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New Structural Data Reveal Benleonardite to Be a Member of the Pearceite-Polybasite Group

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New Structural Data Reveal Benleonardite to Be a Member of the Pearceite-Polybasite Group Mineralogical Magazine, October 2015, Vol. 79(5), pp. 1213–1221 New structural data reveal benleonardite to be a member of the pearceite-polybasite group 1,* 2 3 LUCA BINDI ,CHRISTOPHER J. STANLEY AND PAUL G. SPRY 1 Dipartimento di Scienze della Terra, Università di Firenze, Via G. La Pira 4, I-50121 Firenze, Italy 2 Natural History Museum, Cromwell Road, London SW7 5BD, UK 3 Department of Geological and Atmospheric Sciences, 253 Science I, Iowa State University, Ames, Iowa 50011-3212, USA [Received 4 January 2015; Accepted 19 February 2015; Associate Editor: A. Christy] ABSTRACT The determination of the crystal structure of benleonardite (P3m1; R = 0.0321 for 1250 reflections and 102 parameters; refined formula Ag15.00Cu1.00Sb1.58As0.42S7.03Te 3.97) obtained using data from a gem-quality, untwinned crystal recovered from the type material, revealed that benleonardite exhibits the structure observed 2– for minerals of the pearceite-polybasite group. The structure consists of the stacking of [Ag6(Sb,As)2S6Te] A 2+ and [Ag9Cu(S,Te)2Te 2] B layer modules in which (Sb,As) forms isolated SbS3 pyramids typically occurring in sulfosalts; Cu links two (S,Te) atoms with linear coordination, and Ag occupies sites with coordination geometries ranging from quasi-linear to almost triangular. The silver ions are found in the B layer module along two-dimensional diffusion paths and their electron densities are evidenced by means of a combination of a Gram- Charlier development of the atom displacement factors and a split model. In the structure, two S positions are completely replaced by Te (i.e. Te3 and Te4) and one is half occupied [S1: S0.514(9)Te 0.486], whereas S2 is completely filled by sulfur. This distribution reflects the crystal-chemical environments of the different cations. On the basis of information gained from this characterization, the crystal-chemical formula of benleonardite was revised according to the structural results, yielding Ag15Cu(Sb,As)2S7Te 4 (Z = 1) instead of Ag8(Sb,As)Te2S3 (Z = 2) as previously reported. Thus, the mineral must be considered a member of the pearceite-polybasite group. A recalculation of the chemical data listed in the scientific literature for benleonardite according to the structural results obtained here leads to excellent agreement. K EYWORDS: benleonardite, crystal structure, pearceite, polybasite, Ag-sulfosalt, Bambolla. Introduction with a = 6.603(5) and c = 12.726(6) Å, but the crystal structure remained unknown. Although BENLEONARDITE, ideally Ag8(Sb,As)Te2S3,was benleonardite-like minerals have been found in identified as a new mineral species by Stanley several occurrences [Zyranov gold deposit, Russia et al. (1986) by studying ore minerals collected (Aksenov et al., 1969), Ivigtut cryolite deposit, from the spoil tips of the abandoned Bambolla Greenland (Karup-Møller and Pauly, 1979), Gies mine, Moctezuma, Sonora (Mexico). It was gold–silver telluride deposit, Montana (Zhang and described as forming thin black powdery crusts Spry, 1994; Spry and Thieben, 1996), Mayflower (1–2 mm thick) with native silver, acanthite, gold–silver telluride deposit, Montana (Spry and hessite, cervelleite, pyrite and sphalerite. By Thieben, 1996), Um Samiuki Zn–Pb–Cu–Ag means of powder X-ray diffraction investigations, volcanogenic massive sulfide deposit, Egypt benleonardite was originally given as tetragonal (Helmy et al., 1999; Pals and Spry, 2003), black smoker chimney fragments from the Yaman Kasy massive sulfide deposit, southern Urals (Herrington * E-mail: [email protected] et al., 1998)], no additional structural investigations DOI: 10.1180/minmag.2015.079.5.15 have been reported in the literature to date. © 2015 The Mineralogical Society LUCA BINDI ET AL. TABLE 1. Details pertaining to the single-crystal X-ray data collection and structure refinement of benleonardite. Crystal data Space group P3 m1 Cell parameters a = 7.623(1) (Å) c = 12.708(1) (Å) V = 639.5(2) (Å3) Z 1 Crystal colour black Crystal shape block Crystal size (mm) 0.031 × 0.045 × 0.062 FIG. 1. Reflected plane-polarized light digital image in oil Data collection immersion illustrating a band of benleonardite on a ragged Diffractometer Oxford Diffraction Xcalibur 3 quartz grain to the right with rosettes of acanthite Radiation type MoKα (λ = 0.71073) intergrown with low reflecting, unidentified phases in a Monochromator oriented graphite (002) mass of creamy white to off-white hessite. The hessite Scan mode j/ω contains a vermiform or myrmekitic intergrowth of fine- Temperature (K) 293 grained cervelleite (pale-greenish-grey). The sample Detector to sample 5 (catalogue number E.1161 BM 1985, 354) is the type distance (cm) specimen for both benleonardite and cervelleite. Number of frames 598 Rotation width 0.15 per frame (°) The simplified formula for benleonardite was Measuring time (s) 90 θ given as Ag SbTe S by Stanley et al. (1986). Maximum covered 2 (°) 75.52 (d = 0.86 Å) 8 2 3 – ≤ ≤ – ≤ ≤ However, such a chemical formula is unbalanced, Range of h, k, l 11 h 11, 13 k 13, –20 ≤ l ≤ 20 which is a very unusual characteristic when dealing Collected reflections 11,025 with Ag sulfosalts (Bindi and Evain, 2007; Moëlo Rint before absorption 0.1054 et al., 2008). Moreover, chemical data reported for correction benleonardite (and benleonardite-like minerals) Rint after absorption 0.0355 in the literature show a general deficiency in correction Ag + Cu (i.e. <8 atoms per formula unit (a.p.f.u.)) coupledwithanexcessinS(i.e.3–4 a.p.f.u.) when Refinement 2 normalized on the basis of 14 atoms, thus reinforcing Refinement coefficient F the suggestion by Spry and Thieben (1996) that the No. of reflctions in 1250 formula proposed by Stanley et al. (1986) needs to be refinement No. of observed 609 modified. reflections Here we present the determination of the crystal No. of refined parameters 102 structure of benleonardite obtained using data from Weighting scheme w =1/ [σ2(I) + (0.044 × I)2 ] a gem-quality, untwinned benleonardite crystal R† (obs) / R† (all) 0.0179 / 0.0321 recovered from the type material. We show that wR2† (obs) / wR2† (all) 0.0167 / 0.0172 – benleonardite exhibits the structure observed for the Difference Fourier (e /Å3)[–1.94, 1.13] minerals of the pearceite-polybasite group (Bindi et al., 2006a,b, 2007a,b,c, 2013; Bindi and Menchetti, 2009; Evain et al., 2006a,b). † 2 2 2 2 R = Σ||Fo| – |Fc|| / Σ|Fo|; wR =[Σw(|Fo| – |Fc| ) /Σw(| 4 1/2 Fo| )] . The holotype A crystal for the X-ray investigation was selected from led by the late Sid Williams. It consists of black the type material (catalogue number E.1161 BM powdery crusts (1–2 mm thick) of benleonardite, 1985, 354). The specimen was collected by the late acanthite, hessite and cervelleite together with gangue Alan Criddle on a field excursion to the Sonora Desert quartz and dolomite (Fig. 1). The hessite contains a 1214 BENLEONARDITE – A MEMBER OF THE PEARCEITE-POLYBASITE GROUP TABLE 2. Wyckoff positions, site occupation factors, fractional atom coordinates, and equivalent isotropic displacement parameters (Å2) for the benleonardite crystal selected. Atom Wyckoff s.o.f. x/ay/bz/cUiso Sb 2d 0.789(4) 0.3333 0.6667 0.38572(2) 0.0251(7) As 2d 0.211 0.3333 0.6667 0.38572(2) 0.0251(7) Ag1 6i 0.677(7) 0.2848(3) 0.1424(3) 0.3804(1) 0.0526(3) Ag2 6i 0.311(7) 0.3422(3) 0.1711(3) 0.3537(3) 0.0700(8) Ag3 12j 0.307(5) 0.252(3) 0.3641(4) 0.1192(3) 0.0555(8) Ag4 12j 0.134(6) 0.382(3) 0.402(3) 0.1065(8) 0.0635(6) Ag5 12j 0.315(7) 0.3503(4) 0.2803(18) 0.1234(4) 0.0670(6) Cu 1a 1.000 0 0 0 0.0320(2) S1 2c 0.514(9) 0 0 0.1823(2) 0.0271(9) Te1 2c 0.486 0 0 0.1823(2) 0.0271(9) S2 6i 1.000 0.01458(5) 0.50729(5) 0.30681(8) 0.0404(2) Te3 2d 1.000 0.6667 0.3333 0.01650(3) 0.0475(1) Te4 1b 1.000 0 0 0.5 0.0422(1) s.o.f. – site occupancy factor. vermiform or myrmekitic intergrowth of fine-grained room-temperature data collection, which was carried cervelleite (pale greenish-grey in Fig. 1). The out on a Oxford Diffraction Xcalibur 3 diffractom- assemblage occupies irregular fractures in a highly eter, fitted with a Sapphire 2 CCD detector (see altered rock described by Williams (1982) as an Table 1 for details) using graphite-monochromatized intensely silicified rhyolitic vitrophyre. MoKα radiation (λ = 0.71069 Å). Because of the typical ionic conductivity observed in Ag sulfosalts and the probable presence of twinning (Bindi et al., X-ray crystallography 2006a), a rather high sinθ/λ cutoff and a full sphere of data were considered. Intensity integration and A benleonardite crystal was hand-picked from a standard Lorentz-polarization correction were per- polished section of the type material and used for the formed with the CrysAlis RED (Oxford Diffraction, 2 TABLE 3. Anisotropic displacement parameters (Å ) for the benleonardite crystal selected. Atom U11 U22 U33 U12 U13 U23 Sb 0.0235(9) 0.0235(9) 0.037(1) 0.0118(5) 0 0 As 0.0235(9) 0.0235(9) 0.037(1) 0.0118(5) 0 0 Ag1 0.0542(5) 0.0502(2) 0.0547(6) 0.0271(3) –0.0002(3) –0.0001(1) Ag2 0.093(1) 0.0447(5) 0.089(1) 0.0464(6) 0.042(2) 0.0208(8) Ag3 0.26(2) 0.0381(7) 0.0516(9) –0.016(3) –0.035(3) 0.0024(4) Ag4 0.169(7) 0.240(9) 0.085(4) 0.170(8) 0.073(5) 0.078(6) Ag5 0.0356(8) 0.29(1) 0.050(1) –0.016(2) 0.0068(5) –0.009(2) Cu 0.0388(2) 0.0388(2) 0.0186(3) 0.0194(1) 0 0 S1 0.038(3) 0.038(3) 0.006(2) 0.019(1) 0 0 Te1 0.038(3) 0.038(3) 0.006(2) 0.019(1) 0 0 S2 0.0227(3) 0.0418(3) 0.0503(4) 0.0113(1) –0.0031(2) –0.0015(1) Te3 0.0454(1) 0.0454(1) 0.0519(2) 0.02269(7) 0 0 Te4 0.0424(2) 0.0424(2) 0.0417(3) 0.02120(8) 0 0 1215 LUCA BINDI ET AL.
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