.Measuredbond-lengthsshorterthan hesumof the onic radii of the elements oncernedwereobserved y Gamble 1974) o be inverselypro-portional o the ractional oniccharacter/) in sulfides,selenides nd elluridesFrg.3), with fi definedas:fi (A - B) - 1- exp -{.25 (xa- xs)2] (l )

@auling1948),whereA andB are he two atomsn-volved in thebond, and1, their electronegativities, scalculated y Pauling 1948).However,use of thesmaller adii observedn sulfides Shannon 981;Table I of this paper) ed to goodagreement etweencalculated rd observed ond-lengthsor MI-Y ardM2-Y bonds in tetrahedrite, ennantite and arseniangoldfieldite.Only heshorterbond-lengthngoldfielditewould be consistent ith the relationship bserved yGamble(L974).Hence, he shorterbond-lengths ngoldfielditemight be due o thevacanciesn fheM2 site,rather han o decreasingonic character f the bond.

T\e X-Y bond lengths show some contradictoryresults. The measured Te-S and As-S distances insynthetic goldfieldite, arsenoan goldfieldite andtennantite are onger than thosecalculated, a fact in con-trast with Gamble's (1974) observation.Thesediscrep-ancies may be the result of analytical errors related tothe chemical composition the mineral, errors in thedetermination of the bond lengths,possiblechanges ncoordination of the semimetals as a result of vacanciesin the structure,or inapplicable ionic radii.The possibility of an analytical error was discussedby Wuensch et al. (1966) in their study of the structureof tennantite; the mineral they used could have con-tained minor amountsof Sb, thus increasing the bondlength. A similar error could not have affected syntheticgoldfieldite. Substantial errors in measurements of thebond lengths are unlikely, though Kalbskopt (1974)reported a relatively large uncertainry of 10.03 A forthe measuredTo-S bond length in synthetic goldfieldite.This leavesthe possibility that the estimatedionic radiusfor mTe is slightly too low at 0.66 A.Vacancies are reportedonly in goldfieldite, but theyshould not affect the coordination of Te (KalbskopfI974). If they did, these would result in an increase inthe radius of Te, which would make the differencebefween calculated and measuredbond-lengths positive(Table 2), in agreementwith Gamble's (1974) generalprediction for all minerals.Thus, with the data currently available, no viablehypothesiscan be proposed to explain why the meas-ured bond-lengths for synthetic goldfieldite, naturalarsenoan goldfieldite and tennantite exceed the calcu-lated values. More accuratemeasurementscan possiblyhelp to solve this problem. The X-Y bond lengths intetrahedrite are n agreementwith Gamble' s (1974) pre-diction, though we used theradii applicable o sulfides.There are very limited published data regarding thebond angles n goldfieldite. Kalbskopf (1974) teportedthat in pure goldfieldite, the S-Te-S angle is 95.7 + 0.7",which is very close to the corresponding value (95.1t 0.2") for tetrahedrite measured by Wuensch (1964),as are reported values for the two S(I0{u(M1)-S(Dangles in arsenoangoldfieldite @mitriyevaet al. 1987).Cell dimensions

Currently available data for the cell dimensions ofgoldfieldite are summarized in Table 3, which also listsreference values for synthetic tetrahedrite and naturaltennantite. The prediction of cell dimensions as afunction of the different types of substitutions intetrahedritess was flust attempted by Charlat & I-Evy(1975) and subsequently refined by Mozgova et al.(1979) arrdJohnsonet aI. (1986,1987). However, onlyJohnsonet al. (1986,1987) provided details on the typeof statistical analysis used.Te-for-(As,Sb) substitutionis only taken into account by the equation given byMozgova et al. (1979):

0 . 1 6

o .12

0.08

0.04

o.00

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GOLDFIELDITE, TIIE TELLURIAN TE'TRAHEDRTES 1l2l

M*

3.0Te

3.5 4.0

TOTALStSeATOMSFrc. 4. Plots showing the effect of the choice of the referencematerial for Scalibration on the analytical data- All the datain this diagram are ftom the Tirad porphyry Cu-Au pros-pect, northwestem Luzon, Philippines (Trudu and Knittel,in press). Legend: analyses with S calibration carried outon ttrahedrite, stibnite andlis6uthinite (circles in upperdiagram and stippling in lower diagram), and on aneno-pyrite (crosses n upper diagram and harching in lower dia-gram). All concentrations are expressed in aplu.

In the second study, a l2lsb Mtissbauer investiga-tion ofseveral Sb-bearingminerals, Hedges& Stephens(1977) determined hat the Sb in goldfieldite is 3-coor-dinated, confirming the result of Kalbskopf (1974).CarmnnrroN oF ELEcTRoN-MrcRopRoBE ATA

The quality of electron-microprobe data on gold-fi eldite,generallyobtained n thewavelength-dispersionmode, surprisingly seems o depend on the choice ofthe reference materials used for calibration, in particu-lar for analysisof the samples or S. Figure 4 shows hatcalibration of S on an arsenopyrite standard gave lowvalues for S in goldfieldite, resulting in too high anumber of atoms per formula unit, when formulae werecalculated on the basis of 13 S. Calibration on stibnite.

bismuthinite and tetrahedrite produced markedly betterresults (Fig. 4). A similar behavior in electron-micro-probe data was reported by Harris (1990) for Ni-Hgsulfides: HgS analyzed with S standardizedon NiS at20 kV yielded valuesup to 7 wt . Vohtgfier than expected,whereas the opposite effect took place when NiS wasanalyzedwith S standardizationon HgS. Harris (1990)ascribed hesepoor results to the correction programs,which inadequatelydealt with the absorption edge ofthe standards.Analytical problems with Cu in tetrahedrite that isnot fully substituted have been discussedby Lind &Makovicky (1982).However, the availabledata suggestthat tellurian tenahedrite"" is in general fi.rlly substituted.It is not known whether Se, he other anionpossiblypresent in goldfieldite, should be calibrated under con-ditions similar to those of S. Se-bearinggoldfieldite isvery rare. The only published data pertain to materialsfrom the Ozernoye Au deposit in Kamchatka, Russia.Spiridonov & Okrugin (1985) did not mention whichSe standard they used. For Te and most of the otherelements, standardization on pure metal pfoduced goodresults (Trudu & Knittel, in press). This shows thatmatrix effects have a negligible effect on the measuredconcentrations of the cations, as already pointed out byHanis (1990).Altogether, 19 elements have been reported to occurin tellurian tehahedrite"",but only the following are rou-tinely detected:S, Cu, Fe,Zn, As, Se, Ag, Sb, Te andBi. Of these,Bi and Se tend to be neglected n routineanalysis of goldfieldite. Bi substitutes or Te, and itstheoretical end-memberwill be used n the new classi-fication scheme or goldfieldite (seebelow). Neglect-ing to analyze or Se,even f present n amountsas owas I wt.%o,can lead to the erroneous assumption thatlow S is caused by a poor calibration of the electronmicroprobe.It must be kept in mind that ow concentra-tions of Se may not appear on an elemental spectrum ofgoldfieldite, becausecritical X-ray absorption lines ofSepartially overlap hoseof As, which is usuallypresentin larger amounts than Se in this mineral.Elements of secondary importance, Au and Pb, andminor or trace amountsof Hg, Mn, Cd, Sn, Co, V andGe have been reportedonly occasionally.

Cer,curanow oFTHESTRUcTuRAL onuureStructural formulae for tetrahedrite"" areusually cal-culated on the basis of 13 anions (S + Se) or 29 totalatoms. Seal et al. (1990) suggested hat a calculationprocedurebasedon 4 semimetal atoms(As, Sb, Bi, Te)be adopted, although they admit that small analyticalerrors for the semimetals can cause larger errors inrecasting the number of sulfur and metal atoms. Thisproblem is also inherent in the calculation of the struc-tural formula on the basisof 13 anions, n particular, ifSe is not sought, but is present n substantial2mounts.In addition, analytical data for S may be problematical,

1 31 2

1 1

1 0

-d"ail"- - o* -9td9*" *..s16?&L**- o \ o - _

2.5.O

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r122as outlined above. It is even less suitable to calculatethe struchral formula for goldfieldite based on 12metal(Cu, Ag, Zn, Fe,etc.) atoms(e.g., Charlat&Itvy 1975)becausevacanciesoccur for more than 2 atoms of Te(apfu). T\e same reason should make the calculationprocedure basedupon a fixed number of total atomssuspect, as this varies from 29 for Te S 2 apfu to27 apfu for end- membergoldfieldite.Keeping in mind all these considerations, we preferto calculate the structural formula of goldfieldite andother tellurian minerals of the tetrahedrite"" using themethod basedon the total number of atoms.This proce-dure has the advantage of spreading the analytical errorover all the elements sought. In order to take into accountthe occurrenceofvacancies forTe>2 apfu"we suggestthat l) for Te/(Te + As + Sb + Bi) < 0.5, the calcularionbe basedon 29 apfu, as no vacanciesoccur n the crystalstructure;2) for Tel(Te + As + Sb + Bi) > 0.5, rhe for-mulae are to be calculatedon the basisof29 - 4lTel(Te + As + Sb + Bi) - O.5lapfu.

Mwenar CrnpusrnYWe have assembled a databasecontaining all resultsof electron-microprobe analyses of tellurian membersofthe tenahedriters group available to us, using an arbi-trary cut-off of 0.3VoTe (ir all, 354 compositions, asshown in Table 4). No attempt was made to evaluatethe qualiry ofthese data,becausevarioustypes oferrorscan affect them, e.g., counting statistics, matrix correc-

tion, use ofinappropriate standards,etc.,blt we haveexcludedall compositions with totals outsidethe range97-1O2Vo.In he following.discussiono e will, how-ever, point out inconsistenciesn some datasets.In the following sectionsowe summarize what isknown about the distribution of the elements on the dif-ferent structural positions. Where no information isavailable, we will discuss the possibilities based onGoldschmidt's rules of substitution, as summarized nFaure (1991), and with reference to other studies oftetrahedrite-group minerals.Thc M sites

In tetrahedrite"", there are two sites occupied bymetals, Ml and,M2, whrchare 4- and 3-fold coordinated,respectively.Cu is the main occupant of the M sites; tmay bereplacedby variableamountsof Fe, Zn, Ag, Mn,Cd and Hg. In the tellurian members of the tetrahedrite"",X-ray-diffraction studies(Kalbskopf 1974,Dmiriyevaet al. 1987)have shown that up to two vacanciesoccurin the M2 site where the Te contents exceed 2 apfu, aspreviously discussed.11s srystal ographic positionsof the metals(Fe, Zn,Ag, Mn, Cd and Hg) substituting for Cu in the tetra-hedrite*, and thus in goldfieldite, must be re-assessedin light of the findings of Charn ock et al. ( t 989a,b), whousedextended X-ray absorption fine structure @XAFS)

TABLE 4. LIST OF SOURGS OF ELECTRON.MIG,OPROBEDATAASSEMBLEDFOR TITIS STTJDE AND NTJMBEROF DATASETS

Borisowsra! (1986)Dditiyffistal (1987)Frwzd,et aI (197,Igum(1986)JCPDS1990)Kmp-MsXq (1992) sydhstio laKre(1986) l0Kdo & Sakumi 1970a) IKato & Sshmi (1970b) 1Ktritol(tEpubl. dda) 2aKnitel (1989) I IKovaldksetal (1989) 27Kovsloks&Rusitrqv(l9EO 9lKovalmko&Trom(1980) 8Koralskqeta, (1980) 18Kovalskseral (1989)* 20Totd' ccluding wa omporitim o.lredynputed in Kmloku & Ruinov (198Q.Tho omplete dcahs mybo obtaiaed m E(CEL file fron the snd ailor.

and Mtissbauer spectroscopy, and Makovicky et al.(1990),who studiedsynthetic etrahedriteby Mdssbauerspectroscopy. These studies in part arrived at differentconclusions relevant to tellurian tetrahedriters.Copper, the most abundant metal in goldfieldite,occupiesboth the Ml and.M2 site. Its oxidation statehas been the source of speculation; however, X-rayphotoelectronspectroscopy fabout twenty sulfidesandsulfosalts, ncluding brahedrite, by Nakai et aI. (1976),asquotedby Kase (1986), indicatesthat Cu tends o bemonovalent in these minerals. Reported concenffationsof Cu in the tellurian members of the tetrahedritersvaryfrom 5.88 to 12.74 apfu, but values of less than cc.9.5 Cu apfuwere obtained only for Au-rich goldfieldite(Figs. 5, 6; Kovalenker & Troneva 1980), which prob-ably is not really goldfieldite but a mixture of differentminerals (seediscussion below), Problems of electron-microprobe analysis of tetrahedrite not fully substitutedwith Cu were discussedby Lind & Makovicky (1982),but the trend displayed in Figure 6 suggests hat Cuinvariably is in excessof lO apfu, where this is requiredby coupled substitution as a result of Te incorporation.The Cu contents are a function of two types ofsubstitutions,Ag + Au for Cu, and Te for (As, Sb, BD.The plot shown as Figure 5 indicates that the fust typeof substitution is of minor importance, as the concen-tration of the precious metals rarely exceeds 0.5 apfu.The data of Charnock er aI. (1989a, b) indicate thatAg-for-Cu substitution occurs at the M2 site in bothnatural and synthetic tehahedrite. Figure 6 shows therelationships between concentrations of monovalentmetals (mainly Cu) and Te; Cu increases rom l0 to12 opfu as Te concentrations reach up to 2 apfu. Forhigher amounts of Te, the Cu content decreases tolO apfu, asvacanciesn the M2 site appear Kase 1986,

Koval@kset4l (1990) 35r.&y(r 7) IIpgircvaal (1983) 4Mozow & Ts'pitr1983) 8N@gorcdmerql (1978) 19Nwiello (19E9) 3Sakhrcwsral (1984) 6Shimia& Stslsy (1991) I ISpiridmov(1984) ISpiridonov(I987) 3Spiidowstal (1984) 6Spiridow&Okrugh(1985) 3Spdrgs(l%g) 2Tru&&Kdttel(mpm) 32Twpt\etql (l9n) IWilga[iseral (1990) 2

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GOLDFIELDITE, TTIE TELLIJRIAN TETRAHEDRITE$ r l23l 31 21 11 0c u 9I765

_i.ji"::-:.b tr_Ut r A

t r - 4aA- - - - - - - - . -a.trA

oacrl.isl;,:A

t r t r

-#"6ffi-"-r 12tr-LFffi. -R: E-S".O-ad- tr -Ea A.o --_!Hufrg.tr- --__{:^:-'-a *o ;-;i>.-\