American Mineralogist, Volume 75, pages 289-294, 1990

Disordered intergrowths in -arsenicsulfide minerals and the paragenesisof the sartorite-groupminerals

AlraN PnrNc Department of Mineralogy, South Australian Museum, North Terrace,Adelaide, South Australia, 5000, Australia

AssrRAcr High-resolution transmission electron microscopy has been used to investigate disor- dered structural intergrowths in lead-arsenicsulfide minerals that occur at kngenbach, Binntal, . Most of the material examined was found to be perfectly ordered; however, somehighly disorderedspecimens were found. The "disordered liveingite," com- positionally intermediate betweendufrenoysite and liveingite, was found to be a disordered intergrowth of sartorite-like units and dufrenoysitelike units. The sartorite-like units are similar to those found in , whereasthe dufrenoysiteJike units are similar to those in liveingite. This disordered intergrowth representsa transitional stagein the re- placement of dufrenoysite by the more As-rich mineral liveingite. The origins of the lead- sulfide minerals at the Lengenbachdeposit are discussed;two paragenetic sequences are proposed to account for the formation of these minerals.

INrnorucrroN closely related and also occur together at Lengenbach, The development of structural classificationsystems in Binntal, Switzerland. Lattice imaging by high-resolution which seeminglycomplicated mineral structuresare con- transmission electron microscopy (nnrnrra)was used to sidered as ordered intergrowths of simple components identify disordered intergrowths. has done much to emphasize the relationships among minerals. Such structural relationships often reflect sim- TnB slnronrrE GRoUP ilar parageneticorigins for members of a group. In ad- The sartorite group comprises five well-characterized dition to ordered intergrowth structures, disordered in- minerals: sartorite, PboAsrS,u;rathite, Pb,rAsroSoo; baum- tergrowths have been found in a number of mineral hauerite, Pb,rAs,uSru;liveingite, Pb,r rAsrrSru;and dufren- systems.The most notable examples are the disordered oysite, Pb,uAs,.Soo;together with a number of other less intergrowhs in the biopyribole minerals,which have been well-defined minerals (Makovicky, 1985). The sartorite studied in considerabledetail by Veblen and others (Veb- minerals have closely related structures,which are based len et al., 1977;Veblen and Buseck,1979; Mallinson et on the ordered intergrowth of two slightly different struc- a1., 1980). Other natural disordered intergrowths have tural units. One is the S unit (S representingsartorite), been observed in the humites (White and Hyde, 1982a, which consistsof a strip 3 AsS polyhedra (trigonal pyra- 1982b)and the manganeseoxides ofthe todorokite group mids) wide, separatedfrom the next by a chain of PbS (Turner and Buseck,1979, 1981).Much recentwork has tricapped trigonal prisms (Fig. la). The sartorite structure focusedon the systematicclassification of sulfide mineral consists of two such units in a zig-zag or chemically systems,particularly those in which the structurescan be twinned arrangement (Hyde et al., 1979). The second consideredas being composedof ordered intergrowths of structural unit is the D unit (D for dufrenoysite),which distinct structuralunits (Makovicky, 1981, 1985; Hyde is 4 AsS polyhedra wide and is again separatedfrom its etal., 1979).Although a number of defect structuresand neighborsby chainsof PbStricapped trigonal prisms (Fig. disorderedintergrowths have been reported from studies lb). [Note that Makovicky (1985)has denotedthe S and of synthetic sulfides (Skowron and Tilley, 1986; Tilley D units as "3" and "4," respectively].Two D-type units and Wright, 1986;Tilley et al., 1986),to date no exam- make up the dufrenoysiteand rathite structures;the com- ples in natural sulfide minerals have been reported. This position of the D-type units in these minerals differs be- is surprising, since many of theseminerals are formed in causeof varying degreesof Pb-As substitution, with those low- to moderate-temperatureenvironments, which fa- in dufrenoysitebeing the most Pb-rich. When Pb is pres- vors the formation of ordered and disordered inter- ent in the D- or S-typeunits, it occupiesseven coordinate groWhs. sites (monocapped trigonal prisms) that are not simply A systematic study of several sulfide mineral systems related to the As sites. The D units in dufrenoysite con- was undertaken in an attempt to identify disordered in- tain 8 As atoms and 4 Pb atoms, whereasin rathite, the tergrowths.This study focusedon the lead-arsenicsulfide D units contain l0 As atoms and 2 Pb atoms. Baum- minerals (the sartorite group), which are structurally hauerite and liveingite have structures based on the or- 0003-O04X/90/03044289$02.00 289 290 PRING: DISORDERED INTERGROWTHS IN LEAD ARSENIC SULFIDES

atoms and the other contains 3 Pb and l0 As atoms. In the following discussions,however, composition 3 Pb, l0 As is used, as the subtle difference in composition does not affect the conclusionsregarding the origins ofthe sar- torite minerals. The S units in sartorite contain 8 As at- oms and no Pb. (The c cell repeat is doubled to corre- spond to the other minerals in the group.) Table I summarizesthe structural data of the sartorite group. The occurrence of the sartorite group minerals as a suite is restricted to the unusual Lengenbachdeposit, in the Binntal, Switzerland, although individual members ofthe group have been reported from other deposits(e.g., Quiruvilca, Peru, (Robinsonand Harris, 1987).At Len- genbachthe sartorite minerals occur as well-formed sin- gle crystals and homogeneousmasses up to several cen- timeters acrossin cavities in white dolomite. It has been suggestedthat these minerals were formed as the result of reactionsbetween As-rich hydrothermal fluids and PbS (galena),causing a progressive replacement of As-poor membersby the more As-rich members of the group, the most As-rich minerals being found toward the top of the deposit (Graeser, 1968a, 1977).

Er,rcrnoN MICRoScoPY Specimenswere prepared for examination by crushing in mortar followed by dispersal in acetoneand Fig. I . (a)Structural diagram of sartoriteshowing the atomic an agate grids, arrangementin theS-type unit. (b) Structuraldiagram ofdufren- deposition onto Cu support which had been coated oysiteshowing the atomicarrangement in the D-typeunit. The with a holey carbon film. It was not possible to prepare circlesin orderof decreasingsize: Pb, As, S. The strip of 4 AsS ion-thinned samples becauseof the small size of most polyhedrain the dufrenoysitestructure contains 4 Pb and 8 As specimensand the difficulties in orienting the rough crys- sites;in liveingiteand baumhauerite the stripscontain 3 Pb sites tal fragments.The minerals were examined in a modified and l0 As sites,and in rathitethe compositionof the layersis JEoL 200cx electron microscope fitted with a top entry 2 Pb and 10As sites. goniometer(C" : 1.9mm). The point-to-pointresolution and the Scherzerdefocus of the instrument in the above configurationswas 2.5 A and -670 A. R ptritips EM43or dered intergrowth of S and D units; baumhauerite is SD electron microscope was also used in the preliminary and liveingite DSDDSD. Again, however, the D and S stagesofthis study. units have compositions slightly different from those in A largenumber of specimenswere examined;most were the other minerals becauseof Pb-As substitution. The D found to be perfectly ordered and free from defects.Fig- units contain 3 Pb atoms and l0 As atoms, and the S ure 2 shows an electron diffraction pattern and lattice units I Pb and 7 As atoms. In reality there are two types image for a perfectly ordered liveingite crystal. However, of D units in liveingite:one contains3 Pb and 10.25As some massive material, which was macroscopically in-

TABLE1. The sartorite-groupminerals

Mineral Comoosition Pb:As M:S Structure Unit cell

Dufrenoysite Pb16As16S40 1.00 1.25 DD a : 7.9O,c : 8.37,b : 25.744,B : 90.35 Liveingite Pbr85As6S56 0.77 1.287 DSDDSD c : 7.9Q,a : 8.37,b : 70.49,8: 90.13 Baumhauerite Pb,rAsl6Ss 0.76 1.286 DS c : 7.90,b: 8.36,a : 22.8OA,a : 90.05, P:97.26,r: 89.92 RathiteI (Pb,Tl)1,(As,Ag),oS@ 0.75 1.25 DD b : 7.87,c : 8.47,a : 25.16A. P : 100.47 Sartorite PbiAs6S16 0.50 1.33 DD b : 7.89,c : 11 x 4.19,a : 3 x 19.62A Rathitelll Pb,rAsaSo 0.75 1.25 ? (DD) b : 7.91,c : 8.47,a : 24.52A,B : 90.00 RathitelV n.o. n.d. b : 7.92,c : 8.47,a: 138.3A Rathitela Pb,4As1oS4 0.78 1.25 ? (DD) c: 7.91,a: 8.43,b : 25.80A, B : 90.00 "Fibroussulfosalt" n.d. ? (DD) c: 7.89,a : 8.47,b: 2 x 25.61A Note.'Modified trom Makovicky (1985). PRING: DISORDERED INTERGROWTHS IN LEAD ARSENIC SULFIDES 291

t.

Fig.2. Lattice image, image simulation, and electron-diffraction pattern of liveingite (specimenBN'f, 1926 1691) down [001]. Note the sharp spots in the ditrraction pattem indicating a perfectly ordered crystal. The image shows the perfectly repeating DSDDSD structural sequence.The image simulation was calculatedfor a foil l0O A thick and a defocusof - 1570 A.

distinguishablefrom liveingite and compositionallybe- between dufrenoysite (DD) and liveingite, which has a tween dufrenoysiteand liveingite (hereaftertermed "dis- D:S ratio of 2:1. Electron microprobe analysesrevealed ordered liveingite"), was found to be highly disordered the "disorderedJiveingite" material to be composition- and twinned (Fie. 3). The fringe spacingsin the image ally homogeneousat the scaleof the electron probe beam indicate that it consists of an intergrowth of S- and D- (5-10 pm) (Table 2). The averagecomposition was de- type units. The images of the "disordered liveingite" are terminedto be Pb,o,Ag, ,As,n,Sbo.Soo, which is consistent difficult to interpret becauseof the similarity of the image with "disordered liveingite" being intermediate to dufre- motifs of the S and D units and becauseof the compo- noysite and liveingite. Ag (and Tl when present) in the sitional variability of the D and S units. Contrast differ- sartorite minerals replaceslead in a coupled substitution encesin Figure 3, however, reveal the presenceof pairs with As such that 2Pb2+: Ag+ + As3+,with Ag entering of SD units similar to those found in baumhauerite and the samesites as As rather than Pb. Severalweight percent liveingite; theselight strips are separatedby darker strips of Ag have been reported in some of the sartorite group of variable width, which are composed of D-type units. minerals,in particular liveingite, baumhauerite,and rath- The individual S and D units are identified on the mi- ite (Engeland Nowacki, 1969, 1970; Marumo and No- crograph, as is the location of a twin plane. Computer- wacki, 1965). image simulations calculatedusing the multislice method (Goodmanand Moodie, 1974)confirm that the SD pairs are similar to those in baumhaueriteand liveingite (Engel Trale2. Electron-microprobeanalysis of "disorderedliveingite" and Nowacki, 1969, 1970)and indicatethat the D-type Element Wt%- Atomicproportionsf units are similar to those found in liveingite (Engel and Pb 49.5 14.1 Nowacki, 1970).The compositionsof the D- and S-type Ag 2.O 1.1 units usedin the structuralmodel proposedare (3 Pb, l0 24.8 19.2 Qh rr'l As) and (l Pb, 7 As) respectively.The image simulations 0.3 D 21.8 40 matching the various structural components of the dis- Total 98.8 ordered intergrowth are shown in Figure 3. Structural Nofe; Analyses were performed by Dr. W. Birch on the electron micro- models based on D units with the composition of those probe at the Departmentof Geology,University of Melbourne.The follow- in dufrenoysiteand rathite werealso testedbut werefound ing standardswere used:galena (Pb), arsenopyrite (As, S), pure Sb, and pureAg. The specimencurrent ranged from 0.015pA to 0.025pA with an to be unsatisfactory. acceleratingvoltage of 20 kV. . The proportions of individual D- and S-type units in Average of six analyses _ the imageare in the ratio 5:1, which placesthe material t Calculatedon the basisof'40 S atoms. 292 PRING: DISORDERED INTERGROWTHS IN LEAD ARSENIC SULFIDES

Fig. 3. Electron-diffraction pattern and lattice image down pairs are similar to those found in baumhauerite and liveingite, the 7.9 A axis of the "disordered liveingite" (M35933). The zone and the other D units are similar in composition to those in correspondsto [001] of liveingite. Note that the reflections in liveingite. The arrow toward the edgeof the image indicates the the diffraction pattern are heavily streaked along the D* direc- location of a twin plane. Computer image simulations of two tion, which correspondsto the stacking direction of the D and componentsof the intergrowth are also shown. The image sim- S units. The disordered intergroMh of D- and S-type units is ulations werecalculated for a foil thicknessof75 A and a defocus clearly apparent in the lattice image. Note the DS pairs, which of -570 A. are lighter in contrast than the remaining D-type units. The DS

P.q.RAcnNnsrs frenoysite and liveingite. A two-stagereplacement mech- Graeser(1968a, 1968b, 1969,197'7)proposed that the anism is indicated. The first step could involve the re- Lengenbachdeposit was originally a syngeneticsulfide- placementof Pb by As in the D-type units. This is required dolomite deposit containing galenaand sphalerite,which to account for the differencesin composition of the D- was invaded by arsenic-rich hydothermal fluids towards type units in dufrenoysite (4 Pb, 8 As), liveingite (3 Pb, the end of the Alpidic metamorphism. These fluids in- l0 As), and rathite (2 Pb, l0 As). This processmay be troduced Cu, Ag, and Tl in addition to As into the de- followed by the stage in which the S units are formed. posit. Graeser(1968a, 1977) suggestedthat the lead-ar- This secondstage requires either the formation of new S senic sulfide minerals were formed during metamorphism units via the difusion of Pb, As, and S through the crystal and hydrothermal alteration with the least As-rich min- or the transformation of D units into S units via the elim- eralsforming first and being progressivelyreplaced by the ination of an As/Pb site in the D unit and further replace- more As-rich minerals as the reactions progressed.The ment of Pb by As. Either mechanism suggeststhat the hydrothermal alteration continued until the minerals second stage of replacement requires more energy than reached saturation in As or the reaction conditions the first. The trapping of the "disordered liveingite" changed, resulting in the crystallization of realgar. Ac- structurally intermediate state indicates that these min- cording to this hypothesis,the first Pb-As mineral to form erals were formed during the retrograde stageof hydro- at Lengenbachwas jordanite, which is not simply struc- thermal alteration of the deposit. turally related to the sartorite group minerals. Jordanite Two separateparagenetic sequences leading to the for- was followed, in sequence,by dufrenoysite, liveingite, mation of the sartorite-group minerals are required to baumhauerite, rathite, and sartorite. In terms of the account for the formation of liveingite and rathite. One structures of the sartorite group minerals, this sequence sequenceinvolves the initial replacementof Pb by As in can be seen,with the exceptionofrathite, as a progressive the D units followed by the formation of S units. The replacementof D-type units by S-type units (an increase other involves the continued replacementof Pb by As in in chemical twinning). the D units without formation of S units until the final The "disordered liveingite" of this study appears to step,ifat all. representa remnant of the transitional state betweendu- The flrst sequence: PRING: DISORDERED INTERGROWTHS IN LEAD ARSENIC SULFIDES 293

dufrenoysite(DD, DD, DD) equilibrium behavior in sulfidesprovides direct evidence Pb,uAs,uSoo ofreplacement and other processesin the paragenesisof - intermediate (DD, DD, DD) sulfide deposits.A number of sulfide systems,when con- Pb,oAs,rSoo sideredfrom a structural point ofview, seemparticularly likely to exhibit this kind of structural behavior; for ex- - liveingite (DS, DD, SD) ample, the lillianite, plagionite, boulangerite, pavonite, Pb,r rAsrrSru and junoite groups. Synthetic studies in several ofthese * baumhauerite(DS, DS, DS) systemshave revealeddisordered intergrowhs (Skowron Pb,rAs,uSru and Tilley, 1986;Tilley and Wright, 1986;Tilley et al., - sartorire (SS,SS, SS) 1986).A closer examination of many sulfide systemsap- PbrAs,.S' pears to be justified. The secondsequence: dufrenoysite (DD, DD, DD) AcxNowr,nncMENTs pb,uAs,uSoo I wish to thank Dr. D. A. Jeffersonand ProfessorJ. M. Thomas of the Dr. - intermediate (DD, Department of Physical Chemistry, University of Cambridge,and J. DD, DD) D. Fitz Gerald of the Research School of Earth Sciences,Australian Na- Pb,oAs,rSoo tional University, for accessto nnrrrvr facilities in their laboratories. I also - rathite (DD, DD, DD) wish to thank Dr. W. D. Birch, Museum of Victoria, for the electron- microprobe analyses and for providing specimens for this study. Speci- Pb,rAsroSoo mens were also provided by the British Museum (Natural History), the - sartorite(ss, ss, ss) Department ofEarth Sciences,University ofCambridge, and the Natur- PbrAs,uS, historischesMuseum, Bern. I thank Dr. Birch and Dr. T. B. Williams for their critical reading of the manuscript and Mr. G. Horr for assistancein the preparation ofFigure 1. The financial support ofthe Australian Re- The first sequenceappears to be dominant at Lengen- search Council is gratefully acknowledged. bach sinceliveingite, baumhauerite,and sartorite are rel- atively common in the deposit, whereas rathite is very RrrnnnNcns crrED rare. The evidence of local associationsat kngenbach Engel, P., and Nowacki, W. (1969) Kristallstruktur von Baumhauerit. also suggeststhat these sequencesare correct. Liveingite Zeitschrift fiir Kristallographie, 129, l7 8-202. has not been found with sartorite, nor baumhaueritewith - (1970) Die Kristallstruktur von Rathit-Il (AsrrSr,I Pb65vtrPb,2lx). dufrenoysite (Graeser,private communication). Zeitschrift fiir Kristallographie, 131, 356-375. Goodman, P., and Moodie A.F. (1974)Numerical Evaluation of N-beam Both sequencesinvolve an intermediate"phase," which wave functions in electron scattering by the multi-slice method. Acta has a structure basedon two D-type units similar in com- Crystallographica,A30, 280-290. position to those in liveingite (3 Pb, l0 As) and inter- Graeser,S. (1968a)Die Sulfosalzedes Binntales: Geochemie und Genese. mediate to those of dufrenoysite (4 Pb, 8 As) and rathite Jahrbuch,Naturhistorisches Museum der Stadt Bern, 1966-1968, 46- 63. (2 Pb, l0 As). The identity of the intermediate phase is -(1968b) I-ead isotopesand minor elementsin galenaand sulpho- unclear, and it may not be a stable mineral at Lengen- salts from Binnatal. Eanh and PlanetaryScience htters, 4,384-392. bach. It may, however, be the mineral called rathite I by -(1969) Minor elements in sphalerite and galena from Binnatal. Le Bihan (1962) and rathite Ia by Nowacki et al. (1964). Contributions to Mineralogy and Petrology,24, 156-163. -(1977) The Mineralogical Record, 8, 275- Le Bihan's rathite I has the formula Pb,oAs,rSoo,which Ifigenbach, Switzerland. 281. has an excessof positive charge; however, the mineral Hyde, B.G., Andersson, S., Bakker, M., Plug, C.M., and O'Keeffe, M. also contains traces of Ag and Sb, which may stabilize (1979) The (twin) composition plane as an extended and structure- the mineral. Rathite also has an excessof positive charge building entity in crystals.Progress in Solid StateChemistry, 12,273- (Pb,rAsroSoo)but is stabilized by the presenceof Tl and 327. k Bihan, M.Th. (1962) Etude structuralede quelquessulfures de plomb Ag. Marumo and Nowacki (1967) consideredLe Bihan's et d'arsenic naturels du gisement de Binn. Bulletin de Soci6t6. francaise rathite I to be identical with dufrenoysite on the basis of de Mineralogie et de Cristallogaphie, 85, 15-47. their powder patterns; however, this matter is worthy of Makovicky, E. (1981) The building principles and classificationofbis- further consideration. muthJead sulphosalts and related compounds. Fortschritte der Miner- It is hoped that further work may para- alogie, 59, 137-190- establish the - (1985) The building principles and classiication ofsulphosalts based genetic roles of Ag and Tl in these minerals and also on the SnSarchetype. Fortschritte der Mineralogie, 63, 45-89. clarify the relationships of some of the poorly character- Mallinson, L.G., Jefferson,D.A., Thomas, J.M., and Hutchinson, J.L. ized members of the group, e.9., rathite III and rathite (1980) The intemal structure of nephrite: Experimental and computa- IV. Pring et al. (unpublished manuscript) found that the tional evidence for the coexistence ofmultiple-chain silicates within an Transactions of the Royal Society of presence amphibole host. Philosophical of Ag in baumhauerite is linked to supercellfor- London, 295,537-552. mation in this mineral. Marumo, F., and Nowacki, W. (1965) The crystal structure of rathite-I. In the wider sense,disordered intergrowths may be quite Zeitschrift liir Kristallographie, 122, 433456. widespreadin sulfide minerals, particularly those that have -(1967) The crystal structureof dufrenoysitePb,.As,uS-. Zeitschrift fiir Kristallographie, 124, 409419. been subjectedto moderate- or lower-temperaturemeta- Nowacki, W., Marumo, F., and Tak6uchi, Y. (1964) Untersuchungen an morphism or hydrothermal alteration during the late Sulfiden aus dem Binntal (Kt. wallis Schweiz.). Schweizerische Mi stagesof their history. The occurrenceof this type of non- neralogischeund PetrographischeMitteilungen, 44, 5-9. 294 PRING: DISORDERED INTERGROWTHS IN LEAD ARSENIC SULFIDES

Robinson, G.W., and Harris, D.C. (1987) A baumhaueritelike mineral Veblen, D.R., and Buseck,P.R. (1979) Chain-width order and disorder from Quiruvilca, Peru. Mineralogical Record, 17,199-201. in biopyriboles. American Mineralogist, 64, 687-7 00. Skowron, A., and Tilley, R.J.D. (1986) The transformation of chemically Veblen,D.R., Buseck,P.R., and Burnham, C.W. (1977)Asbestiform chain twinned phasesin the Pb-BirS, systemto the galenastructure. Chemica silicates:New minerals and structural groups.Science, 198, 359-363. Scripta,26, 353-358. White, T.J., and Hyde, B.G. (1982a)An electron microscopestudy of the Tilley, R.J.D., and Wright, A.C. (1986)Nonstoichiometric forms of Sb2S3 humite minerals: I Mg-humites. Physicsand Chemistry of Minerals, 8, occuring in the PbSbrSn-SbrS,region of the Pb-Sb-Sphase diagram. 55-63. Journal of Solid StateChemistry, 64, 1-21. -(1982b) An electron microscopestudy of the humite minerals: II Tilley, R.J.D., Wright, A.C., and Smith, D.J. (1986) The structureof non- Mn-Humites. Physicsand Chemistry of Minerals, 8, 167-174' stoichiometric phasesin the SbrSs-PbSsystem close to Sb2Sr.Proceed- ings of the Royal Societyof lrndon, A404, 9-22. Turner, S., and Buseck,P.R. (1979) Manganeseoxide tunnel structur€s and their intergrowths.Science, 203, 456458. -(1981) Todorokites: A new family of naturally occurring man- MlNuscnrrr RECETVEDJurv 3. 1989 ganeseoxides. Science,212, 1024-1027- MeNuscnrrr AccEprED NowMsrR. 20, 1989