207 Thz CanadianMineralogist Vol. 36,pp. 207-213(1998)
SOLIDSOLUTIONIN SYNTHETICZINKENITE,ROBINSONITEAND MENEGHINITEIN THE SYSTEM CurS-PbS-Sb2S3 KAMAL L. PRUSETHI ANDBISWAJITMISHRA Deparxnent of Geology and Geophysics, Indian lwilrute
of Technoktgy, Kharagpu7 721302 India
HEINZJ. BERNHARDT Insritut RuhrUniversitiit Bochum" D44780Germany fiir Mineralogie, ABSTRAcT The extent of solid solution in zinkenite, robinsonite and meneghinite has been deternined from electron-microprobeestablished compositions of their synthetic analogs in pertinent assemblagesin the course of a phase-equilibrium study of the system CurS-PbS-Sb2S3.All three solid-solution fields are elongate more or less parallel to the PbS-SbrS3 binary join, and are relatively broader at their PbS-rich ends. The positions of the PbS-rich ends are practically insensifive to variations in temperature. Robinsonite and meneghinilg are not stable at 300"C. Plots of molar propoftions of Sb2S3yersus PbS suggest a cofirmon scheme of substitution, 3Pb2+=2Sb3+. In PbS-rich compositions of the solid solutions, the proportion of Cu is substantial, and Cu must be incorporated on the left-hand side of the substitution scheme.Natural compositions of meneghinite cluster at one point in the CurS-PbS-Sb2S3temary system, but zinkenite and robinsonite have variable compositions. The most appropriate formula for zinkenite appearsto be 6PbS.7SbrS3.Likewise,4PbS.3Sb2S3 is the preferred formula for robinsonite. Keryords: solid solution, zinkenite, robinsonite, meneghinite, substitution scheme, phase-equilibrium study.
Sowerne Nousavonsd6termin6 l'6tendue desolutionsolidepourlesmindraux zinkenite, robinsonite et m6n6ghinite d partirde donndes obtenuespar microsonde 6lectronique sur la composition d'dquivalents synthdtiques, dans le contexte d'une 6tude du systbmeCurS-PbS-ShS3. Les trois ghamps de solution solide sont allongds plus ou moins le long de Ia s6rie binaire PbS-SgS., et deviennent plus lmges d leur extr6mit6 riche en PbS. La posidon de ces extr6mit6s est pratiquement insensible aux variations en temp6ratue. La robinsonite et la m6ndghinite ne sont pas stables A 300oC. La relation entre fractions molaires de Sb2S3et de PbS fait penser que le schdma de substitution 3Pb2*+ 2Sbk s'applique aux trois phases.Dans les compositions proches du p6le PbS, la teneur en Cu est importante, de sorte que le Cu doit participer avec le Pb dans ce sch6ma de substitution. Les compositions de mdn6ghinite natuelle sont concentrdes i un point dans le systbme ternaire Cu2S-PbS-Sb2S3, mais la zinkenite et la robinsonite ont des compositions variables. La formule 6PbS.7Sb2S3semble 6re la plus appropri6e pour Ia zbkenite. De m6me, nous pr6conisons la formule 4PbS.3Sb2S3pour la robinsonite. (Traduit par la Rddaction) Mots-clds: solution solide, zinkenite, robinsonite, m6n6ghinite, sch6ma de substitution, 6tude de 1'6quilibre des phases.
hnnolucnoN Evidence from nature does not seem to support the generally accepted structural formulae of zinkenite (PbS.SbrS3) and robinsonite (7PbS.6Sb2S3).Both zinkenite and robinsonite contain minor elements, like Cu, Zn and Fe, in addition to Pb and Sb. Our experimental study of the pseudoternary system CurS-PbS-Sb2S3 proves that these phases show a considerable 4mount of solid solution. I E-mail address:
[email protected]
We hereprovideresultsof experimentalstudiesin Theseresultssupportthe the systemCurS-PbS-Sb2Sr. formulae6PbS.7SbrS3 and4PbS.3Sb2S3, respectively, for zinkenitearrdrobinsonite.We alsoattemptto draw a parallelismin the natureof solid solutionin zinkenite, robinsonite and meneghinite.Phaserelations were determinedat 500"0 440" an'd 300"C (Pruseth er al. 1997).Becauserobinsoniteandmeneghinitewere found to be unstable at 300'C, we restrict our discussionto the 500oanld440"Cisothermsonlv.
208
TIIE CANADIAN MINERALOGIST
B acrcRour.p hmonvterroN Garvin (1973) failed to synthesizehomogeneous zinkenite from the starting material PbS.SbzS:. Robinsonite was found to form with zinkenite. However, Hanis (1965) obtained a homogeneousphase of zinkenite from 6PbS.7Sb2S3.Lebas & Le Bihan (1976) proposed the formula Pb'*oSbo-nS, for zinkenite,with 0.50 3n.@Zc-nsZs Chrgs Balee x lql, wh@ nc, ns, Ze ud Zs re the fmrla otole md &rge of the md mlfir, epetively. metsls (+ @imetab)
All structural formulae, accordingly, have been calculated on the basis of 27 sulfur atoms for zinkenite and 13 sulfur atoms for robinsonite. The absolute charge-balancevalues for all compositions also are consistently low (Table 1). The only set ofanalyses of robinsonite from a single run-product at 500oC does not show the general trend of negative charge-balance. The meneghinite solid-solution field at 500oC extends beyond the phase-Z field and partially enclosesit. The PbS-rich ends are relatively stable, and the shrinkage at
lower temperature occurs from the PbS-poor ends (Fig. 1). Copper tends to be enriched with increasing PbS, and the maximum possible content of copper seemsto be unaffected by change of temperature. Drscusstox A compilation of compositions of natural zinkenite, robinsonite and meneghinite from various localities is presented in Table 2. Figure I shows the same
210
TIIE CANADIAN
-
44OoC 550"C Msnoghlnlte l
Robinaonlte
o Zink6nlte
FIG. 1. Compositions of zinkenite, robinsonite and meneghinitefrom selectedsulfide ore depositsplotted on a portion of the systemCu2S-PbS-Sb2S3. Mineral abbreviationsare same as in Table 1. Symbols: M meneghinite,ZphaseZ. information plotted on a portion of the 440'C isotherm, experimentally determined in the CurS-PbS-Sb2S3 system (Pruseth et al. 1997). It is clear that natural samples of zinkenib and robinsonite tend to have variable compositions, whereas meneghinite has a more or less constant composition despite its relatively larger extent of solid solution, as determined experimentally. One composition that does not fall within the scope of the zinkenite solid-solution field is the PbS.SbrS, phase ofJambor & Owens (1982), referred to in the introduction. The two compositions of meneghinite that plot off the main cluster are from Rajpura-Dariba, lndia, and contain considerable T1. kevious investigators, e.9., Salanci & Moh (1970), Garvin (1973), Craig et al. (1973), Hoda & Chang (1975) and Salanci (1979) reported no solid solution in either zinkenite or robinsonite. On the basis of compositions of natural phases,Mo6lo er al. (1984) showed zinkenite to have compositions ranging from approximately 44 mole 7oPbS on the PbS-Sb2S3binary join to a point with nearly 47 mole VoPbS, but with approximately 3 mole VoCurS. However, they did not indicate the probability of a solid-solution field for zinkenite. Experimental results of Pruseth et al. (1997) show that the zinkenite solid-solution field is the largest at 500'C and gradually shrinks with a decrease of temperature (Fig. l).
MINERALOGIST
The ternary composition of zinkenite that is stable with any fwo of the phaseschalcostibite, phase Z, and robinsonite has the composition 0.49Cu2S. 5.74PbS . 6.92Sb2S3.The most PbS-rich Cu-free zinkenite is 5.75PbS. 7.80Sb2$. Paradoxically,the ratio PbS:Sb2S3 in the former equals 0.83, whereas in the latter it is 0.74.To attain the ratio PbS:SbrS: = l, thus still more copper has to enter the structure of zinkenite. Conversely, there seems to be an upper limit to the amount of copper that can be incorporated in the mineral. At 500'C, the maximum Cu2S content of zinkenite is approximately 4.3 mole 7o (Pruseth 1996). Therefore, zinkenite of the composition PbS.SbzS: is impossible becausesuch a phaserequires a considerable amount of a minor element (e.9., Cu) to maintain the ratio of PbS to SbrS. close to 1. Garvin (1973) also could not synthesize zinkenite of the composition PbS.Sb2S3.In nature, temary compositions of zinkenite should be more abundantthan their binary counterparts. Failure to recognize the role of a minor element in stabilizing the crystal structure of ternary zinkenite may have led to the acceptanceof the formula PbS.Sb2S3, Unequivocal support in favor of the formula 6PbS.7Sb2S3can only be obtained from further work on crystal structure of zinkenite.
TABLE 2. COMPOSITIONS OF ZINKEMTE, ROBINSONITE AND FROM VARJOUS POLYMEIATLIC SUIJIDE ORE MENFfgtr.qTD DEPOSITs
Atrkenit€ Vall de R.lbe, Spah
Rrjre,
Yogelavio
Robh&nib vall de Ribe, Spala
R!Jeh., Yug6lavle Thtlno, Yukou Red Blrd Mile, Nevada
M€n€ghinite H!,llefom, S*edu Dhurode, IFIUd Vall de Rtbe, Spab Botlao, Italy Anglesea Tp., Ontarlo P6rry Silv€r Mlg Ontqlo Mmora, Onbrlo Planacte MIae, Australla lidla RajpurDerlbq Bordq Mslto Tomorc'
100.49 s9.s7 09.ir5 100.10 99.90 9S.90
(1) (1) (1) (2) (2) (3)
0.00 0.00 0.0t 3 o.0o 10 0.00 o.oo o.oo 0.00 o-00 0.00 0.00
4il.r? 3E.N n.r2 s8.n 42.59 311.80m.93 100.32 4L.u 36.73 21.0E 99.06 41.40 36.00 20.07 0E.97 4it.93 36.37 20.23 100.63 42.07 36.41 20.16 98.e4 42.92 lB.N 20.44 59.71 4220 36.60 21.20 100.00 N-44 36.89 21.14 9E.41 45.6 35.4 Z).5 101.4 42.6 365 20.9 99.0
o) (1) (1) (4) (4) (4) (4) (3) (6) (6) (7)
1J0 1.35 1.56 rm r.46 r.49 t,62 1.18 1.35 1.30 1.20
61.90 18.90 17.20 99.60 61.62 19.34 1?.12 993tt 61.08 18.52 r?.14 98.30 il.62 19.63 r.69 S984 6r.30 19.65 1?.88 100.19 0r.m 18.65 17.69 99.66 92.44 19.47 1?.49 100.v2 61.49 rq.U 16.79 98.70 59.94 20.10 r?.99 99.38 58.89 .32 18.12 98.72 61.09 19.84 17.i16 99.48
(E) (9) (1) (10) (10) (10) (10) (11) 02) (12) (13)
0.46 0.00 0.00 0.00 0.00 0.00
3f'.7E 43,80 n.4 32.69 44.86 22.53 30.0.:|(,.31 23.01 32.70 M.40 .00 t/.40 4L.N 20.80 31t.60 4:1.20 23.10
@ hdtctt€d bY "n". Numb6 of ualys '1. Ayon & Pfrilps (rgS1), 2. Jmbor & Oreo (1982)' 3. Moeb d sl' (198i1)' a. AFe & Gall (19s1),6. MoAo et d. (1984),6. J@bor (1967)'7. Jmbor & Pldt (19?6),8. zekre*h & Nu8f3rcn (1984),9. Wen et at. (1901)' 10. Htcks & Nufield (19?8), 11. McQoq (1984), 12. Bm (1081), 13. Botr4 & Jordeq (1983)
SYNTHETIC ZNKENIIE,
ROBINSONITE AND MENEGHTNITE
2ll
lrbneghinite(50OoC) Slope= -9.33 4
a
uf -d
-d v, ^gl
o
s
$
10 11 12 1314 Mde PbSi
Mole PHi
7.2 D znkenib(4ooc) a Slope= -9.34
7.1
a 7.O -s o
.tf .d o
g 6.9
$
6.8 6.7
s.2 5.6 6.0 MolePHi
5.2 5.6 6.0 6.4 MolePbS
Ftc. 2. Plotsof molarproportionof SbrS,verszsthatof PbSin meneghiniteandzintenite from phase-equilibrium experiments at 500'and 440'C. MolesSb2S,in theformulae havebeenrecalculated as [S - (Cr:/2)- Pb]/3.
Robinsonite coexisting with either phase Z and zinkenite or phaseZ and meneghinitehas a composition close to 0.06Cu2S. 3.93PbS . 3.00Sb2S3,which is alrnost identical with 4PbS.3Sb2S..This formula is the most acceptable as the ideal formula for robinsonite because the individual phases in a three-phase field have unique compositions. At still lower temperatures, robinsonite most likely will match the proposed formula exactly because it would be entirely free of copper. As shown in Figures 2a andb, the plots of molar proportion of Sb2S3against that of PbS at both 500' and 440oC can be approximatedby straight lines. At 500oC, all points, excluding the compositions of CPS15/5 and CPS16/5, describe a near-perfecrsrraight line. Similar straight lines also fit to the data of
zinkenite (Figs. 2c, d). The near-perfect linear relationship between the molar proportions of PbS and SbrS, in the formulae of thesephasesshows the strong negative correlation between PbS and Sb2S3content. Ideally, compositions on the Cu-rich and Cu-poor boundaries of the solid-solution fields should plot on different straight lines. The longer the solid-solution field, the closer will be the two straight lines and the less scattered will be the plotted points. For the same reason, the data at lower temperatures will be more scattered than those at higher temperatures. This is evident from Figures 2c and d. The slope for meneghinite is -0.33 at 500oC, and that at 44OoC is -0.35. The straight line for zinkenite has a slope of 4.43 at 500'C, and -0.34 at 440"C. The Cu2S-rich boundary runs through almost the entire length of
2t2 the zinkenite solid-solution field (Fig. l), parallel to the Fficrs, W.D. & NunT'run, E.W. (1978): Natural and syntletrc meneghinite. Can. Mineral. 16, 393-395. PbS-Sb2S3 binary join. Thus there is an underrepresentation from the Cu2S-poor zinkenite compositions and hence the higher slope. A small Hone, S.N. & CnaNc, L.L.Y. (1975): Phase relations in the pseudo-ternarysystem Pb${u2S-Sb2S3 and the synthesis amount of Cu2Sis releasedfrom the structure as Sb2S3 of meneghinite. Can. Mineral. 13, 388-393. substitutes for PbS. However, the substitution can be approximated with a scheme of 3Pb2*= 2Sb3+,on the Jeunon, l.L. (1967): New lead sulfantimonides from basis of observed slopes. Insufficient data exist for Madoc, Ontario. II. Mineral descriptrors. Can. Mineral.9, robinsonite. Nevertheless, the same substitution 191-213. scheme,3PbP*= 2Sb:*, may explain the nature of solid solution ir robinsonite, considering the similarity of the & OwpNs, D.R. (1982): Re-examination of shapeof its solid-solution field with those of zinkenite robinsonite from Vall de Ribes, Spain. Can. Mineral. ?4, and meneghinite. 97-tOO. AcrNowrmcsMENTs Prof. Michael Raith provided access to the electron-microprobe analytical facility at the Mineralogisch-Petrologisches Institut, University of Bonn, and Prof. Richard O. Sack of Purdue University provided the synthetic tetrahedrite standard. We are extremely grateful to Dr. J. William Milleq Jr. for his detailed review of the manuscript, and Dr. Robert F. Martin for his valuable editorial suggestions. This study was financially supported by a research scheme [24(204)/90194/EMR-II] to B.M. and a research associateship 19I 8 | (2'79)| 9 6-EMR-II to K.L.P. from the Council of Scientific and Industrial Research. India.
& Pr-ANr,A.G. (1975): The composition of the lead sulphantimonide, robinsonite. Can. M ineral. 13, 4L5417. LEBAS, c. & Lp Brtetrl, M.T. (1976): Etude chimique et structurale d'un sulfure naturel: lazilnckenite. Bull. Soc.fn M ind ral. Cristallo gr 99, 351-360. McQUEEN, K.G. (1984): Meneghinite, boulangerite and associatedminerals from the Pinnacles mine, Broken Hill, New South Wales. Neues Jahrb. Mineral., Monatsh., 323-336. K.L. (1994): Phaseequilibrium study Mrsrne, B. & PRUSETH, in the system CurS-PbS-Sb2S3: non-stoichiometry in sulfosalts and isothermal variation in sulfur fugacity. Contrib. M ine ral. P etrol. 118, 92-98.
MoELo, Y. (1978): Minor constituents of acicular lead sulphantimonides; their part in conditions of formation of these sulphosalts. Int. Mineral. Assoc., XI Gen. Meet. Avona, C. & Gerr, S. (1981):Additionaldataon robinsonite. (Novosibirsk) L, 138- 139 (abstr.). Can.Mineral. 19, 415-417.
REFERENcES
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SYNTTIETIC ZINKENTTE. ROBINSONITE AND MENEGHINITE
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2r3
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