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

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 %, lambda 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) degrees, b 4.117(1) angstrom, c 24.056(5) angstrom, beta 118.84(3)degrees, V 4192(3) angstrom(3), Z = 2, D-calc. = 6.00 g/cm(3); a 2b superstructure is present. Main lines of the X-ray diffraction powder pattern [d (in angstrom), I (main hkl)]: 4.02, 33 ((12) over bar .0.3, 802); 3.480, 64 (803, 604); 3.418, 88 ((6) over bar 07, 10.0.2, (3) over bar 14); 3.106, 31 ((9) over bar .1.4); 2.994, 100 ((11) over bar 1.2); 2.922, 41 ((11) over bar1.1) 2.056, 52 (020); 1.764, 41 ((6) over bar 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 Cu0.21Hg2.98Pb15.46Sb18.44S45.87Cl0.13, 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 angstrom). 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 similar to c; beta similar to 120 degrees). 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.