American Mineralogist, Volume 75, pages 915-922, 1990

Baumhauerite-2aiA silver-bearingmineral with a baumhauerite-likesupercell from Lengenbach,Switzerland

Ar,r,.q.NPnrNc South Australian Museum, North Terrace,Adelaide, South Australia, 5000, Australia Wrr-r-r.lntD. Brncrr Museum of Victoria, 285 Russell Street,Melbourne, Victoria, 3000, Australia Dlvro Srwnr.l Department of Geology, University of Melbourne, Parkville, Victoria,3052, Australia Srnr'.q.NGurspn Natural History Museum, CH 4001 Basel,Switzerland and Mineralogical Institute, University of Basel,CH 4056, Switzerland ANonnls EonNHanrrn Mineralogisch-KristallographischesInstitut, Universitiit G

AssrRAcr Baumhauerite-2a rs a silver-bearing having a superstructurederived from a baumhaueritelike structure,with an averagecomposition Pb,rAgorAs,r.Sboo536. Baum- hauerite-2a occurs as an intergrowth with a silver-free baumhauerite in at Len- genbach,Binntal, Valais, Switzerland.Baumhauerite-2ais massive,opaque, and steelgray with a metallic luster and a dark reddish brown . Baumhauerite-2ahas one perfect cleavageon (100), has a conchoidal , and is brittle. The microhardnessVHNro is 156-165,mean 159,corresponding to a Mohs hardnessof 3. In reflectedplane-polarized light, the mineral varies in color from white to off white; the mineral is bireflectant and nonpleochroic. Reflectancevalues are tabulated and compared with those of baumhauer- ite. Powder X-ray diffraction data were indexed on a monoclinic cell having a: 44.74(4), b:8.471(4),c:7.910(6)A,B:93.37'(5)andZ:2.Strongestreflectionsinthepowder diffracrionpattern [d in A (D hkn 4.22,(80), 120;3.68,(60), 9l 1,221 3.21,(60), 13.0.1; 3.01,(100), 13.1.1; 2.95,(70),921 2.75, (90), 11.0.2; 2.73,(70),622;Z.ZS, (90),332. The mineral's superstructureis believed to be due to the ordering of Pb, Ag, or As atoms over two baumhauerite-like cells. High-resolution lattice images show that baumhateite-2a and baumhauerite can intergrow at the unit cell level. The mineral is named for its rela- tionship to baumhauerite.

INrnonucrroN of which appear to be superstructuresof the better-de- Lengenbach,Binntal, Canton Valais, Switzerland,is well fined species.The nature of supercellformation in these known for the occurrenceof rare sulfosalt , par- minerals is poorly understood, and there is a lack of ac- ticularly the Pb-As sulfides,many of which have not been curate chemical and X-ray data for most of them. Phys- found elsewhere.The sulfosalts occur with and ically and optically they are very similar to their parent in cavities in a Triassic dolomite that lies at the phases. outer edge of the Monte-Leone Nappe (Graeser, 1965, We have undertaken a systematic study of the lead- 1977).Six well-characterizedPb-As sulfide minerals have arsenic sulfide minerals with powder X-ray diffraction, been identified from the deposit: , rathite, baum- electron microprobe analysesand high-resolution trans- hauerite, liveingite, dufrenoysite,and jordanite. With the mission electron microscopy (HRTEM) techniquesin or- exception ofjordanite, these minerals are chemically and der to characterizethese supercell phasesand to try to structurally very closely related, having two unit-cell re- establish the nature of supercell ordering. In this paper peats, 8.4 A and 7.9 A, in common (Makovicky, 1985). we will confine ourselvesto baumhaueriteand the closely In addition to thesewell-characterized phases, a number related new mineral baumhauerite-2a. The mineral and of poorly defined phaseshave also been reported, most name, baumhateite-2a, were approved by the Commis- 0003-o04x/90/0708-09l 5s02.00 9l 5 9t6 PRING ET AL.: BAUMHAUERITE-2a FROM LENGENBACH

Trau 1. R and '.F valuesfor baumhauerite-2aand baum- hauerite(specimen M3010)

Baumhauerite-2a Baumhauerite

t^ tmF| R1 R2 ,.F, R" H1 R2 ^R" 400 38.9 41.9 23.2 26.6 38.0 42.1 22.4 26.7 420 38.4 41.6 22.6 26.0 37.6 41.9 21.95 26.2 440 37.85 41.3 22.05 25.5 37.2 41.6 21.55 25.7 460 37.3 40.9 21.4 25.O 36.8 41.2 21.1 25.3 470 37.1 40.7 2't.2 24.8 36,6 41.0 20.9 25.05 480 36.85 40.5 20.9 24.6 36.4 44.7 20.7 24.8 500 36.2 40.2 20.3 24.2 36.0 40.4 20.2 24.3 s20 35.6 39.6 19.7 23.65 35.5 39.8 19.8 23.8 s40 34.9 39.0 19.1 23.0 35.1 39.2 19.4 23.2 546 34.7 38.7 18.9 22.7 35.0 39.05 19.3 23.O 560 34.15 38.1 18.5 22.2 34.6 38.6 18.9 22.5 580 33.5 37.3 17.9 21.5 u.2 37.9 18.5 21.9 589 33.1 36.9 17.6 21.1 33.9 37.5 18.3 21.6 600 32.8 36.4 17.3 20.65 33.7 37.2 18.1 21.3 620 32j 35.6 16.7 19.9 33.2 36.5 17.6 20.6 640 31.35 34.7 16.1 19.2 32.5 35.7 17.O 19.9 Fig. l. Sketchshowing the textural relationshipbetween 650 31.05 u.4 15.9 18.9 32-2 34.3 16.7 19.6 baumhauerite-2a and batrnhaueritein a massiveintergrowth. 660 30.6 34.0 15.6 18.4 31.8 34.9 16.4 19.3 680 30.1 33.3 15.0 17.8 31.1 34.2 15.8 18.6 700 29.6 32.65 14.65 17.3 30.5 33.5 15.3 18.0 sion on New Minerals and Mineral Names, I.M.A., in l 989. the baumhauerite associated with baumhauerite-2a is S,c,]upr.n DESCRTPTToN somewhat higher, being in the range 177-184 with an A number of specimens containing baumhauerite-2a averageof l8l basedon sevenindentations. and baumhaueritewere examined.Specimens M3010 and The reflectedlight properties of baumhauerite-2awere M30980 from the Museum of Victoria and BM 1926 determined on fragments from specimen M3010, which 1654 from the British Museum (Natural History) are contained an intergrowth of baumhaueite-2a and baum- massive and contain a heterogeneousintergrowth of hauerite. The specimenwas polished for examination us- baumhauerite-2a and baumhauerite. Three additional ing the proceduresas describedby Criddle et al. (1983). specimensexamined, L7228, L9501, and L15621, from In plane polarized light, baumhaueite-2a and baum- the collection of the Naturhistorisches Museum, Basel, hauerite are indistinguishable, varying in color fiom white are composed of pure baumhauerite-2a. Specimens to off white; both minerals are moderately anisotropic M3010, BM 19261654 and L7228 aredesignated as being with subtly different rotation tints; from extinction the cotypes of baumhaueite-2a; several fragments from sequencefor baumhaueite-2a is brownish gray, greenish specimen M3010 are also preserved in the collection of gray, very pale bluish gray and pale greenishgray; those the South Australian Museum (Catalogue number G of the accompanyingbaumhauerite are dark brown, light 15547). All of the specimens were from the quarry at brown and light brownish gray. The differencesin tints Lengenbach, Binntal, Switzerland. The exact locality between the minerals are subtle, and although it is pos- within the quarry is unknown, but the mineral may be sible to distinguish one mineral from the other when both fairly widespread. are present, ifonly one were present, the best that could be hoped for was that one could say that it could be Or"rrcar, AND PHYSTCALPRoPERTTES baumhauerite or baumhaueite- 2a. The physical and optical properties of baumhauerite- In the specimenexamined, baumhauerite is character- 2aand baumhauerite differ only subtly, and the two min- ized by fine lamellar twinning, which is absent from erals are difficult to distinguish without recourseto pow- baumhauerite-2a. The latter occurswithin baumhauerite der X-ray diffraction methods. In hand specimen, both as lenses,some of which are flamelike in shapeand others minerals are opaque and steelgray with a metallic luster. arcuate and sigmoidal (Fig. l). The red internal reflec- The streakis dark reddish brown. The minerals are brittle tions common in baumhauerite were not observed in with a conchoidal fracture, and baumhaueite-2a has one baumhauerite-2a. perfect ,probably on (100), similar to that of Reflectancemeasurements were made using the pro- baumhauerite. It was not possibleto measurethe density ceduresand equipment describedby Criddle (in Cabri et of baumhaueite-2a owing to the nature of the inter- al., l98l), except that air and oil l6x objectiveswere growth of baumhaueite-2a and baumhauerite, but the used. The effective numerical apertures of these objec- calculated density is 5.31 g,/cm3. tives were adjusted to 0. 15 by means of the illuminator The mineral has a Mohs hardnessof 3 and a micro- ap€rture diaphragm. hardnessVHN5. in the range 156-165, with an average Four areas were measured: two of baumhauerite and of 159 based on 5 indentations. The microhardness of two of baumhateite-2a. The reflectancespectra (Fig. 2) PRING ET AL.: BAUMHAUERITE-2a FROM LENGENBACH 9t7

RZ 25

q.f7( 4AA 45A JU9 554 6A@ 65A 7AA Lombdo rrm Fig.2. R,, R, and '-R,, t-R, spectrafor baumhauerite(dashed lines) and baumhauerite-2a.

confirm the visual impression that the two minerals are ular surfacefor our measurements,some of our data are optically very similar, but it is clear that there are slight in error (by as much as 0.10/oabsolute). Both baumhauer- differencesin the dispersion oftheir reflectances,as there ite-2 a andbaumhauerite are essentiallytransparent in the should be, if the rotation tints differ. Data for two of the orange-to-redregion of the visible spectmm, and it is in measuredareas are summarized in Table l. Theseresults this region that our R andi-R data produce erroneous are consistently lower than those of the baumhauerite absorption coefficients (cf. Embrey and Criddle, 1978). reported by Picot and Johan (1982), and the dispersion Given this fact, it may well be that, in larger, less well- of their reflectancespectra match much more closelythose polished samples, or on fractured surfaces,baumhauer- of baumhaueile-2a than baumhauerite. It is worth re- ite-2awlll also display red internal reflections. calling that some samples from Binntal consist entirely of baumhaueite-2a and that the distinction between CHrnnrsrny pi- baumhauerite and baumhaueite-2a was unknown to Quantitative electron microprobe analyses of baum- cot and Johan(1982). haueile-2a and the associatedbaumhauerite were ini- Although great care was taken to obtain a perfect spec- tially obtained using the electron microprobes at the De- 918 PRING ET AL.: BAUMHAUERITE-2a FROM LENGENBACH

TABLE2. Summary of baumhauerite-2aand baumhaueritecompositional data

Baumhauerite Baumhauerite-2a BM1926,1654 M3010 BM 1926,1654 Ele- ments 1. 2. 1. 1. 2. wt06 Pb 48.3 47.5 52.8 51.3 48.2 47.1 48.1 46.2 46.2 46.3 Ag 0.1 1.5 1.4 1.6 1.6 1.4 1.4 TI n.o. 0.8 n.d. 0.4 n.d. 0.5 n.o. 0.8 AS 26.5 27.9 24.1 25.1 25.5 26.8 25.9 27.4 26.3 26.9 Sb 1.2 0.6 1.1 0.8 0.5 0.5 s 23.2 24.6 21.9 23.1 22.6 23.9 22.6 24.2 25.6 24.3 Total 99.2 100.8 99.5 99.9 98.9 99.7 99.0 100.2 100.0 99.4 Atomic proportions Pb 11.6 10.7 13.5 12.4 11.9 10.9 11.8 10.6 10.0 10.8 Ag o.7 0.6 0.8 o.7 0.6 0.6 TI o.2 0.1 0.1 o.2 o.2 AS 17.6 17.4 17.0 16.7 17.4 17.3 17.6 17.4 15.8 17.3 J 36 36 36 36 36 36 36 36 36 36 Nofe.'n.d. : not detected. Analyses 1. Universityof Melbourne,W. D Birch and D. Sewell analysts.Analysis 1'. E'T.H. Zurich' A. Edenharteranalyst. Analyses 2. Universityof Gottingen, A. Edenharteranalyst.

partment of Geology, University of Melbourne (M3010 X-n-q.v DTFFRACTTONDATA and BM 1926 1654) and at EidgeniissischeTechnische Hochschule, Zuich(L7228). A duplicate set of analyses Precessionand Weissenbergsingle- X-ray pho- was undertakenat the University of Gdttingen to confirm tographswere obtained using a small fragment of baum- the significant variation in composition of the specimens. haueite-2a from specimen L7228. These photographs Analytical standardsused in Melbourne were galena(Pb), showed that baumhaueite-2a has a monoclinic unit cell : arsenopyrite(As,S), pure Sb, and pure Ag. The specimen witha: 44.41(r),b:8.49(2), c:7.92(r) A, and0 current rangedfrom 0.015 pA to 0.025 pA with an ac- 9341',. celeratingvoltage of 20 kV. Analysesat Zuich and Giit- The powder X-ray diffraction pattern ofbaumhauerite- tingen used the following standardsgalena (Pb), stibnite 2awas recordedusing a Guinier-Hfiggcamera (100-mm (Sb and S), synthetic lorandite (Tl and As), and synthetic diameter) with monochromated CuKa radiation and sil- pyrargyrite (Ag), with an acceleratingvoltage of 15 kV icon as an internal standard.The powder pattern contains and a specimen current of about 8 aA. The analysesof some lines from baumhauerite, as the intimacy of the the specimensexamined are presented in Table 2. The intergrowth betweenthe two forms of baumhauerite pre- compositionswithin samplesfall into two distinct groups: vents their separation.The pattern also contains a num- Ag-free compositions defined as having Ag < 0.20loand ber oflines fromjordanite. The powder diffraction data compositions having Ag - 1.50/0.Intermediate compo- for baumhaueite-2a are given in Table 3; lines due to sitions were not found, suggestingthat there are two com- jordanite and baumhauerite have been omitted. positionally distinct phasesrather than a singlephase that As they are chemically and physically similar, powder is compositionally zoned. This conclusion is confirmed X-ray diffraction offers the simplest method to diferen- optically. Energy-dispersiveX-ray analyses (EDA) and tiate betweenthe two forms ofbaumhauerite. The pattern electron diffraction examination revealed that the higher of baumhaueite-2a was indexed on a monoclinic cell Ag contents are associatedwith baumhaueite-2a and the with the aid of single-crystalphotographs and ref,ned by accompanyingbaumhauerite is essentiallyAg-free. least-squaresmethods using 54 reflections with 20 < 40. The electron microprobe analysesgave the averagefor- The following cell parameterswere obtained:a: 44.74(4), mula (the mean of three specimens,both setsof analyses b:8.477(4), c:7.910(Q A, andP:93.37"(5). X-ray calculatedon the basis of 36 S atoms) Pb,,Ago'- diffraction data were recorded for three specimens of As,rrSbooS.ufor baumhaueite-2a and Pb,rAs,rrSrufor baumhauerite-2a, and only minor variations in cell di- baumhauerite. The compositional relations suggestthat mensionswere found (Table 4). Comparison with the tri- : Ag would replaceAs. The analysesshow a range of com- clinic cell of baumhaueile-a : 22.80, b : 8.357, c positions for baumhaueite-2a, with the duplication of 7.89A, a:90'03', 0:97"16', y: 89"55'(Engeland the analysesat the University of G

TAaLe3. Powder X-ray diffractiondata for baumhauerite-2aBv TABLE4. Cell parameters for baumhaue(ile-2a 19261654 Specimen no. aA bA cA A d*', A 4*, BM(MH)1926 1654 44.74(4) 8.477(4) 7.e10(6) 93.3715) 20 1111 11.171 400 M3010 44.2(1) 8.4s(4) 7.91(3) e3.612) 5 8.49 8.476 010 L7228 44.25(6) 8.497(11) 7.934(10) 93.912) 10 7.72 7.697 101 L7228 44.41(1) 8.49(2) 7.92(1) 93.68" 5 7.38 7.367 310 - photographs. 20 7.16 7.151 301 Fromsingle-crystal 5 6.40 6.383 700 10 6.16 6.150 510 10 6.09 6.096 501 10 5.84 5.777 011 (?) 10 5.73 5.751 501 5 ((o 5.594 610 Er,ncrnoN MrcRoscoPY 10 4.80 4.827 70'l 80 4.22 4.219 120 Sampleswere prepared for the electron microscopeby 40 4.16 4.164 220 grinding in acetone and by depositing a drop of the re- 10 3.95 3.947 002 sulting suspensiononto holey carbon films. These frag- 30 3.87 3.872 302 40 3.79 3.792 402 ments were examined at a magrrificationof 360 000 x in 60 3.68 3.683 911 a modified JEM-200CX electron microscope fitted with 3.665 221 a top-entry goniometer and an energy-dispersiveX-ray 50 3.65 rt.occ 402 20 3.60 3.599 321 detector. The characteristicsofthe objective lens at 200 10 3.57 3.571 421 kV are C" : 1.9 mm and C" : 1.9 mm, giving a point- 30 3.50 3.504 212 30 3.47 3.480 521 to-point resolution of approximately 2.5 A at the so-called 30 3.46 3.462 412 Scherzerfocus (Scherzer,1949). 20 3.41 3.409 12.1.0 Figures 3a and 3b shows electron diffraction patterns 3.406 602 20 3.39 3.392 11.1 .1 for baumhauerite and baumhauerite-2a, projected down 10 3.36 3.356 412 [001]. The doubling of the a cell repeat is apparent when 10 3.31 3.316 802 patterns The distribu- 60 3.21 3.221 13.0.1 the two diffraction are compared. 5 3.14 3.138 802 tion of the strong subcell reflections is common to both 5 3.O7 3.070 12.'t.1 patterns, indicating that the structures of the two min- 3.065 821 10 3.05 3.054 712 erals are composed of the same basic subcell units. 100 3.01 3.011 13.1.1 High-resolution lattice images of baumhauerite and 70 2.95 2.945 921 baumhauerite-2a confrrm the close relationship between 40 2.93 2.932 11.2.0 50 2.90 2.905 10.2.1 the two phases(Figs. 4a and 4b). The baumhauerite im- 20 2.89 2.890 122 age shows that the a repeat is composed of two distinct 2.889 o22 lattice motifs (Fig. 4a), whereas four motifs are visible 30 2.82 2.820 130 20 2.78 2.776 330 along the arepeat of baumhaueite-2a (Fig. ab). The dou- 20 2.77 2.768 422 bling of the arcpeatcar be seenby comparison of Figures 90 2.75 2.752 11.0.2 2.73 2.733 622 4a and 4b; the 8.4-A repeat along [010] is the same in 20 2.71 2.713 11.2.1 both images.Baumhauerite and baumhaueite-2a clearly 50 2.65 2.652 16.1.0 have a common structural framework. i.e.. the order of 10 2.64 2.641 630 20 2.63 2.628 331 the structural units is the same in both phases,and the 20 2.61 2.610 331 structure repeats differ, probably becauseof cation or- 20 2.56 2.563 531 motifs in images 20 2.42 2.423 413 dering. The details of the lattice the two 20 2.38 2.380 10.2.2 are similar but not identical. This is not unexpected,as 30 2.34 2.342 613 the structures not only contain slightly different cation- 20 2.33 2.325 931 90 2.28 2.282 332 ordering schemes,but the images also were recorded at 30 2.26 2.261 16.1.2 different objective lens defocus values. Within an image the effectsof thickness on lattice motifs can be great; for A/ore:Cell a : 44.74(41,b : 8.477(4),c : 7.910(6)A, B : 93.37.(5). * Intensitiesestimated visually. example,in Figures 4aand 4b, crystal thicknessdecreases from the top of the image to the bottom. The doubled- cell repeat of baumhaueite-2a is distinct in regions of intermediatethickness (- 100 A). At greaterthickness and This is clearly a 2a slpercell of baumhauerite, although in very thin regions the contrast differencesamong the the reflectionat d : I l.l I A, which is sometimesabsent, four layers in the doubled cell are much less distinct. cannot be indexed with the cell with a: 45.7 A. Baum- Figure 5 shows an irregular intergrowth of the two haueite-2amay also be triclinic, but, if so, the deviations forms; baumhaueite-2a is the dominant phasewith small of a and "yfrom 90' are very small, and the uncertainties strips of baumhauerite intergrown. The intergrowth of in assigning indices to higher angle reflections prevent the two minerals at the unit-cell level indicates that the accuraterefinement of a triclinic cell. structural difference between baumhauerite and baum- 920 PRING ET AL.: BAUMHAUERITE-2a FROM LENGENBACH

Fig. 3. Electron difraction patterns of the baumhauerite minerals projected down [001]: (a) baumhauerite,(b) baumhauerite- 2a (M30980). The distinctive distribution of subcell reflections is indicated in the two patterns.

haueite-2a is the result of chemical variation by means ofcation ordering.

DrscussroN Baumhauerite from Lengenbach, Switzerland, was originally describedby Solly (1902) on the basis of chem- ical and morphological data. The original analysis gave PboAsuS,r,or recalculatedto 36 S atoms,Pb,,,As,uuS.u, an Ag-free composition, but Solly also noted an earlier analysis of "rathite" by Uhrlaub, which Solly considered to be Ag-bearingbaumhauerite. Engel and Nowacki (1969) proposed Pb,rAs,uSruas the structural formula based on their crystal-structuredetermination. The structural role of Ag was consideredby Engel and Nowacki, as the crystal giving rise to their intensity data had an Ag-rich composition (Pb,,uAgouAs,srS.u).They assignedAg to partially replaceAs on one site and a small amount of As was found to occupy a Pb site. These sub- stitutions, which appear to be linked, indicate that a de- greeof chemical variation is possiblein the baumhauerite structure. This is a general feature of the Pb-As-sulfide minerals, as rathite, Pb.AsrS,o, and dufrenoysite, PbrAsrSro,have essentially the same structural frame- work with Pb and As atoms replacing each other, though they occupy slightly different sites (Marumo and No- wacki, 1965; Ribir et al., 1969).The electron microprobe Fig. 4. High-resolution lattice images of (a) baumhauerite analysesreported here show a wide compositional range (M30980) and (b) baumhauerite-2a (M3Ol0). Both images are of baumhauerite-2afrom Pb,oAgo oSruto projected down [001] with a horizontal and D vertical. Note that uAs,rrSbo variation the lattice repeat of baumhauerite-2a is tluv.rcethat of baum- Pb,, nAgo,Asr? 4Sb0 5536. Although some of this hauerite along a but equal along b, the 8.4 A translation. Note may be a reflection of the uncertainties associatedwith also in the regions where the image detail is greatest(circled) electron microprobe analysesof these minerals, the anal- that the lattice motifs are similar. yses were performed independently in two laboratories, PRING ET AL.: BAUMHAUERITE-2a FROM LENGENBACH 921

Fig. 5. High resolution image of baumhauerite-2a(2a) intergrown with thin strips of baumhauerite (a), [00U zone. The width of the strip of baumhauerite is 2 unit cells, and it forms a coherent intergrouth. The crystal has been slightly tilted away from the zone axis so as to highlight contrast differencesbetween the baumhaueite-2a arrdbaumhauerite units. and the two sets of data are in good agreement.Few of ordering, the exact nature ofwhich could not be identified the many published analysesof the Pb-As-sulfide min- from the images,but ordering of Ag, As, or Pb over the erals correlate closely with the formulae determined by two baumhauerite cells seemsmost likely. A full crystal- crystal-structure analyses(see Stalder et al., 1978). The structure analysis or more extensive and higher-resolu- structures offer considerableflexibility to accommodate tion lattice imagesare required to reveal the exact nature stoichiometric variation. In addition to cation substitu- ofthe cation ordering. tions, disorderedstructural intergrowths also occur; Pring The literature on the minerals from the Lengenbach ( I 990) found disordered intergrowth of sartoriteJike and quarry contains many referencesto poorly defined min- dufrenoysite-like structural units in a disordered liveing- erals. In addition to rathite, also known as rathite I, there ite. In the case of baumhauerite-2a, an extended com- are rathite II, rathite III, and rathite IV, for all of which positional field is observed,and this is due to cation sub- unit-cell parameters have been determined. Nowacki stitution rather than structural intergrowth. Roesch and (1967) showedthat liveingite and material formally known Hellner (1959) synthesizedbaumhauerite and baum- as rathite II are equivalent. The name liveingite has been hauerite-2a (they identified this mineral as baumhauerite adopted for this mineral, as it doesnot have a superstruc- II) from Ag-free compositions at 280 oC and 200 atm. ture that is derived from the structure of rathite. It is an The results ofthe current study and those oflaroussi and ordered intergrowth of dufrenoysite and sartorite-type Moelo (1988)indicate that Ag appearsto be an essential units (Pring, 1990). Rathite IV, on the basis of cell pa- component of baumhaueite-2a. A recent find of baum- rameters, appearsto have a superstructurebased on the haueite-2a at Quiruvilca, Peru, however, suggeststhat structure of liveingite with a doubled. A mineral also this may not be so (Robinson and Harris, 1987).The Qui- identified as rathite IV, with a : 45.9 A (Salder et al., ruvilca material contains Tl rather than Ag, and although 1978) seemsto be identical to baumhaueite-2a. Grae- it may appear that the presenceof monovalent cations is ser's (1983) rathite 44 was examined during this study responsiblefor supercellformation, it is possiblethat su- and found to be baumhaueite-2a and compositionally at percell formation is due to ordering of Pb and As. the edgeof the baumhauerite field. The so-calledAg-rich The lattice images of intergrowths show that super- baumhauerite of I-aroussi and Moelo (1988) is also structure formation results from some form of chemical baumhauerite-2a. A baumhauerite-like mineral from 922 PRING ET AL.: BAUMHAUERITE-2a FROM LENGENBACH

- Lengenbach,Switzerland. Min- Quiruvilca, Peru, has recently been reported (Robinson (1977) Famous mineral localities: 275-281. and Harris, 1987).Electron diffraction studiesof this ma- eralogicalRecord, 8, - 0983) Die wissenschaftlicheBedeutung der Mineralfundstelle terial have shown it to be baumhaueite-2a, possibly in- kngenbach. Mineralienfreund, I 983, 8zt-87' tergrown with baumhauerite (Pring and Robinson, un- Laroussi, A., and Moelo, Y. (198E)Argent et thallium dans des sulfosels published data). de la s6rie de la sartorite (grisement de Lengenbach, Vall6e de Binn' Further studies of the Pb-As sulfide minerals are cur- Suisse).Bulletin de Min6ralogie (supplement),1 I l, 18. Makovicky, E. (1985) The building principles and classificationof sul- rently underway, and it is hoped that they will clarify the phosaltsbased on the SnS archetype.Fortschritte der Mineralogie, 63, mineralogicalrelations of this system. 45-89. Marumo. F., and Nowacki, W. (1965) The of rathite-I' AcrNowr-nncMENTs Zeitschrift fiir Kristallographie, 122, 433456. Nowacki, W. (1967) Uber die miigliche identitat von "Liveingit" mit We wish to thank ProfessorJ.M. Thomas and Dr. D.A. Jefferson,who Rathit-Il. NeuesJahrbuch Iiir Mineralogie Monatshefte,353-354. provided accessto the electron microscopefacilities at the Department Picot, P, and Johan, Z. (1982) Atlas of ore minerals, 458 p. Elsevier, of Physical Chemistry, University of Cambridge. The financial support Amsterdam. (for A.P.) was provided by the Australian ResearchGrants Committee. Pring, A. (1990) Disordered interglowths in lead arsenicsulfide minerals The comments of B.F. konard and an anonymousreviewer were greatly and the paragenesisof the sartonte group minerals. American Miner- appreciated alogist, 75, 289-294. Ribir, 8., Nicca, Ch., and Nowacki, W. (1969) DreidimensionaleVerfei- Rnrnnolqcns crrED nerung der Kristallstrukur von Dufrenoysit, PbrAs.Sro.Zeitschrift fiir Cabri, L, Criddle, A.J., La Flamme, J.H.G., Bearne,G.S., and Harris, D. Kristallographie, 130, 15-40. (1981) Mineralogical study of complex Pt-Fe nuggetsfrom Ethiopia' Robinson, G., and Harris, D. (1987) A baumhauerite-likemineral from Bulletin de Min6ralogie, 104, 508-525. Quiruvilca, Peru. The Mineralogical Record, 18, 199-201 Criddle,A.J., Stanley,C.J., Chisholm, J.E., and Fejer,E.E. (1983)Hen- Roesch,H., and Hellner, E. (1959) Hydrothermale Untersuchungenam Die Naturwissenschaften,46, 72. ryite, a new copper-silver telluride from Bisbee,Arizona. Bulletin de SystemPbS-AsrS'. Min€ralogie,106, 5l l-517 Scherzer,O. (1949) The theoreticalresolution limit ofthe electronmicro- Embrey, P.G., and Criddle, A.J. (1978) Enor problems in the two media scope Journal ofApplied Physics,20, 20-29. method of deriving the optical conslants n and k from measuredre- Solly, R. H. (1902)Baumhauerite, a new mineral; and Dufrenoysite.Min- flectances.American Mineralogist, 63, 853-862. eralogicalMagazine, I 3, I 51-171. (1978) Engel,P., and Nowacki, W. (1969) Die Ikistallstruktur von Baumhauerit. Stalder,H.A., Embrey,P., Graeser,S, and Nowacki,W. Die Mi- Zeitschrift fiir Kristallographie, 129, 178-202. neralien des Binntales.Naturhistorisches Museum der Stadt Bern. Graeser,S. (1965) Die Mineralfunde im Dolomit des Binnatales.Schwei- zerischeMineralogische und PetrographischeMitteilungen, 45, 597- MnNuscrrpt REcEIVEDOcrosen 23, 1989 796. Mnuuscnrrr AccEmEDAprur 19, 1990