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The Crystal Chemistry of Duftite, Pbcuaso4(OH)

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The Crystal Chemistry of Duftite, Pbcuaso4(OH) 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
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