The Distribution of Ag and Sb in Galena: Inclusions Versus Solid Solution

American Mineralogist, Volume 78, pages 85-95, 1993

The distribution of Ag and Sb in galena: Inclusions versus solid solution

Tnorras G. Sn.lnpx Departmentof Geology,Arizona StateUniversity, Tempe,Arizona 85287-1404,U.S.A. PBrBn R. Busncr Departments of Geology and Chemistry, Arizona State University, Tempe, Arizona 85287-1404,U.S.A.

Ansrnacr The distributions of Ag and Sb in galena samplesfrom La Paz and Zacatecas,Mexico, were investigatedusing electron microprobe analysis,backscattered-electron imaging, and high-resolution transmission electron microscopy. Both samples contain numerous rod- shapedinclusions of diaphorite (PbrAg3Sb3S8),and the LaPaz samplesalso contain franckeite [(Pb,Sn).SnrSbrFeS,o].Although diaphorite is a common Ag-bearing inclusion in galena, franckeite has not been previously reported. Both sampleshave Ag concentrations nearly equal to those of Sb, indicating coupled substitution of Ag* and Sb3*for 2 Pb,*. The average Ago,Sbo,S contents of the bulk La Paz and Zacatecassamples (including inclusions)are approximately1.37 and0.44 molo/0,respectively. Of thesetotal AgorSborS amounts, 0.48 and 0.32 molo/ooccur in solid solution in galenain the La Paz andZacatecas samples,respectively. Inclusions ofdiaphorite are from 20 nm to 50 pm long, but no evidencefor Ag-Sb atom clusterswas found. Rod-shapeddiaphorite inclusions are oriented with their c axes (rod axes)parallel to the ( 100) directions ofgalena and their a and b axesparallel to the ( I l0) galenadirections. The diaphorite-galenainterfaces are semicoherent,with misfit dislocations every 6 to 10 nm. Disk-shaped inclusions of franckeite in the LaPaz samplesare oriented with their a*, b, and c axes parallel to the (100) directions of galena. The top and bottom interfacesofthe disks are parallel to the (100) layers offranckeite and appear to be coherentwith the galena.The franckeite inclusions have a modulated-layer structure with a wavelength of 3.55 nm, which is significantly smaller than that of previously de- scribed franckeite. Both diaphorite and franckeite inclusions appearto have resulted from coherent exsolution at low temperatures.Diaphorite appearsto occur metastably relative to freieslebenite(AgPbSbSr), suggesting that there is a coherent solvus betweendiaphorite and galena.

INrnooucrroN from material that is structurally bound in solid solution The distribution of precious metals in sulfide minerals is crucial for understandingthe distributions ofprecious is of economic, crystallographic, and geologic impor- metals and the processesof their incorporation and re- tance. Accurate knowledge of precious metal distribu- covery. This is particularly difficult for invisible Au, which tions is of economic importance for maximizing metal occurs as either submicroscopicparticles or in solid so- recovery and improving metallurgical techniques. The lution with sulfides, sulfarsenides,and arsenides.Ag oc- crystallographic significanceof precious metal distribu- curs in more minerals than Au and is common in many tions is in understanding how "foreign" atoms are ac- base-metalminerals, either in the form of Ag-bearing in- commodated in sulfide structures and in the crystallo- clusions or in solid solution. By overlooking Ag in such graphiccontrols on exsolution.Understanding the primary common minerals, significant amounts of Ag are lost in vs. secondary nature of precious metals, as well as the processing(Cabri, 1987, 1992). processesand conditions of ore deposition, is of signifi- Microbeam techniquesare very useful for determining cant geologicimportance. the concentrations and distributions of precious metals The occurrenceof precious metals in sulfide minerals in minerals (Cabri, 1992), but such techniques cannot and their economic importance has been addressedby, always distinguish precious metals in solid solution from among others, Cabri (1987, 1992) and Chryssoulis and microscopic inclusions of precious-metal-bearingminer- Cabri (1990). Distinguishing submicroscopic inclusions als. If such inclusions are not accountedfor in analyses, the result could be unrealistically high solid-solution es- - p..*"t BayerischesGeoinstitut, Universitiit Bay- timates or incorrectly interpreted substitution mecha- "Odress: reuth,Postfach 10 12 51,8580 Bayreuth, Germany. nisms. SEM imaging of samplescan be usedto determine 0003-o04x/93/0102-o085$02.00 85 86 SHARP AND BUSECK: Ag AND Sb IN GALENA the presenceof microinclusions that are larger than about galena solid solution below 420 to 390 "C (Hoda and 100 nm, but the high spatial resolution of transmission Chang, 1975; Amcofl 1976) thar may be useful as an electron microscopy (TEM) is neededto observe smaller indicator of deposition temperatures.However, our re- particles.In addition, the structural relations betweenin- sults, as well as previous studies (Czamanskeand Hall, clusions and the host mineral can be obtained with TEM 1976;Laflammeand Cabri, 1986a,1986b), suggest that to interpret better the origins of inclusions. High-resolu- diaphorite (PbrAg.SbrSr),rather than freieslebenite,is the tion transmission electron microscopy (HRTEM) has re- major inclusion type in Ag- and Sb-bearinggalena. cently been applied to the problem of invisible Au in The purpose ofthe present study is to investigate the arsenopyrite(Cabri et al., 1989) and disseminatedAu in distribution of Ag and Sb between solid solution and in- Carlin-type deposits(Bakken et al., I 989). In the first case clusions in galena samples from two Ag deposits. The (Cabri et al., 1989),Au particles were not observed,sug- compositions of the galenaand inclusions are determined gestingthat the Au is structurally bound, whereasBakken with EMPA. The type and size distribution of the inclu- et al. (1989) found Au particles less than 20 nm in di- sions are determined with TEM, as are the structural re- ameter that had not been previously observed. lations to the host galena.The origins of the inclusions High-sensitivity analytical techniques,such as proton- are discussedin terms of their morphologies and crystal- induced X-ray emission (Cabri, 1987; Cabri et al., 1984, lographic relations to galena. 1985)and secondary-ionmass spectrometry (Mclntyre et al., 1984;Chryssoulis et al., 1986; Chryssoulisand Cabri, AND CHARACTERIZATToNTECHNTQUES 1990; Cabri et al., 1989, l99l), combined with careful S,l'rvrpr-rs inspection for inclusions, have been used to show that The Ag- and Sb-bearing galena samples used in this trace concentrations of precious metals occur in many study were from the La Paz and Zacalecas Ag districts, sulfide minerals. Samplesof galena containing relatively Mexico. Most publishedstudies of Ag-bearinggalena have high concentrationsofAg, Bi, and Sb have been studied focused on coupled substitution of Ag with Bi; for this using electron-microprobe analysis (EMPA) (Laflamme study sampleswere chosenthat contain up to I wto/oAg and Cabri, I 986a,I 986b;Foord et al., I 988),but HRTEM and Sb, with no detectableBi and only trace amounts of (Sharp and Buseck, 1989) and scanning tunneling mi- As, Sn, and Te. Both samplescontain Ag-Sb inclusions, croscopy(STM) (Sharpet al., 1990)have only recently predominantly diaphorite, and minor amounts of other been applied to Ag-bearing galenas. sulfosalts.These galena samples were also chosenfor their ConcentrationsofAg in galenaare variable and depend large crystal size, which was required for previously re- on the presenceof other elementssuch as Sb and Bi. The ported STM investigations(Sharp et al., 1990). simple substitution of Ag for Pb (2Ag- : Pb'*) is limited In addition to the Ag- and Sb-bearingsamples, Ag-free to a maximum of 0.4 molo/oAg.S at 700 oC and less at galena (from an unknown location) and synthetic Ag- lower temperaturesbecause half of the Ag must go into bearing and Sb-Bi-free galena(provided by Louis Cabri) interstitial sites(Van Hook, 1960;Karup-Moller, 1977). were examined. These sampleswere used as controls to Where Ag* substitution for Pb2* is coupled with substi- determine whether observed defects correlate with Ag and tution of Sb3*or Bi3* [Ag* + (Sb,BD3*: 2Pb"f, the charge Sb concentrations. and cation-anion ratio are balanced, allowing higher Ag Chemical analyseswere obtained with a Jeol JXA 8600 concentrations. Galena with combined Ag, Sb, and Bi Superprobeelectron microprobe using an acceleratingpo- concentrationsgreater than 0.5 wto/ois called galenasolid tential of 15 kV. Cleavage fragments of galena were solutionby Foord et al. (1988),who report samplescon- mounted in epoxy resin, polished,and C coated,resulting taining as much as 9.4 and 18.5 wto/oAg and Bi, respec- in polished surfacesnearly parallel to (100) planes.Back- tively. Whereas excessAg relative to Sb or Bi is limited scattered-electronimaging (BEI) was used to locate sul- by interstitial substitution of Ag, excessSb or Bi is more fosalt inclusions and to observetheir textural relations to easilyaccommodated by the substitutionmechanism 2(Sb, the galena host. Inclusions and the galenahost were an- Bi;'* * tr : 3Pb'?*),resulting in vacancieson Pb sites. alyzed with wavelength-dispersive X-ray spectroscopyfor The lattice constantsfor the AgSbSr-galenasolid solution Pb, S, Ag, Sb, Bi, As, and Sn, using common sulfidesand show a negative deviation from ideality, which is inter- Bi metal as standards.The galena host (between inclu- preted as evidence for clustering of the solute atoms in sions), was analyzedusing a 50-nA beam current to ob- the solid solution (Wernick, 1960; Godovikov and Nena- tain high count rates for Ag and Sb. Basedon 1o counting sheva,1968). statistics, the precision of these analysesis +0.02 wto/0. Galena solid solution and the sulfosalt inclusions may Analysesof the host galenaplus inclusions were obtained be useful as indicators of depositional conditions. The by rastering the beam over areas 120 x 120 pm. Because coexistenceofgalena, galenasolid solution, and a variety the rastered-beamanalyses and those of the micrometer- of sulfosalts was interpreted as evidence for multiple stages sized inclusions have contributions from inclusions and of mineralization in the Round Mountain and Manhattan matrix, the results are only semiquantitative. Au districts, Nevada (Foord et al., 1988).Experimentally For HRTEM studies,cleaved slabs of galenawere glued determined phaserelations in the AgrS-SbrS'-PbSsystem to 3-mm Cu grids, mechanically thinned to approxi- indicate a solvus between freieslebenite(AgPbSbSr) and mately 50 p.m, and ion milled with 5-kV Ar ions until SHARP AND BUSECK: AS AND Sb IN GALENA 87

termined by HRTEM imaging of the inclusions and their boundaries. Ag-Sb DISTRIBUTIoNBETwEEN GALENA AND INCLUSIONS Although the Ag- and Sb-bearing inclusions in these samplesare not visible by reflected-lightmicroscopy, they f stand out in high-magnification and high-contrast BEI micrographs.The predominant inclusionsin both sam- features , ples are diaphorite, which appear as dark linear and nearlyround spots(Fig. 1).This texturesuggests that the inclusions are orthogonally oriented rods within the galena.Most rods appearto be 0.5 to 2.0 1rmin diameter 1Oirm, and as long as 50 prm,although larger and less regularly shapeddiaphorite inclusions occur in the LaPaz sample. Fig. l A backscattered-electronimage of La Pazgalena with Diaphoriteinclusions of similar sizeand shapealso occur diaphorite(Dph) and Sn-bearing(Sn) inclusions. A large,irreg- in galena from the Brunswick 12 mine (Laflamme and ularlyshaped diaphorite grain, as well as the typicalrod-shaped Cabri, 1986a,1986b). inclusions,are indicated. The orientationofthe diaphoriterods In addition to diaphorite,the La Paz samplecontains alongthe galena( 100)directions produces the dark orthogonal These in- Iinesand spotsdepicted here. An exampleof the Sn-bearing significantamounts of Sn-bearinginclusions. materialoccurs at the end ofthe largediaphorite grain clusionsappear in BEI micrographs(Fig. l) as irregularly shapedgrains that are commonly associatedwith diaphorite. Many display variable contrastin such images,indicating chemical heterogeneity and possibly a multi- the slabswere perforated.Low-voltage milling (1.6 kV) phasecharacter. The Sn-bearinginclusions seen with BEI was done to remove excessbombardment-damaged ma- werenot observedwith HRTEM, but the Sn-Ag-Sbmin- terial. Electrical contact betweenthe samplesand the Cu eral, franckeite,was commonly encounteredin the La Paz grids was achievedby either applying a light C coat or by samplewith HRTEM. Additional inclusionssuch as bou- making a bridge with conductive C paint. Crushed sam- langerite(PbrSboS,,) and someAg-rich tellurideswere ob- ples,suspended on holey-carbongrids, were examined to servedin the La Paz andZacatecassamples, but they are monitor ion-bombardmentdamage in the milled samples. rare. HRTEM experiments were conducted with a Jeol Electron microprobe analysis(EMPA) of the inclusions 2000FX using an acceleratingpotential of 200 kV. Im- is limited by their small sizes;many analysesof the di- aging and selected-areaelectron diffraction experiments aphorite inclusions have excessPb, indicating that even (SAED)were done alongthe ( I 00) axesof galena.SAED for the larger inclusions there can be contributions from was used to identify inclusions and to determine their the surroundinggalena. The compositionsof diaphorite, orientation and crystallographicrelations to host galena. Sn-bearinginclusions, and boulangerite are presentedin Structural and interfacial relations with salena were de- Table 1, togetherwith the stoichiometriccompositions

TreLe 1. Electronmicroprobe data for inclusionsin La Paz galenaand Ag and Sb contentsof La Paz and Zacatecasgalena (+ inclusions)

La Paz Zacatecas Diaphorite ldeal Sn ldealPb- Boulang ldeal + inclu- + Inctu- t6l diaphorite phase[3] franckeite t11 boulang. Galena[21] sionsn 2l Galena[24] sions[12]

J 1I 9s(030) 18.86 16.44(2.0) 21 88 18 13 18.80 Ag 22 92(0.53) 23 80 8.44(10) 0.02 022(012]' 0.62(006) 0.14(0.08) 0.19(0.07) Sb 26.55(0.41) 26 86 1s.51(12) 1187 2524 2596 0 28(014) 0.81(0.06) 0 1e(0.09) 0.27(006) Pb 30.86(073) 30 48 59.90(21) 40 39 55 57 5523 5n 1.13(0.84) 23.14 0.32 Fe 272 Total 99.28(0.60) 10000 10142e.0\ 10000 9928 9999 fncfusion stoichiometry based on givenno. ot S atomsand ato/o Ag andSb J 8.00 8.00 14.00 14.00 1100 1100 Ag 288 3.00 1.98 0.24(013) 0.69(0.07) 0 16(0.09) 0 22(008) Sb 295 3.00 3.28 200 403 400 0.27(013) 0.79(0.06) 0 18(0.0s) 0 27(006) Pb 2.02 2.00 8.21 4 00 5.22 5 00 Sn 0.27 4.00 0 05 Fe 1.00 Total 1585 16.00 27 74 25.00 1930 1900 Note.The numbersin bracketscorrespond to the numberof analysespresented, whereas the numbersin parenthesescorrespond to the standard deviationsof the data. 88 SHARP AND BUSECK: AE AND Sb IN GALENA

a b 0.020 La Paz qgzs Ac = Acanthite AgSbPb-z Bl = Boulangerite Pbssb4sl1 0.015

Dph = Diaphorite Pb2Ag3Sb3SB

Fri = Freieslebenite PbAgSbS, 0.010 AgC Pb Gn = Galena PbS -0.5 -0.s 0.005 Mr = Miargyrite AgSbS,

Stb = Stibnite sb2s3 0,000 0.000 0.005 0.010 0.015 0.020 sbtr Pb AgSbPb-, 0.5 -1.5 1.0 c 0.020 calecas AgE .rPb*.u AgSbPb., 0.015

0.010 AgQo.uP9o.

0.005 iF PbS 0.33 0.67 Sb.67S 0.000 sbtro.sPhl.s 0.000 0.005 0.010 0.015 0.020 toEo.rto-r.u

Fig. 2. Chemographicand substitution relations (a) in the AgrS-PbS-SbourS system showing substitution vectors relating galena to acanthite, stibnite, miargyrite, boulangerite,freieslebenite, and diaphorite. The PbS corner of the triangle is enlargedto show the substitutions of Ag and Sb in the (b) La Paz and (c) Zacatecassamples. Many data plot along or slightly below the AgSbPb ' coupled-substitutionvector.

of these minerals and Pb-rich franckeite for comparison. tion vectorsAgtr orPb-., and Sbtro'Pb-'' (Fig. 2). The The analysesofdiaphorite and boulangeriteare near their chemographic and substitution relations among galena, stoichiometric compositions, but that of the Sn-bearing stibnite, acanthite, miargyrite, diaphorite, and freiesle- materials does not closely resemble franckeite. The Sn- benite are illustrated (Fig.2a), with the right triangle de- bearing inclusions contain high concentrationsofAg and fined by the components AgrS, PbS, and SbourS,(nor- Sb and little Sn relative to franckeite and may represent malized to one S atom). In this diagram, application of a mixture of minerals. the Agtr orPb-., vector two times transforms PbS into The Ag and Sb concentrationswere quite variable, re- AgrS (acanthite)and application of the Sblo.'Pb , , vec- flecting the distribution of dissolved Ag and Sb, as well tor 0.67 times transformsPbS into Sbou,S(stibnite). The as inclusions too small to be resolved with BEI, and av- galena corner of this triangle is enlarged in Figures 2b eragevalues are presentedin Table l. Concentrationsof arrd 2c to illustrate the correlation between Ag and Sb Ag rangedfrom 0.11 to 0.69 wto/o(mean : 0.22 ! 0.12 substitutions in the galenasolid solution. The Ag and Sb wto/o)for the La Paz sample and from 0.04 to 0.36 wto/o substitution (y and x, respectively) are calculated from (mean : 0. 14 + 0.08 wt0/0)for the Zacalecassample. The the atom percentsusing the generalformula Pb'-'r"-orr- Sb concentrationsrange from 0. l8 to 0.84 wto/o(mean : Ag"Sb"Sfor the Ag'S-PbS-SbourSsystem. Because the cat- 0.28 + 0.14 wto/o)for the LaPaz sampleand from 0.08 ion-anion ratios changein this system, except along the to 0.45 wto/o(mean : 0. 19 + 0.09 wto/o)for the Zacatecas coupled-substitution vector, the substitution is not lin- sample. early related to the atom percent.However, at the low Ag The mechanismsof Ag and Sb substitution aro exam- and Sb contents of these samples,the substitution is ap- ined by plotting the concentrationsin terms of substitu- proximately two times the Ag and Sb atom percent. As SHARP AND BUSECK: Ag AND Sb IN GALENA 89

TABLE2. Dimensionalrelations among galena,diaphorite, freieslebenite, and franckeite

Unit cell(nm) Subcell- % misfit relativeto galena Space Gp. Galena(1) F4lm32lm 0.594 0.594 0.594 9e a\f212 a\f212 Diaph.(2) Q'la 1.584 3.208 0.590 90.10' al4' blg' -4.5-. -0.51 Freies.(3) P'ln 0 753 1.279 0.588 9214', al2. bl3 104 1.4* -1.01 Frnk.T (4) 1.72 0.579 0.581 -2.51 -2.01 't.72 Frnk.H (4) 0.365 0.63 -38.61 6.7t

/VotejCrystallographic data from (1)Wycoff, 1 963, (2) Helner,1 957a, (3) Helner,1957b, and (4) Williamsand Hyde,1988. . Subcellshown in Figure3. -. Misfitrelative to galena410. t Misfit relativeto galena dloo.

can be seenin Figure 2, Ag vs. Sb substitutions for both Franckeite [(PbSn)uSnrSbrFeS,o]is a layered mineral samplesplot along but slightly below the AgSbPb_, vec- consisting of alternating (along a) pseudotetragonal(T) tor, indicating coupled substitution with a slight excess and pseudohexagonal(H) sheetsthat have an incommen- of Sb relative to Ag, presumably accompaniedby vacan- suraterelation (Makovicky, 1974, 1976; Moh, I 984, I 987; cies on Pb sites. Williams and Hyde, 1988; Wang and Kuo, 1991).The The results ofthe rastered-beamanalyses (galena plus T-sheet structure is either PbS-like or slightly sheared, inclusions, Table l) indicate that the total Ag and Sb making it SnSJike, with Sb3* and Ag* substituting for concentrations in the l-a Paz sample are approximately Pb2*or Sn2t;its thicknessis approximately 1.2 nm along 0.62 (+9.66; and 0.81 (+0.06) wt0/0,respectively, where- a. The cell parametersthat are parallel to the sheet,D' : as the total Ag and Sb concentrations in the Zacatecas 0.579 and cr : 0.581nm (Williams and Hyde, 1988),are sampleare approximately 0.19 (+0.07) and0.27 (+0.06) wto/0,respectively. The distribution of Ag and Sb between galenaand inclusions can be estimatedby subtractingthe AgrSborS contents of the galenafrom those determined Galena by rastered-beamanalyses. In the l-aPaz sample, the total AgorSborScontent is 1.37 mol0/0,with 0.48 molo/oin 00loph 00lFrl the solid solution and 0.89 molo/o inclusions, princi- [1 [1 as -r-* pally diaphorite. The total Ago,SborScontent of theZa- catecassample is 0.44 mol0/0,with 0.32 molo/oin solid [1lolcn solution, leaving only 0.12 molo/oAg"rSborS to account for diaphorite inclusions.

Srnucrun-q.L RELATIoNSBETwEEN GALENA AND INCLUSIONS Freieslebenite The structuresof galena,diaphorite, and freieslebenite a = 0.75nm are closely related, allowing coherent intergrowth and in- b = 1.28nm terface relations such as those observed in HRTEM images.Unit-cell and subcellrelations among theseminerals Diaphorite are summarized in Table 2 and illustrated in Figure 3. a = 1.58nm Galena has the NaCl structure, with octahedrally coor- b = 3.21nm dinated Pb and S defining a face-centeredcubic lattice. The structuresofdiaphorite and freieslebeniteare deriv- atives of galena, with the same octahedral arrangement of cations and S anions, but with Ag* and Sb3*substituted lilolcn totoloon for Pb2*.Both diaphorite and freieslebenitehave galena- I like subcells(Hellner, 1957a, 1957b),as illustrated in Fig- Y lololFri ure 3. The presenceof Sb3*(radius 0.89 nm; Shannon, Fig. 3. illustration 1976), which is considerably smaller than Pb2* (radius Schematic of the unit-cell relations among galena,diaphorite, and freieslebeniteas viewed along ther c axes. 1.33 nm), results in the diaphorite subcell parameters(a" : : The box illustrating the diaphorite unit cell is truncated to save a/4, b": b/8, and c. c) somewhatsmaller than a"t/2/ space.The cation (small circles)and anion (largecircles) arrange- 2, and a" of galena. The freieslebenitesubcell (a": a/2, ment and the subcell (shadedbox) common to all three struc- : b": b/3, and c" c) is also smaller than the primitive tures are illustrated. The galena face diagonal, along [110], is galena subcell, but with most of the misfit along the significantly longer than the freieslebenitea parameter,illustrat- freieslebenitea axis. ing the large misfit betweenthese minerals along a. 90 SHARP AND BUSECK: Ag AND Sb IN GALENA

HRTEM OBSERVATIONS Diaphorite inclusions Diaphorite inclusions are abundant in both theL,aPaz arrd Zacatecassamples. Most are rod shaped, with the rod axis parallel to the diaphorite c axis; many are nearly round, but some have partial {120} faces (Fig. 4a) or irregular shapes.Diaphorite rods in the La Paz sample are commonly 300-600 nm in diameter, and those in the Zacatecassample 100-200 nm, but inclusions as large as several micrometers in diameter and as small as 20 x 200 nm have been observed.The smallestof theseinclusions are not visible by BEI and would therefore be in- corporated into analysesof the solid solution. The rare occurrenceofthese inclusions on the scaleof20-200 nm suggeststhat galenaanalyses reflect primarily Ag and Sb in true solid solution. The orientation relation suggestedby the textures in BEI micrographs is confirmed by TEM observations. Diaphorite rods (c axis : rod axis) are parallel to the three ( I 00) directions in galena.This relation is a result of the almost identical values of the c cell parameter of diaphorite (0.590nm) and a cell parameterof galena(0.594 nm). SAED patterns of the diaphorite [001] and the galena [001] zone axes (Fig. ab) show that the a* and b* directionsofdiaphorite are parallel to the (ll0)* directions ofgalena. The substructureofdiaphorite is indicated in the [001] SAED pattern (Fig. ab) by high-intensity reflectionsthat occur very near the galenareflections. The NaCl arrangement of cations and anions in diaphorite results in a substructurediffraction pattern that is essen- tially the same as that of galena.The diaphorite 400 and 080 difraction spots are examples of subcell reflections that are located adjacent to the correspondinggalena re- (a) Fig. 4. TEM imageof a diaphoriterod viewedalong the flections, indicative of a diaphorite subcell that is some- rod axisand (b) the correspondingSAED pattern. The traceof what smaller than that of galena. the diaphorite-galenainterface and dislocationsextending from theinterface appear as dark lines in theimage. The interface has interfaces flattenedsides that are subparallelto the {120} planesof dia- Diaphorite-galena phoriteand { 100}planes ofgalena. The SAEDpattern along the The interfacesbetween diaphorite and galenaare semi- diaphoriteand galena [001] zone axes illustrates that the a* and coherent, with periodic steps (Fig. 5a) and misfit dislo- b* directionsofdiaphorite are parallel to the (110) directions of cations (Fig. 5b). Becauseof the smaller diaphorite sub- galena.The intensesubstructure reflections of diaphorite400 cell, a semicoherentrelation requires extra atomic planes and 080 arenearly coincident with the D0 and220 reflections on the diaphorite side of the boundary. The terminations of galena. ofthese layersat the boundary resembledislocations and can be describedin terms of Burgersvectors and circuits. Burgers circuits drawn around misfit dislocations (Fig. only slightly smaller than the galenacell parameter4s and 5b) indicate extra (240) and (240) atomic planesin diaph- can fit well into the galena structure. The H sheet is a orite analogousto the {200} planes of galena. The cor- derivative ofthe berndite (SnSr)structure, with Fe3"sub- respondingBurgers vectors are 0.28 nm along diaphorite stituted for Sno*;it is approximately 0.5 nm thick, with I l0] or [1T0], where only one extra atomic plane occurs, cell parametersb, : 0.365 nm and cH: 0.63 nm parallel and 0.40 nm along [00] or [010], wheretwo orthogonal to the sheet (Williams and Hyde, 1988). The large misfit extra planes occur together. In Figure 5b, the Burgers between the H and T sheetsresults in the incommensu- circuit on the left side of the figure revealsonly one extra rate structure. An alternating sequenceof the T and H atomic plane, whereas the circuits in the center and on sheetsproduces a layered structure 1.72 nm thick, with the right side show two extra planes.A singleextra atom- a sinusoidal modulation along c* and with a variable ic plane (0.28-nm Burgersvector) would result in anions wavelengthof 4.2 nm (Williams and Hyde, 1988)to 4.7 on cation sites(Fig. 6) and a chargedstacking fault, which nm (Wangand Kuo, 1991). would be highly unlikely in a galenalike structure. The SHARP AND BUSECK: AS AND Sb IN GALENA 91

Fig. 5. HRTEM images of a diaphorite (Dph) inclusion in galena (Gn), viewed along [001], illustrating the structure and morphology ofthe diaphorite-galenainterface. The orientation relationship (a) is illustrated by the a and b lattice vectors ofboth minerals. The interface is curved with facets(at arrows) parallel to the {100} planes of galena.A higher magnification image (b) of this interface shows misfit dislocations illustrated by the Burgers circuits (boxes).The gaps at the bottoms ofthe Burgers circuits representprojected Burgersvectors of |za[100] (0.28 nm) and,(y2/2)all101 (0.40 nm) relative to the galenastructure. 92 SHARP AND BUSECK: Ae AND Sb IN GALENA

indicates nearly identical atomic spacings in the two structures. Streaking along a* in the T-sheet rows indi- catesa stackingdisorder ofthe sheets. The closely spaced reflections along the modulation vector q (crossingthe rows of T-sheet reflections) correspond to a 3.55-nm modulation of the T sheets.This 3.55-nmwavelength is considerablysmaller than the 4.2- 4.7 values reported for franckeite (Williams and Hyde, 1988;Wang and Kuo, l99l) and may be a result of high Ag and Sb contents.Although Ag*, Sn2*and Pb'* all have comparableionic radii in sixfold coordination (=0.130 nm), the radiusof Sb3*is only 0.089nm (Shannon,1976). Excesssubstitution of Sb3*(and thus Ag*) for Pb2* and Subcell Sn'?*in the pseudotetragonalsheet of franckeite would misfit between T and H sheets,requiring a Fig.6. Schematicillustration ofthe apparentBurgers vectors increasethe for the misfit dislocationsobserved at the galena-diaphoritein- greatermodulation of the structure. terfaceofFig. 5.The small dark circles correspond to thecations, The layers offranckeite are further resolved into their the lightly shadedcircles correspond to the anions,and the box T and H sheetsin Figure 8. The images show ofsets of indicatesthe subcell.The v, Burgersvectors, along the galena the lattice fringes acrossboth sheets,indicating sheared ( 100)directions, are unlikely because they connect anion to cat- structures. The problem of the PbS-like vs. SnS-like ion sites.The v, vector,along the galena[ 10]direction, is 0.40 structure of the T sheetwas investigatedby Williams and nm longand connectsequivalent ion sites. Hyde (1988),who presentedimage calculations that can be used to distinguish between the PbS- and SnSlike apparent0.28-nm dislocation probably has an equivalent structures.Their image simulations indicate fringes nor- 0.28-nm screwcomponent along c that is invisible in this mal to the T layers for the PbS-typestructure and fringes projection. If this model is correct, all the misfit dislo- oblique to the T layers for the SnS-type structure. The cations are equivalent to the 0.40-nm type, but with some image presentedhere (Fig. 8a) resolvesthe lattice fringes accommodating misfit along c as well as along a or b. in the T sheet,showing that they are oblique to the sheets, The spacingsof the misfit dislocations are consistent similar to the image calculationsof the SnSJike structure with the values expectedfrom the calculated misfit be- presentedby Williams and Hyde (1988). tween diaphorite and galena. Dislocations with Burgers vectorsof0.40 nm along a on a diaphorite(010) bound- Franckeite-galenainterfaces ary would accommodate the 5.7o/omisfit if spacedevery The franckeite inclusions are coherently intergrown with 7.0 nm along a. The distance between the two disloca- galena.The most extensive of the boundaries is parallel tions with 0.40-nm Burgers vectors in Figure 5b is 7.8 to the franckeite (100) layersand the galena(100) planes. nm. The separation expectedbetween a 0.28-nm dislo- Here the (020) lattice fringesof galenaare continuous into cation and a 0.40-nm dislocationis 5.3 nm, as compared the franckeite, with minor offsets but with no apparent with the 5.6-nm value observedin Figure 5b. On a larger misfit dislocations (Fig. 8). This observation implies that scale(Fig. 5a), the diaphorite (010) boundary consistsof a small amount of misfit at the interface is accommodat- small {ll0} facetsthat are associatedwith misfit dislo- ed by homogeneousstrain. An interesting form of inter- cations.The spacingof thesefacets is 6.3-8.3 nm, which facial strain is evident in franckeite inclusions that are is comparable to the distance between dislocations with imaged along [010] (Fig. 8a), where the amplitude of the 0.40-nm Burgersvectors. modulations decreasestoward the (100) boundary. Such a flattening of the modulation apparently resultsin a bet- Franckeite inclusions ter fit betweenthe H sheetoffranckeite and the galenaat In addition to diaphorite, franckeite has been observed the interface. A step is present along the boundary in in the La Paz sample(Fig. 7). Theseinclusions have disk- Figure 8a, where a 1.7-nm layer of franckeite terminates like morphologiesparallel to their (100) layers.The disks into the galena.The step is evident as the termination of are 60-150 nm thick and occur parallel to the galena an H sheet,but with a smooth transition from the galena {100} planes.In the HRTEM imagesof franckeite inclu- to the T sheet below the terminated H sheet.The pres- sionsviewed along [0 I 0] (Figs.7a, 8a),the I .72-nmlayers enceof such stepsat these interfacessuggests a layer-by- and their sinusoidal modulations are clearly visible. The layer growth mechanism for the franckeite inclusions. SAED pattern for this orientation (Fig. 7b) suggestsa topotaxial relation betweengalena and franckeite, where Defects in galena b, a*, and c* offranckeite are nearly parallel to the ( 100) Defectsthat resembleGuinier-Preston (G-P) zones(Fig. directions of galena and the T-sheet structure nearly 5a) are abundant in ion-milled galena,but are absent in matchesthat of galena.The fact that the T-sheet diffrac- all of the crushed samples,suggesting that they are arti- tion rows along a* coincide with the galena reflections facts ofion thinning. SHARP AND BUSECK: Ae AND Sb IN GALENA 93

Fig. 7. (a) TEM image of a franckeite(Fk) inclusion in galena Fig. 8. HRTEM imagesof interfacesbetween galena (Gn) (Gn) and (b) the correspondingSAED pattern. The image shows andfranckeite (Fk) viewed along the franckeite(a) [01 0] and(b) the end of a plate-shapedinclusion of franckeite viewed along [001] zoneaxes. In both imagesthe pseudotetragonal(T) and the [010] zone axis. Fringes that are visible in this inclusion pseudohexagonal(H) sheetsof franckeiteare resolved;the H correspondto the 1.7-nmlayers along a and their 3.55-nmmod- sheetsappear smeared and perhapsbeam damaged and the T ulations. The SAED pattern is a superpositionofthe franckeite sheetsappear galena-like but slightlysheared. The galena-franck- [010] and galena [001] pattems. The a,* and c'* directions and eiteinterfaces are coherent with no apparentmisfit dislocations, the modulation vector qr of franckeite are nearly parallel to the althoughthere is somedistortion ofthe galena(020) fringes at (100) directions ofgalena. The streaksalong franckeite ar* in- the interfacesand a decrease(a) in the modulationamplitude of dicate layer-stackingdisorder. the franckeitelayers. A stepalong the interface(a) is evidentas a termination(t) of an H sheet. DrscussroN Ag and Sb are distributed between galena solid-solu- trast. We have seen no evidence of inclusions or atom tion and microscopic inclusions, most of which are di- clusters smaller than 20 nm in the HRTEM results, but aphorite, in proportions dictated by bulk concentrations STM experiments on cleavagesurfaces have shown dis- and the extent of exsolution. Microscopic inclusions of torted structure that may be a result of AgSb clusters diaphorite may be common in Ag-Sb-bearinggalena but (Sharpet al., 1990). overlooked becauseof their small size and optical simi- The rodlike morphology, orthogonal orientation, and larities to galena.Most diaphorite inclusions that we have homogeneousdistribution of the diaphorite inclusionsare observedare rods less than I pm in diameter, but larger strong evidencefor exsolution. Although franckeite exso- diaphorite inclusions have been reported by Laflamme lution in galena has not been reported previously, disk- and Cabri (1986a, 1986b) and Czamanskeand Hall shapedinclusions with topotaxial relations to the galena (1976). Although the optical properties ofdiaphorite are suggestthat they are also products of exsolution. Both similar to those of galena(Ramdohr, 1980),they are eas- franckeite and diaphorite inclusions have topotaxial re- ily observedusing BEI with high magnification and con- lations to galena,with coherent or semicoherentinterfac- 94 SHARP AND BUSECK: Ae AND Sb IN GALENA

es. Thesecrystallographic relations result from the struc- Wang for discussionsconcerning franckeite structure and JamesClark for tural similarities between galena and the galena-derivative assistancewith the electron microprobe This work was funded by NSF grant facility is funded by NSF structuresofdiaphorite T EAR-8708 529. The electron microprobe and the layer in franckeite. The grant EAR-8408529and the HRTEM at Arizona StateUniversity is fund- interfaces and orientations of exsolution features are a ed by the NSF and ASU function of the misfit between the two phasesand their elastic properties (Yund and Tullis, 1983, and references RnrnnpNcns ctrno therein). Without knowledge of the elastic constants for Amcoff, O. (1976) The solubility of silver and antimony in galena Neues diaphorite and franckeite, we can only consider the re- Jahrbuch fur Mineralogie Monatshefte,6, 247-261. lations in terms of misfit. Bakken,B.M., Hochella,M.F., Marshall,A.F, and Turner,A.M. (1989) Because the diaphorite substructure is only slightly High-resolution microscopy of gold in unoxidized ore from the Carlin mine, Nevada.Economic Geology, 84,171-179. smaller than the galenastructure, the misfit at diaphorite- Cabri, L.J (1987)The mineralogy of preciousmetals: New developments galenainterfaces is relatively small and is accommodated and metallurgical implications. Canadian Mineralogist, 25, l-7. by misfit dislocations.The interfacial strain is minimized - (1992) The distribution of trace precious metals in minerals and by the rod morphology of the inclusions along c, the axis mineral products: The 23rd Hallimond Lecture (1991) Mineralogical 289-308. with the leastmisfit. The relativelyAgo,Sbo,S-rich galena Magazine,56, Cabri, L J., Harris, D.C., and Nobtng, R. (1984) Trace silver analysisby from La Paz (l.37 molo/o)probably experienceddiapho- proton microprobe in ore evaluation In U. Kudryk, D.A. Corrigan, rite exsolution at a higher temperaturethan the Z,acalecas and W.W. Liang, Eds., Preciousmetals: Mining, extraction, and pro- sample,which containsonly 0.44 molo/oAgo rSbo rS. As a cessing,Metallurgical Society of AIME Proceedings,p. 93-100. War- result of higher Ag and Sb concentrations and tempera- rendale,Pennsylvania Cabri, L J., Campbell,J.L, Laflamrne,J.H.G., Leigh, R.G, Maxwell, tures, the La Paz sample contains coarser and more ir- J A, and Scott,J.D (1985)Proton-microprobe analysis of trace ele- regularly shaped diaphorite inclusions. Comparing rod ments in sulfidesfrom some massive sulfide deposits.Canadian Min- lengths and widths in backscattered-electronimages, an eralogist,23, 133-148. average aspect ratio for diaphorite rods in the La Paz Cabri, L.J.,Chryssoulis, S.L., De Villiers,J.P.R, Laflamme,J.H.G., and (1989) The nature of "invisible" gold in arsenopyrite. galenais 16:l, whereas Buseck, P R. that determined for the Zacalecas Canadian Mineralogist, 27, 353-362 galena is 47:1. The higher aspectratios of Zacatecasdi- Cabri, L.J., Chryssoulis,S.L., Campbell, J.L., and Teasdale,W.J. (1991) aphorite reflect the increased importance of interfacial Comparison of in-situ gold analysesin arsenianplrite. Applied Geo- strain energy during exsolution in the samples less rich chemistry,6,225-230. in Ag and Sb. Chryssoulis,S.L., and Cabri, L.J (l 990) Significanceof gold mineral bal- ancesin mineral processes.Transactions of the Institution of Mining The exsolution of diaphorite in both samplessuggests and Metallurgy,Section C99, C1-C10. that a solvus exists betweengalena and diaphorite in the Chryssoulis,S.L., Chauvin, W J., and Surges,L.J. (1986)Trace element PbS-AgSbS, system. However, experimental investiga- analysisby secondaryion mass spectrometrywith particular reference tions of phaseequilibria in the PbS-AgSbSr-SbrS.system to silver in Brunswick sphalerite CanadianMetallurgrcal Quarterly, 25, above 300'C (Hoda 1975;Amcofi, 1976) 233-239. and Chang, Czamanske,G K., and Hall, W.E (1976)The Ag-Bi-Pb-Sb-S-Se-Temin- indicate a solvus between galena and freieslebenite eralogy of the Darwin lead-silver-zinc deposit, southern California. (AgPbSbS,).One explanation is that equilibrium was not EconomicGeology, 70, 1092-l I 10. attained in the experimental studiesand that freiesleben- Foord,E.E , Shawe,D.R., and Concklin,N.M (1988)Coexisting galena, ite exsolvedmetastably. This seemsunlikely becauseboth PbSand sulfosalts:Evidence for multiple episodesofmineralization in the Round Mountain and Manhattan gold distncts, Nevada. Canadian studies found immiscibility between galena and freiesle- Mineralogist, 26, 355-37 6. benitein PbS-richsamples, and Hoda and Chang(1975) Godovikov, A.A., and Nenasheva,S.N. (1969)The AgSbS'-PbSsystem found a second miscibility gap between diaphorite and above480'C. Dokilady AcademieNauk SSSR,185, 76-79. freieslebenite.A secondexplanation for the occurrenceof Hellner, E.V (1957a)Uber komplex zusammengesetzteSpie Glanze. III. Pb,Ag,Sb,Sr.Zeitschrift fiiLrKristallogra- in galenais Zur Struktur des Diaphorite diaphorite that it exsolvesmetastably because phie,ll0, 169-174 of a coherentsolvus at lower temperaturesthan the freies- -(195?b) Uber komplex zusammengesetzteErze. II. Zur Structur lebenite-galenasolvus. This explanation is possibleifdi- desFreieslebenite PbAgSbS,. Zeitschrift fiir Kristallographie,109, 284- aphorite meshes with the galena structure more easily 29s than freieslebenite.A comparison of the misfit values for Hoda,S.N., and Chang,L.L.Y. (1975)Phase relations in the systemsPbS- AgSr-SbrS,and PbS-AgSrBi,S. American Mineralogist, 60, 621-633. diaphorite and freieslebenite(Table 2) indicates that the Karup-Moller, S (1977) Mineralogy of some Ag-(Cu)-Pb-Bi sulfide as- strain is more evenly distributed along a and b for diaph- sociations.Bulletin of the GeologicalSociety of Denmark, 26, 4l-68. orite-galenaintergrowths. Although the misfit differences Laflamme, J.H G., and Cabri, L.J (1986a)Silver and antimony contents appear small, they are apparently large enough to reduce of galena from the Brunswick No. 12 mine. CANMET Mineral Sci- Division Report MSL, 86-138, p. l-13. the nucleation energy for diaphorite relative freiesle- encesLaboratories to - ( I 986b) Silver and bismuth contentsof galenafrom the Brunswick benite in some Ag-Sb-bearinggalena. No. 12 mine Project 30.77.01: Silver recovery in the zinc industry. CANMET Mineral SciencesLaboratories Division Report MSL, 86- AcxNowr-rocMENTS 91,p. l-16. Makovicky, E. (1974) Mineralogical data on cylindrite and incaite. Neues we thank Donald Burt and Julio Pinto for thel,aPaz sample,Michael Jahrbuch fiir Mineralogie Monatshefte,6, 23 5-256. Sheridanfor the Zacatecassample, and Louis Cabri for the syntheticAg- -(1976) Crystallographyof cylindrite. Part I. Crystal lattices of cyl- bearing samples.We also thank Nancy Brown, Gerry Czamanske,and indrite and incaite. Neues Jahrbuch fiir Mineralogie Abhandlungen, Tamsin McCormick for helpful reviews of the manuscript, as well as Su 126.304-326. SHARP AND BUSECK: Ae AND Sb IN GALENA 95

Mclntyre,N.S., Cabri,L.J., Chauvn, W.J., and Laflamme,J.H.G. (1984) Sharp,T.G , Zheng,N.J., Tsong,I S.T.,and Buseck,P.R. (1990)Scanning Secondaryion massspectrometry study ofdissolved silver and indium tunneling microscopy of defectsin Ag- and Sb-bearinggalena. Ameri- in sulfide minerals ScanningElectron Microscopy, 3, 1139-1147. can Mineralogist, 75, | 438- | 442. Moh, G.H. (1984) Sulfosalts:Observations and mineral descriptions,ex- Van Hook, H.J.K (1960) The ternary systemAg'S-Bi'S,-PbS. Economic perimentsand applications.Neues Jahrbuch fiir Mineralogie Abhand- Geology, 55, 7 59-787 lungen,150, 25-64 Wang, S., and Kuo, K.H. (1991) Crystal lattices and crystal chemistry of -(1987) Current ore petrography:Microscopy, genesis, analysis, and cylindrite and franckeite.Acta Crystallographica,47 4., 381-392. experimentation.Neues Jahrbuch fiiLrMineralogie Abhandlungen, I 53, Wernick, J H. (1960) Constitution of the AgBiSlAgBiSe' systems.Amer- 245-324. ican Mineralogist,45, 591-598. Ramdohr, P. (1980) The ore minerals and their inter$owths. In Inter- Williams, T.B., and Hyde, B.G. (1988) Electron microscopy of cylindrite national seriesin earth sciences,vol. 35 (2nd edition), 1207 p. Perga- and franckeite Physicsand Chemistry ofMinerals, 15,521-544 mon, New York. Wycofl R.W.G. (1963) Crystal structuresI. 467 p Wiley, New York. Shannon,R.D. (l 976) Revisedeffective ionic radii and systematicstudies Yund, R.A., and Tullis, J. (1983) Subsolidusphase relations in the alkali ofinteratomic distancesin halides and chalcogenides.Acta Crystallo- feldsparswith emphasison coherent phases.In Mineralogical Society graphica,432,751-767 of America Reviews in Mineralogy, 2, 141-176 Sharp, T G., and Buseck,P.R (l 989) Distribution of silver in galena:A high spatial resolution study. GeologicalSociety ofAmerica Abstracts M.lxuscmsr REcETVEDNowrrlsen 1, 1991 with Pro$ams, 2l-6, A248. Mervuscnrp'rAccEprED Aucusr 24, 1992