American Mineralogist, Volume 92, pages 918–925, 2007 The pearceite-polybasite group of minerals: Crystal chemistry and new nomenclature rules LUCA BINDI,1,* MICHEL EVAIN,2 PAUL G. SPRY,3 AND SILVIO MENCHETTI1 1Dipartimento di Scienze della Terra, Università di Firenze, Via La Pira 4, I-50121 Firenze, Italy 2Laboratoire de Chimie des Solides, IMN, UMR C6502 CNRS, Université de Nantes, 2 rue de la Houssinière, BP32229, 44322 Nantes CEDEX 3, France 3Department of Geological and Atmospheric Sciences, 253 Science I, Iowa State University, Ames, Iowa 50011–3210, U.S.A. ABSTRACT The present paper reports changes to the existing nomenclature for minerals belonging to the pearceite-polybasite group. Thirty-one samples of minerals in this group from different localities, with variable chemical composition, and showing the 111, 221, and 222 unit-cell types, were studied by means of X-ray single-crystal diffraction and electron microprobe. The unit-cell parameters were modeled using a multiple regression method as a function of the Ag, Sb, and Se contents. The deter- mination of the crystal structures for all the members of the group permits them to be considered as a family of polytypes and for all members to be named pearceite or polybasite. The main reason for doubling the unit-cell parameters is linked to the ordering of silver. The distinction between pearceite and polybasite is easily done with an electron microprobe analysis (As/Sb ratio). A hyphenated italic suffi x indicating the crystal system and the cell-type symbol should be added, if crystallographic data are available. Given this designation, the old names antimonpearceite and arsenpolybasite are abandoned here and the old names pearceite and polybasite, previously defi ned on a structural basis (i.e., 111 and 222), are redefi ned on a chemical basis. The old name pearceite will be replaced by pearceite-Tac, antimonpearceite by polybasite-Tac, arsenpolybasite-221 by pearceite-T2ac, arsenpolybasite-222 by pearceite-M2a2b2c, polybasite-221 by polybasite-T2ac, and polybasite-222 by polybasite-M2a2b2c. Since all polytypes are composed of two different layers stacked along [001]: layer A, with general 2– 2+ composition [(Ag,Cu)6(As,Sb)2S7] , and layer B, with general composition [Ag9CuS4] , the chemi- cal formulae of pearceite and polybasite should be written as [Ag9CuS4][(Ag,Cu)6(As,Sb)2S7] and [Ag9CuS4][(Ag,Cu)6(Sb,As)2S7], respectively, instead of (Ag,Cu)16(As,Sb)2S11 and (Ag,Cu)16(Sb,As)2S11, as is currently accepted. The new nomenclature rules were approved by the Commission on New Minerals and Mineral Names of the International Mineralogical Association. Keywords: Pearceite-polybasite, nomenclature rules, crystal chemistry, X-ray data, chemical composition INTRODUCTION mediate type of unit cell labeled 221 for a sample of polybasite Sulfosalts belonging to the pearceite-polybasite group are from the Las Chispas deposit, Sonora, Mexico. They observed relatively common in nature and were originally discovered in that the intermediate cell 221 was present in some areas of the the 19th century (pearceite, Penfi eld 1896; polybasite, Rose 1829), sample analyzed and was closely associated with the small cell while the new names antimonpearceite and arsenpolybasite were 111, which is present in other seemingly identical areas of the introduced by Frondel in 1963. The names pearceite and polyba- same sample. The same intermediate type of unit cell (i.e., 221) site are grandfathered whereas the names antimonpearceite and was subsequently reported by Edenharter et al. (1971) and by arsenpolybasite were approved by the IMA Commission (see re- Minčeva-Stefanova et al. (1979). Phase relations for the system port of the IMA-CNMMN, 1967). According to Frondel (1963), (Ag,Cu)16(As,Sb)2S11–(Ag,Cu)16(Sb,As)2S11 were investigated these four minerals can be divided on structural basis into two experimentally by Hall (1967) who hypothesized that the varia- tion of Cu content in different samples might play a key role in series: the fi rst one, formed by pearceite (Ag,Cu)16(As,Sb)2S11 and favoring chemical order-disorder phenomena and, therefore, to antimonpearceite (Ag,Cu)16(Sb,As)2S11, characterized by “small” unit cell (labeled 111) and high Cu content, and, the second one, be the driving force in stabilizing the different unit cells. From a chemical standpoint, the members of both series are formed by polybasite (Ag,Cu)16(Sb,As)2S11 and arsenpolybasite generally pure containing only minor amounts of Bi, Pb, Zn, and (Ag,Cu)16(As,Sb)2S11, with double cell parameters (labeled 222) and low Cu content. The chemical and crystallographic details of Fe. However, Harris et al. (1965) reported the occurrence of an the four minerals are given in Table 1. Soon after Frondel’s (1963) antimonpearceite sample from the San Carlos mine, Guanajuato, study, Harris et al. (1965) pointed out the existence of an inter- Mexico, containing up to 8.7 wt% Se but limited their study to the determination of the unit-cell parameters and chemical * E-mail: [email protected]fi .it characterization only. 0003-004X/07/0506–918$05.00/DOI: 10.2138/am.2007.2440 918 BINDI ET AL.: NEW NOMENCLATURE RULES FOR THE PEARCEITE-POLYBASITE GROUP 919 TABLE 1. List of minerals belonging to the pearceite-polybasite group Name Chemical formula Unit-cell parameters Range of Cu content monoclinic setting hexagonal setting pearceite (Ag,Cu)16(As,Sb)2S11 a ≈ 12.59, b ≈ 7.27, c ≈ 11.81 Å, β = 90.0° a ≈ 7.27, c ≈ 11.81 Å 5.5–19.7 wt% antimonpearceite (Ag,Cu)16(Sb,As)2S11 a ≈12.81, b ≈7.41, c ≈11.91 Å, β = 90.0° a ≈ 7.41, c ≈ 11.91 Å 7.9–18.2 wt% arsenpolybasite (Ag,Cu)16(As,Sb)2S11 a ≈ 26.08, b ≈ 15.04, c ≈ 23.84 Å, β = 90.0° a ≈ 15.04, c ≈ 23.84 Å 3.0–5.2 wt% polybasite (Ag,Cu)16(Sb,As)2S11 a ≈ 26.17, b ≈ 15.11, c ≈ 23.89 Å, β = 90.0° a ≈ 15.11, c ≈ 23.89 Å 3.1–7.6 wt% Notes: The transformation matrix from monoclinic to the trigonal unit cell is [½–½0, 010, 001]. The unit-cell values are taken from Strunz and Nickel (2001). The ranges of the copper content are taken from Hall (1967). Several authors (Peacock and Berry 1947; Frondel 1963; EXPERIMENTAL METHODS Harris et al. 1965; Hall 1967; Sugaki et al. 1983) considered A list of the samples investigated here together with their provenance and the the members of the pearceite-polybasite group as monoclinic, unit-cell type (expressed, for clarity, in hexagonal setting for all the members) is space group C2/m, although dimensionally pseudo-hexagonal. given in Table 2. We studied six samples of pearceite, fi ve samples of antimon- On the other hand, Edenharter et al. (1971) determined the Laue pearceite, fi ve samples of arsenpolybasite, and 15 samples of polybasite. Among the specimens of arsenpolybasite and polybasite we found several with the 221 symmetry as trigonal 3m. Despite the fact that these minerals unit-cell type (Table 2). All crystals were analyzed by X-ray single-crystal diffrac- are relatively common in nature, their crystal structure has yet tion and with an electron microprobe. to be characterized. Harris et al. (1965), Minčeva-Stefanova (1979), and Guinier X-ray single-crystal diffraction et al. (1984) have all pointed out problems associated with the The diffraction quality of the single crystals was initially checked by classifi cation scheme of Frondel (1963). Harris et al. (1965), for means of an Enraf-Nonius CAD4 single-crystal diffractometer equipped with a conventional point detector. Then, the unit-cell type was carefully investigated example, noted that the doubled dimensions, manifested as weak by means of an Oxford Diffraction “Xcalibur 2” single-crystal diffractometer intermediate layer lines on rotation photographs, represented (enhanced X-ray source, X-ray radiation MoKα, λ = 0.71073 Å) fi tted with a less-than-fundamental differences; the basic structural unit and Sapphire 2 CCD detector. A total of 100 frames of data were collected at room the external crystal form are the same for all the members of temperature as 3 sets of omega runs with an exposure time of 100 s per frame the pearceite-polybasite group. Therefore, Harris et al. (1965) and a frame width of 0.75°. Data frames were processed using the CrysAlis software package running on the Xcalibur 2 control PC. For samples 45895/G proposed to maintain the original classifi cation with the addition (pearceite), 171541 (antimonpearceite), 2453/I (Se-rich antimonpearceite), of a symbol to signify the type of the cell. Pearceite (1–1–1) has 17002/38 (polybasite-222), 2503/I (polybasite-221), AW634 (arsenpolybasite- As > Sb and unit-cell parameters of the basic structural unit (i.e., 222), and AB6829 (arsenpolybasite-221), a complete collection of X-ray data a ~ 13, b ~ 7.5, c ~ 12 Å, β ~ 90°), polybasite (2–2–2) has Sb was obtained. > As and unit-cell parameters doubled with respect to the basic Electron microprobe analyses structural unit (i.e., a ~ 26, b ~ 15, c ~ 24 Å, β ~ 90°), whereas Qualitative chemical analysis using energy dispersive spectrometry, performed polybasite (2–2–1) has a and b doubled with respect to the basic on the crystal fragments used for the structural study, did not indicate the pres- structural unit (i.e., a ~ 26, b ~ 15, c ~ 12 Å, β ~ 90°). ence of elements (Z > 9) other than S, Fe, Cu, Zn, As, Se, Ag, Sb, Te, Au, Pb, and More recently, Guinier et al. (1984), in a report for the Inter- Bi.