Mineralogical Magazine, February 1998, Vol. 62(1), pp. 121–130

The crystalchemistry of duftite,

PbCuAsO4(OH) and the b-duftiteproblem

KHARISUN*, MAX R. TAYLOR, D. J. M BEVAN Department of Chemistry, The Flinders University of South Australia, GPOBox 2100 Adelaide, S.A.5001, Australia

AND

ALLAN PRING Department of Mineralogy, South Australian Museum, North Terrace, S.A.5000, Australia

ABSTRACT

Duftite,PbCu(AsO 4)(OH)is orthorhombic, space group P212121 with a = 7.768(1), b = 9. 211(1), c = 5.999(1) AÊ ,Z=4;the structure has been refined to R =4.6 % and Rw = 6.5% using640 observed reflections[F> 2 s(F)].The structure consists of chainsof edge-sharingCuO 6 ‘octahedra’,parallelto c; whichare linked via AsO 4 tetrahedraand Pb atoms in distorted square antiprismatic co-ordination to forma threedimensional network. The CuO 6 ‘octahedra’ showJahn-Teller distortion with the elongationrunning approximately along <627>. The hydrogen bonding network in the structure was characterizedusing bond valence calculations. ‘b-duftite’ isan intermediate in the duftite– conichalcite series,which has a modulatedstructure based on the intergrowth of the two structures in domains of approximately50 A Ê .Theorigin of the modulation is thought to beassociatedwith displacements in the oxygenlattice and is related to the orientation of the Jahn-Teller distortion of CuO 6 ‘octahedra’. Approximatelyhalf of the strips show an elongation parallel to <627> while the other strips are elongatedparallel to [010 ].This ordering results in an increase in the b cellrepeat compared to duftite andconichalcite.

KEY WORDS: duftite,conichalcite, crystal structure, crystal chemistry, modulated structure.

hadearlier tentatively proposed the point group Introduction 222,on the basis of morphological observations. Guillemin(1956) identified two distinct poly- DUFTITE,aleadc uprich ydroxyarsenate, morphsof duftite on specimens from Tsumeb, PbCu(AsO4)(OH)was originally described by whichhe denoted as ‘a-dufite’ withthe unit cell a Pufahl(1920) from the Tsumeb mine, in what is = 7.81, b = 9.19, c = 6.08 AÊ and ‘b-duftite’ which nowNamibia. The mineral occurs in some hasthe slightly different unit cell a = 7.49, b = abundancewith other lead and copper arsenates, 9.36, c = 5.91 AÊ .Heproposed that ‘a-dufite’ is calciteand azurite throughout the oxidized zones isomorphouswith descloizite, PbZn(VO 4)(OH), ofthiscomplex deposit (Keller, 1977). Richmond andhas the space group Pnam while ‘b-duftite’ is (1940)determined the unit cell parameters for isomorphouswithconichalcite, duftite as ao = 7.50 bo = 9.12 co =5.90kX butwas CaCu(AsO4)(OH),which has the space group unableto assign the space group; Pufahl (1920) P212121.Hischemical analyses suggested that the a-formis almost pure PbCu(AsO 4)(OH) while the b-formhas approximately 20 % Casubstituted *Permanent address: Fakultas Pertanian, Universitas forPb. While Guillemin (1956) was able to Djenderal Soedirman, P.O.Box 125, Purwokerto 53123, prepare the a-formsynthetically his attempts to Jawa-Tengah, Indonesia prepare the b-formwere unsucces sful.The

# 1998The Mineralogical Society KHARISUN ETAL.

duftitesare members of the adelite group which wereproduced. Similar methods were employed includes,adelite, CaMn(AsO 4)(OH),conichalcite, inattempts to synthesize b-duftite;in these CaCu(AsO4)(OH),austinite, CaZn(AsO 4)(OH), experimentsbetween 5 and30 mol. % of the Pb andgabrielsonite, PbFe(AsO 4)(OH),and exten- wasreplaced by Ca. Calcium was added to the sivesolid solution fields have been reported reactionmixture in the form of CaCO 3. Despite betweenmembers of this series (Radcliffe and numerousexperiments over a 2yearperiod, Simmons,1971; Jambor et al.,1980;Taggart and wherethe conditions were systematically varied, Foord,1980). Jambor et al. (1980)examined a b-duftiteo rcalcianduftitec ouldnotbe seriesof calcianduftite and plumbian conichalcite synthesized. specimensfrom Tsumeb and concluded that Syntheticproducts and natural samples were completesolid solution exists between the Ca identifiedusingpowd erX -raydiffraction andPb end-members and that b-duftiteis not a methods.Diffractio npatternswere recorded distinctpolymorph but rather a calcianduftite. usingan 80 mm, Ha¨gg-Guiniercamera with Giventhe space group assignments of Guillemin monochromatedCu- Ka1 (l = 1.54051 AÊ ) radia- (1956)it appearedthat there would be a changein tion,with elemental Si asan internal standard. symmetryat some point along the duftite– Preliminaryidentification was made by referring conichalcitecompositional join. tothe JCPDS powderdiffraction file or Guillemin Sokolova et al.,(1982)reported the crystal (1956).Subsequently, the patterns were fully structureof duftite in space group P212121 but indexedand the unit cell parameters of the wereonly able to achieve an R factorof 12.2 %. mineralswere refined by least squares methods. Effenbergerand Pertlik (1988), in a conference Forexamination in the transmission electron abstract,reported a refinementof synthetic duftite microscope,crystals or crystal fragments were inspace group P212121 butapparently did not groundunder an organic solvent in an agate publishfull details. The crystal structure of mortarand dispersed on Cu grids coated with conichalcitewas determined by Qurashi and holeycarbon support films. The fragments were Barnes(1963). In order to establish the relation- examinedin a PhilipsCM200 transmiss ion shipsbetween duftite and conichalcite and to electronmicroscope. clarifythe nature of the calcium/ leadsubstitution Chemicalanalyses of natural ’b-duftite’ were inthis series, and the nature of b-duftite,we performedusing a JEOL electronprobe micro- undertooka crystalstructure refinement of duftite analyserwith a wavelengthdispersive analysis andan examination of ‘b-duftite’ by powder X- system,operating under the following conditions: raydiffraction methods and electron diffraction. acceleratingvoltage, 15 kV, beam current, 20 nA, Inthis paper we will use the name duftite to countingtime 10 s onthe peak, 5 sforthe representGuillemin ’s ‘a-dufite’ and use ‘b- background.The standar dsemployed were: duftite’ forthe Ca bearing duftite-like mineral galena(Pb ),hematite(Fe), fluorapat ite(P), whereappropriate. arsenopyrite(As), wollastonite ( Ca),sphalerite (Zn,S), Cu metal (Cu). Experimental Refinementof the structureof duftite Anumberof duftiteand b-duftitespecimens from BrokenHill, N.S.W, MountBonnie, Northern Examinationof synthetic duftite crystals by Territory,and Tsumeb, Namibia, were examined petrographicmicroscope revealed that they were butno crystals suitable for single crystal structure suitablefor the single-crystal X-ray analysis; they analysiscould be found. Attempts were made to showedclear and sharp extinction. Precession synthesizeboth duftite and b-duftite. photographsconfirmed orthorhombic symmetry Solutionswere prepared following the method andthe systematic absences were h00, h = 2n + 1; ofGuillemin(1956). Aqueous solutions of 0.01 M 0k0, k = 2n + 1; 00l, l = 2n +1,uniquelydefining Cu(CH3CO2)2 H2O, 0.01 M Pb(CH3CO2)2.3H2O thespace group P212121.Adataset was collected and 0.01 M Na2H(AsO4) 7H2O,weremixed and onan Enraf-Nonius CAD4 turbodiffractometer at 0.1 M HClO4 wasadded to adjust the pH to 3. theUniversity of New SouthWales. The unit cell Whenthe reaction mixture was heated in teflon- dimensionswere obtained by aleastsquares fit of linedhydrothermal bombs at 200 8Cand15.3 atm thesetting angles for 25 selected and centred for120 hours well-formed single crystals of reflections.Intensities of 1554 reflections were duftiteof dimensions 0.1 6 0.05 6 0.03 mm collectedwith graphite-monochroma tizedMo- Ka

122 CRYSTALCHEMISTRYOFDU FTITE

radiation (l = 0.7107 AÊ ) using a y/2y scan. Data TABLE 1.Crystal data for duftite forthe crystal, the intensity data collection and structurerefinement are summarized in Table 1. Formula PbCu(AsO4)(OH) Calculationsrelating to the structure solution and Molecular Weight 426.7 refinementwere done with the XTAL 3.2 Space group P212121 programsystem (Hall et al.,1992).An absorption a(AÊ ) 7.768(1) correctioncalculated from the crystal shape b(AÊ ) 9.211(1) (Gaussianquadrature) was applied to the data c(AÊ ) 5.999(1) V(AÊ 3) 429.2 setand the reflections were sorted and merged to ­ 3 averagesymmetrically equivalent reflections. Dcalc(g cm ) 6.602 Initialattempts to refine the structure starting Z 4 m(mm ­ 1) 51.64 withthe coordinates from Sokolova et al., (1982), Ê l (Mo Ka)(A) 0.7101 werenot satisfactory, converging with R =0.15. Crystal dimensions 0.1 6 0.05 6 0.03 mm Theheavy atom method was then employed to Tmin, Tmax 0.006, 0.112 solvethe structure in the space group P212121 fromthe intensity data. A Pattersonmap was Data Collection: calculatedto identify the position of thePb atom. Diffractometer Enraf-Nonius CAD4 Thepositions of the next heavy atoms, As and Cu, Turbo werethen obtained from a differenceFourier and ymax (8) 30 subsequentdifference maps revealed the positions h ­ 10 ? 10 k 0 12 ofthe O atoms. ? l ­ 8 ? 8 Thepositional parameters for the atoms were Total reflections measured 1554 refinedwith isotropic displacement parameters; Number after averaging 680 thisgave an R =0.07.Further refinement with the Rfor averaging 0.084 displacementparameters for metal atoms allowed tovary anisotropically gave R =0.046. Refinement: Refinement on F 2 Weight 1/s (hkl) Descriptionofthe structureof duftite R 0.046 Thefinal atomic coordinates and displacement Rw 0.065 Reflections used in parametersfrom the refinement are given in refinement (F>2 s) 640 Table2 andselected interatomic distance and Number of parameters bondangles are provided in Table 3. A summary refined 48 ofthe bond valence calculations for the duftite Hatoms not located structureis given in Table 4. The final list of Goodness of fit S 1.76(5) observedand calculated structure factor ampli- (Ds)max 0.002 Ê ­ 3 tudesis given in Table 5 (depositedwith the Dqmax, Dqmin (e A ) ­ 2.66, 3.25 Editor). Thestructure of duftiteconsists of edgesharing chainsof Cu(O,OH) 6 ‘octahedra’ runningparallel to c .TheCu(O,OH) 6 chainsare linked by AsO 4 [627¯],that is atanangleof about71 8 away from b tetrahedrawhich share three corners with the towards c.(Fig.1). Similar Jahn-Teller distortions chainsof octahedra and thus form a three- ofCu co-ordination have been observed in a dimensionalnetwork. Lead atoms occupy square numberof other copper minerals including: antiprismaticcavities in the network (Fig. 1). malachite,lindgrenite and linarite (Table 6). The ‘octahedral’ co-ordinationof Cuby Oand Induftite the Pb is coordinated by eightoxygen OHshowsa characteristicJahn-Teller distortion atomsarranged at thecorners of adistortedsquare around Cu2+.Theinteratomic distances of theCu- antiprism.The range of Pb ­ Obondlengths is octahedronfallinto two groups: four short 2.50­ 2.81 AÊ withan average of 2.66 A Ê . Within equatorialbonds,ranging between 1.92 and each AsO4 tetrahedronthere are three short and 2.13 AÊ andtwo long apical bonds, 2.31 and one long As­ O bonds. 2.38 AÊ .Theelongation of each octahedron lies Thebond valence sums (BVS) forO(1) and approximatelyparallel to [627 ]andthe three O(5)are low, being 1.40 and 1.64 valence unit symmetricallyrelated directions [6 ¯27], [62¯7] and (vu)respectivelywhile those of theother O atoms

123 KHARISUN ETAL.

TABLE 2.Atomic coordinates and displacement parameters for duftite

x y z

Pb 0.3764(1) 0.1668(1) 0.4793(2) Cu 0.2556(4) 0.4963(4) 0.7537(4) As 0.3686(2) ­ 0.1934(2) 0.5076(4) O(1) 0.385(2) 0.438(1) 0.497(3) O(2) 0.142(2) 0.285(2) 0.765(3) O(3) 0.126(2) 0.305(2) 0.241(3) O(4) 0.541(2) ­ 0.085(2) 0.494(3) O(5) 0.197(2) ­ 0.085(2) 0.538(3)

U11 U22 U33 U12 U13 U23

Pb 0.0102(4) 0.0091(4) 0.0142(4) 0.0018(3) ­ 0.0002(3) 0.0002(3) Cu 0.014(1) 0.0071(9) 0.002(1) ­ 0.0006(8) ­ 0.001(1) ­ 0.0032(9) As 0.0078(8) 0.0047(7) 0.007(1) ­ 0.0004(6) 0.0020(9) ­ 0.0003(7) O(1) 0.008(2) O(2) 0.017(3) O(3) 0.012(3) O(4) 0.012(3) O(5) 0.020(3)

TABLE 3.Selectedbond lengths and angles for duftite

Pb ­ O Cu ­ O As ­ O

O(1) 2.50(1) O(1) 1.92(1) O(2)ii 1.68(2) O(2) 2.73(2) O(1)iv 1.93(1) O(3)iii 1.74(2) O(2)i 2.57(2) O(2) 2.13(2) O(4) 1.67(1) O(3) 2.73(2) O(3) 2.05(2) O(5) 1.67(2) O(3)i 2.58(2) O(4)v 2.31(1) O(4) 2.65(1) O(4)vi 2.38(1) O(5) 2.73(2) O(5)ii 2.81(2)

O(5)­ As­ O(3) 106.508 O(4)­ As­ O(5) 106.658 O(4)­ As­ O(2) 107.228 O(5)­ As­ O(2) 110.718 O(4)­ As­ O(3) 111.918 O(2)­ As­ O(3) 113.658

Symmetry —equivalent positions. (i) 1/2+x, 1/2 ­ y, 1­ z (ii) 1/2 ­ x, ­ y, ­ 1/2 ­ z (iii) 1/2 ­ x, ­ y, 1/2 ­ z (iv) 1/2 ­ x, 1 ­ y, 1/2 ­ z (v) 1/2 ­ x, 1/2+y, 1/2 ­ z (vi) ­ 1/2 + x, 1/2 ­ y, 1­ z

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TABLE 4.Summary of bond-valence analysis for duftite

Pb Cu As SUM

O(1) 0.36 0.52 1.40 0.52 O(2) 0.30 0.29 1.25 2.03 0.19 O(3) 0.28 0.37 1.08 1.92 0.19 O(4) 0.23 0.18 1.27 1.97 0.16 O(5) 0.18 1.30 1.64 0.16 SUM 1.89 2.04 4.90

areclose to 2.00 vu (seeTable 4). Stereochemical andBVS considerationssuggest that O(1) is an OHgroup.It is the only oxygen atom that is coordinatedby Pb and two Cu cations but not As andhas the lowest BVS. Inthe PbO 8 polyhedra IG and CuO6 octahedra,Pb ­ O(1) and Cu­ O(1) are F .1Schematic diagram of the duftite structure down theshortest metal– oxygen distances. By assuming [100] showing the polyhedral linkages. The Pb atoms an average O­ Hbondlength of 0.98 A Ê for the are shown as open circles. The arrows indicate the direction of elongation of the CuO 6 octahedra. O(1)­ Hcontactand using the standard r o value forH bondedto O(0.882) (Brownand Attermat, 1985)the bond valence contribution is 0.76 vu andthe sum for O(1) is then 2.16 vu. The H bond space group Pnam,canbe considered as the mustthen be to O(5), as the second lowest BVS. archetypeor parentstructure for the group. In the The O(1)­ O(5)distance is 2.80 A Ê and by parentstructure all atoms, except one of the O assuminglinearity of the linkage O(1) ­ H_O(5) ionsoccupy special positions on a mirrorat z = and the O­ Hbondlength 0.98 A Ê then the 0.25.The cation distributi onin duftite and O(5)_Hcontacthas a lengthof 1.82 A Ê . This wouldgive a bondvalence contribution of 0.08 vu bringingthe bond valence sum for O5 to 1.72 vu. Thisresult is consistent with the correlation of O ­ H_Obondingreportedby Brown and Altermatt(1985), however a slighlylonger O(1)­ Hbondlength and a shorterH _O(5) contactwould give bond valence sums for O(1) andO(5) closer to 2. The position of the H bondingin the structure is shown in Fig. 2.

Comparisonof duftite and conichalci te structures Asnoted in the introduction duftite is a member oftheadelite structural group, and is also related FIG.2Schematic diagram of the duftite structure down toa numberof vanadates including descloizite [001] showing the polyhedral linkages and the proposed (Hawthorneand Faggiani, 1979) and cechite hydrogen bonding network. The Pb atoms are shown as (Pertlik,1989). The descloizite structure, in open circles.

125 KHARISUN ETAL.

TABLE 6.Jahn-Tellerdistorted CuO 6 octahedrain copper oxysalt minerals

Mineral Equatorial Apical Ref duftite PbCu(AsO4)(OH) 1.92 1.93 2.13 2.05 2.31 2.38 1 conichalcite CaCu(AsO 4)(OH) 1.95 1.95 2.04 2.09 2.38 2.28 2 malachite Cu 2(CO3)(OH)2 2.00 2.05 1.90 1.91 2.51 2.64 3 lindgrenite Cu 3(MoO4)2(OH)2 1.96 1.93 1.98 2.00 2.30 2.47 4 " " 1.95 1.95 1.98 1.98 2.42 2.42 linarite PbCu(SO4)(OH)2 1.93 1.93 1.98 1.98 2.54 2.54 5 schmiederite Pb 2Cu2(OH)4(SeO3)(SeO4)1.95 1.95 1.97 1.98 2.57 2.63 5

(1) this work; (2) Qurashi and Barnes, 1963; (3) Zigan et al.,1977; (4) Hawthorne and Eby, 1985; (5) Effenberger, 1987

conichalciteclosely approximate special positions isdisplaced along the b axisand O(5) along a in Pnam,howeverseveral of the O ionsare directionapproximately 25 8 to the c axis. These significantlydisplaced from the mirror plane displacementsresult in a lessdistorted co- requiringthe lowering of symmetry to P212121. ordinationsphere than the Pb(O,OH) 8 square Thusduftite can be considered as a displacively antiprism. modulatedsuperstructure of the Pnam parent in whichthe primary modulation vector qprim = 0 The natureof b-duftite (seePerez-Mato et al.,1987;Withers, 1989; Pring et al.,1993for discussion of thenomenclature of Guillemin(1956) identified b-duftiteas a distinct modulatedstructures). polymorphof duftite on anumberspecimens from Because b-duftiteis compositiona llyinter- Tsumeb,later Jambor et al.(1980)suggested that mediatebetween duftite and conicha lcitea itwas merely an intermediatein the solid solution detailedcomparison of these two structures is seriesbetween duftite and conichalcite. mostapprop riate.Th ecrystalstructureof Anapparently well-crystallized specimen of b- conichalcite,CaCu(AsO 4)(OH)was reported by duftitefrom Tsumeb, Namibia, from the Museum Qurashiand Barnes (1963). The space group is ofthe Institute for Mineralogy and Petrology, P212121,thesame as duftite, but the unit cell is Universityof Hamburg (MinMusHH-83/ 5),was slightlysmaller a = 7.393, b = 9.220 c = 5.830 AÊ located.On this specimen the b-duftiteappeared V = 397.4 AÊ 3 (Radcliffeand Simmons, 1971). tobe well formed transparent single crystals up to Thisdifference reflects the difference in the size 0.5mm inlength, however precession photo- ofPband Ca cations. It too can be consideredas a graphsrevealed that the ‘singlecrystals ’ were displacivelymodulated superstruc tureof the disordered.Electron probe microana lysisof Pnam parentin which the primary modulation crystalsfrom this specimen showed that the vector qprim =0.The oxygen co-ordination around Pb:Cara tiowas 59: 41(Tab le7 )andthe Asin conichalcite is more distorted than it is in simplifiedformula, based on 5 oxygenatoms, is duftiteand the AsO 4 tetrahedraare rotated 10 8 (Pb0.59Ca0.41)(Cu0.89Zn0.11)(AsO4)(OH).This is anti-clockwiseabout a.TheCu co-ordination in somewhatmoreCa -richthan the b-duftite bothstructures shows classic Jahn-Teller distor- analysedfrom the same locality by Guillemin tion.However, there is a distinctdifference (1956)(i.e Pb:Ca = 76:24). betweenthe direction of the elongation of the Theleast-squares refinement of the b-duftite Cu(O,OH)6 octahedra;in conichalcite it runs powderX-ray diffraction data on the orthorhombic along the b axiswhereas in duftite the octahedra cell gave a =7.4870(5), b =9.4451(5), c = areelongated approximately along <627>. 5.9271(5) AÊ andV =419.14A Ê 3. Thus a and c cell Theco-ordination Ca(O,OH) 8 inconichalcite is repeats of b-duftiteare shorter than those of duftite verysimilar to Pb(O,OH) 8 induftite. However andlonger than those of conichalcite; however the thereare substantial shifts in thepositions of two b cellrepeat is significantly longer than those of oftheco-ordinating O atomsin conichalcite; O(3) end-memberduftite or conichalcite (Table 8 ).

126 CRYSTALCHEMISTRYOFDU FTITE

TABLE 7.Electron microprobe analyses of b-duftite [100]zoneelectron diffraction patterns. Note the distributionof intensity in the Bragg reflections. The[100 ]zonepattern shows 0 kl, k + l = 2n while OxideRange (wt. %)Average(wt. %)* Atoms the[010 ]zoneshows h00, h = 2n; 00l, l = 2n and As+P = 1 h0l, h =2n,theset of systematic absences which PbO 33.77 ­ 38.23 35.68 0.588 correspondsto space group Pnam inthe setting CaO 5.41 ­ 7.36 6.17 0.405 usedhere. It was noted above that the cation CuO 20.07 ­ 22.16 21.52 0.997 distributionin duftite and conichalcite closely ZnO 2.26 ­ 2.93 2.64 0.120 approximatespecial positions in Pnam while the As2O5 30.12 ­ 31.41 30.82 0.987 oxygenatomsare displa civelymod ulated, P2O5 0.14 ­ 0.55 0.25 0.013 loweringthe symmetry to P212121. In b-duftite H2O not0. determined themodulation in no longer commensurate. In additionto the Bragg reflections G,superlattice *Mean of 10 analyses. reflectionsa rea lsopr esent,but they are distributedonly around the reflections which are forbiddenby the n and a glidesin the parent Thereis only a smalldeviation from a linear structure.On the [010 ]zone(Fig. 4 a) faint trendbetween the Ca and Pb end members for a- superlatticereflections occur at h0l, h = 2n + 1 prim and c-repeats,but a substantialdeparture in the witha modulationvector G + q & 3/13 d001* case of the b-repeats(Fig. 3). Overall, however, correspondingto a realspace wavelength of ~ thecell volume shows no deviation from a linear 25.7 AÊ .Inthiscase the superlattice reflections are trend.The small deviation in a and c may be due veryweak and diffuse. In the [100] zone (Fig b) tothe partial substitution of Zn for Cu in some pairsof superlattice reflections are distributed compositions.There is a substantialdeviation in around the 0kl, k + l = 2n +1reflections with a prim the b-cellrepeat; it increases significantly at modulationvector G + q & 3/26 d001* which compositionsaround the mid point in the series. correspondsto a realspace wavelength of 51.4A Ê . Jambor et al.(1980)noted a similar,though not as Herethe superlattice reflections are relatively pronouncedtrend in their investigations which strongand sharp and are in factslightly canted off theyattributed to compositional zoning. Evidence [001],the actual direction being close to [013]. forcompositional zoning in the specimen used in Theorigin of the structural modulation super- thecurrent study was sought by electron probe latticesis a questionof some importa nce. microanalysisbut the grains were found to be Structuralmodulations can be eithercompositional homogeneousat the resolution offered by the ordisplacive in origin, and in the case of gross electronmicroprobe (> 1 mm). compositionalmodulation, there is usually an Electrondiffraction patterns of b-duftiteshow associateddisplacive component. Withers (1989) evidenceof an incommensur atelymodulated suggestedthat it ispossible to distinguishbetween superstructure.Figure 4 showsthe [010] and compositionaland displacively modulations on the

TABLE 8.Cell parameters of members of theduftite– conichalcite series*

Pb:Ca Cu:Zn a (AÊ ) b (AÊ ) c (AÊ ) V (AÊ 3) Ref

100:0 100:0 7.768 9.211 5.999 429.2 1 100:0 100:0 7.778 9.207 6.000 429.7 2 93:7 96:4 7.716 9.178 5.954 421.6 2 59:41 89:11 7.487 9.445 5.927 419.1 1 41:59 90:10 7.446 9.255 5.881 405.3 2 40:60 89:11 7.461 9.202 5.864 402.6 2 20:80 69:31 7.465 9.185 5.892 404.0 2 0:100 100:0 7.393 9.220 5.830 397.4 3

(1) this work (2) Jambor et al.(1980) (3) Radcliffe and Simmons (1971) *cell data have been reset to acommon orientation

127 KHARISUN ETAL.

basisof the intensity distribution in the superlattice reflections.Thus, in the case of compositional modulation,the satellite reflections are strongest nearthe centre of the diffraction pattern, whereas fora displacivemodulation, the intensity of the satellitereflections initially increase as | G* + mq| increasesuntil the contribution of thetemperature factorcomponent of intensity dominates, and the intensitydecays. The intensity distribution in the satellitereflections in Fig. 4 b,theclearer of the diffractionpatterns, suggests to us a displacive originfor the modulation. Unfortunatelyit is not possible to resolve the natureof the modulated structure in b-duftite unequivocallyon the basis of these electron diffractionpatterns. However comparison of the structuraldata suggests a possibleorigin. The distinctdeviation of the b-cell,at the mid-member composition,is linked to changes in the orienta- tionof the tetragonal distortion of the CuO 6 octahedron.As noted above, the orientation of the Jahn-Tellerelongation of theCuO 6 octahedronin duftiteis approximately 71 8 from the c axis whereasin conichalcite the orientation of the Jahn-Tellerdistortion is parallel to the b axis. This differencein orientation results in a substantial differencein the length of the octahedra parallel to the b axis(duftite, 4.18A Ê ;conichalcite,4.66A Ê ). Thesubstitution of 41 % Ca for Pb in b-duftite resultsin approximately one half of thechains of octahedrabeing composed of octahedra with Jahn-Tellerelongation of conichalcite-type and onehalf with octahedra of duftite-type. The fact thatthe superlattice reflections occur only about Pnam forbiddenreflections suggests that the Pb andCa are not ordered on a supercell.If metal distributionwere also modulated one would expectsuperlattice reflections around reflections with 0kl, k+l = 2n and h0l, h = 2n.Itseemsmost likelythat the modulation is linked to displace- mentsof the oxygen atoms in b-duftite.The size ofthe domains could be expected to vary greatly withsmall changes in composition as they do in theplagioclase feldspars (Putnis, 1992). Clearly b-duftiteis nota distinctmineral but is acompositionor a seriesof compositions in the duftite–conichalcite series. It is also evident that FIG. 3. (a)Comparison of the trends of the three unit cell theduftite– conichalcite series is notasimplesolid repeats for members in the duftite–conichalcite series. solutionbut is rather more complex. A detailed (b)plot of composition against cell volume for members studyof other compositions in the series is of the duftite–conichalcite series. requiredbefore a detailedunderstanding of the seriescan be obtained. It is also clear, given the modulatedstructure of the b-duftite,why synth- esiswas not possible.

128 CRYSTALCHEMISTRYOFDU FTITE

FIG.4. Electron diffraction patterns of b-duftite showing an apparently incommensurate superlattice. ( a, left) The prim [010] zone with faint superlattice reflections at h0l, h = 2n +1with amodulation vector G + q & 3/13 d001*. (b, right) The [100] zone showing pairs of superlattice reflections are distributed around the 0 kl, k + l = 2n + 1 prim reflections with amodulation vector G + q & 3/26 d001.

Acknowledgements Pb(Fe,Mn)(VO 4 )(OH)=cechiteand Wewish to thank AusAID for the award of a PbCu(AsO4)(OH) =duftite. Z.Kristallogr., 185, 610 (abstract). scholarshipto one of us (K) andMr D. Craigof Guillemin, C.(1956) Contribution ala mine´ralogie des theUniversity of New SouthWales for assistance arse´niates, phosphates et vanadates de cuivre. 1 withthe single crystal data collection. Dr Jochum Arse´niates de cuivre. Bull. Soc. fr. Mine´ral. Schluter,Institute of Mineralogy, University of Cristallogr. , 79, 7­ 95. Hamburgsupplied the sample of -duftiteand Dr b Hall, S.,R., Flack, H.D.and Stewart, J.M. (Eds) (1992). W.D.Birch, Museum of Victoria undertook the Xtal3.2 Reference Manual. University of Western electronmicroprobe analyses. We also would like Australia, Perth, Australia. tothankthe anonymous reviewer who pointed out Hawthorne, F.C.and Eby, R.K.(1985) Refinement of anumberof errors in an earlier version of the the crystal structure of lindgrenite, Neues Jahrb. manuscript. Mineral. Mh. , 234­ 40. Hawthorne, F.C.and Faggiani, R.(1979) Refinement of References the structure of descloizite. Acta Crystallogr., B35, 717­ 20. Jambor, J.L., Owens, D.R.and Dutrizac, J.E. (1980) Brown, I.D. and Altermatt, D.(1985) Bond valence Solid solution in the adelite group of arsenates. parameters obtained from asystematic analysis of Canad. Mineral., 18, 191­ 5. the inorganic crystal structure database. Acta Keller, P.(1977) Tsumeb, VIParagenesis, Miner. Crystallogr., B41, 241 ­ 7. Record, 8, 36­ 47. Effenberger, H.(1987) Crystal structure and chemical Perez-Mato, G., Madariaga, G., Zun~iga, F.J. and Garcia formula of schmiederite, Pb 2Cu2(OH)4(SeO3)(SeO4), Arribas, S.A.(1987) On the structure and symmetry with acomparison to linarite, PbCu(OH) 2(SO4). of incommensurate Phases. apractical formulation. Mineral. Petrol ., 36, 3­ 12. Acta Crystallogr ., A43, 216 ­ 26. Effenberger, H.and Pertlik, F.(1988) Comparison of the Pertlik, F.(1989) The crystal structure of cechite, 2+ 2+ homeomorphiccrystalstructuresof Pb(Fe ,Mn )(VO4)(OH) with Fe>Mn. Amineral

129 KHARISUN ETAL.

of the descloizite group. Neues Jahrb. Mineral. Mh., phosphates, arsenates and vanadates of the type 34­ 40. A2XO4(Z) Amer. Mineral., 25, 441­ 78. Pring, A., Williams, T.B.and Withers R.L.(1993) Sokolova, E.V.,Egorov-Tismenko, Y.K.and Structural modulation in sartorite: an electron Yakhonoptova P.K.(1982) Research on crystal microscope study. Amer. Mineral. , 78, 619­ 26. structure of rare arsenates: duftite and mimetite Pufahl, O.(1920) Mitteilungen u¨ber Mineralien und Vest. Mosk Univ. Ser. , 4, 50 ­ 6(Russian). Erze von Su¨dwestafrika, besonders solche von Taggart, J.E. and Foord, E.E.(1980) Conichalcite, Tsumeb. Centralblatt fu¨rMineralogie, Geologie, cuprian austinite, and plumbian conichalcite from und Pala¨ontologie, 289­ 96. La Plata County, Colorado. Mineral. Record, 11, Putnis A.(1992) Introduction to Mineral Sciences. 37 ­ 8. Cambridge University Press, 434 ­ 7. Withers, R.L.(1989) The characterization of modulated Qurashi, M.M.and Barnes, W.H. (1963) The structures structures via their diffraction patterns. Prog. Crystal of the minerals of the descloizite and adelite groups: Growth Character ., 18, 139­ 204. IV-Descloizite and conichalcite (Part 2) the Zigan, F., Joswig, W.,Schuster, H.D.and Mason, S.A. structure of conichalcite. Canad. Mineral., 7, (1977) Verfeinerung der structure von malachit, 561 ­ 77. Cu2(OH)2CO3 durch Neutronenbeugung. Z. Radcliffe, D.and Simmons, W.B. Jr (1971) Austinite: Kristallogr., 145, 412­ 26. chemistry and physical properties in relation to chonichalcite. Amer. Mineral ., 56, 1359 ­ 65. [Manuscript received 6January: Richmond, W.E.(1940) Crystal chemistry of the revised 27 February 1997 ]

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