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Lead-antimony sulfosalts from Tuscany (Italy). IX. Marrucciite, Hg₃Pb₁₆Sb₁₈S₄₆, a new sulfosalt from Buca della Vena mine, Apuan Alps: Definition and crystal structure

¹ Dipartimento di Scienze della Terra, Università di Pisa, Via S. Maria 53, 56126 Pisa, Italy
² Institut des Matériaux Jean Rouxel, UMR 6502 CNRS-Université de Nantes, Laboratoire de Chimie des Solides, 2, rue de la Houssinière, 44 322 Nantes Cedex 3, France
³ Dipartimento di Chimica Strutturale e Stereochimica Inorganica, Via Venezian 21, 20133 Milano, Italia

^* Corresponding authors, e-mail: orlandi{at}dst.unipi.it, Yves.Moelo{at}cnrs-imn.fr

Marrucciite, Hg₃Pb₁₆Sb₁₈S₄₆, is a new sulfosalt discovered in the Fe-Ba deposit of Buca della Vena, Apuan Alps (Italy). It occurs as black acicular metallic crystals in calcite veins; reflectance values in air are (R %, nm): 33.1 (470), 30.0 (546), 28.8 (589), 26.5 (650). Marrucciite is monoclinic, space group C2/m, with a 48.32(1) Å, b 4.117(1) Å, c 24.056(5) Å, ß 118.84(3)°, V 4192(3) ų, Z = 2, D_calc. = 6.00 g/cm³; a 2b superstructure is present. Main lines of the X-ray diffraction powder pattern [d (in Å), I (main hkl)]: 4.02, 33 (.0.3, 802); 3.480, 64 (803, 604); 3.418, 88 (07, 10.0.2, 14) ; 3.106, 31 (.1.4); 2.994, 100 (.1.2); 2.922, 41 (.1.1); 2.056, 52 (020); 1.764, 41 (27). Electron microprobe analysis gave (wt.%; mean of 4 spot analyses): Cu 0.18(12), Hg 7.90(9), Pb 42.41(7), Sb 29.71(7), S 19.47(18), Cl 0.06(2), sum 99.73(14). The chemical formula (basis: 46 (S + Cl) at.) is Cu_0.21Hg_2.98Pb_15.46Sb_18.44S_45.87Cl_0.13, giving the stoichiometric one Hg₃Pb₁₆Sb₁₈S₄₆ that fulfils the charge balance. X-ray single crystal study (R = 9.56 %) revealed a complex structure, with Hg in two specific distorted octahedral coordinations (2 short distances each, close to 2.36 Å). There are 40 other independent positions: 7 Pb, 2 (Pb,Sb), 8 Sb and 23 S. The general architecture of the structure is strongly related to that of hexagonal Ba₁₂Bi₂₄S₄₈, with three types of Me₁₀S₁₄ rods (x 2) around a HgSb₂Pb₄S₄ central column. It explains the pseudo-hexagonal symmetry (a/2 ~ c; ß ~ 120°). A strong topologic relationship exists also between marrucciite and scainiite. Like rouxelite, the other coexisting Hg bearing sulfosalt, marrucciite was formed by hydrothermal process during the Apenninic tectono-metamorphic event.

Key-words: marrucciite, new mineral, sulfosalt, lead, antimony, mercury, crystal structure, Tuscany, Italy.

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