LEAD-ANTIMONY SULFOSALTS FROM TUSCANY (ITALY). XI. THE NEW MINERAL SPECIES PARASTERRYITE, Ag4Pb20(Sb14.5As9.5) 24S58, AND ASSOCIATED STERRYITE, Cu(Ag,Cu)3Pb19(Sb,As)22(As-As)S56, FROM THE POLLONE MINE, TUSCANY, ITALY
International audienceThe new mineral species parasterryite has been discovered in the Pollone barite - pyrite - (Pb-Zn-Ag) deposit at Valdicastello Carducci, near Pietrasanta, in the Apuan Alps, Tuscany, Italy. It forms acicular crystals up to 4 × 0.3 mm in vugs in quartz-barite veins. Sterryite (second world occurrence) also is present. The associated sulfides and sulfosalts are pyrite, sphalerite and various Sb-As sulfosalts. Parasterryite is black and metallic. Under the microscope, it is white with weak pleochroism; anisotropism is distinct, with greenish and brownish tints. Data on maximum and minimum reflectances for the COM wavelengths are [λ (nm), Rair, Roil (%)]: 470: 35.1, 40.8 and 19.4, 22.2, 546: 33.5, 39.3 and 19.3, 21.7, 589: 32.7, 38.2 and 18.7, 21.1, 650: 31.4, 36.5 and 17.3, 19.5. Hardness VHN10 = 238 kg/mm2. An electron-microprobe analysis gave (in wt.%, mean result of 32 spot analyses): Cu 0.09(2), Ag 4.36(6), Hg 0.15(2), Pb 47.00(18), Sb 19.57(30), As 7.73(21), S 20.56(7), total 99.46(23). A single-crystal X-ray study of parasterryite indicates monoclinic symmetry, space group P21/c, with a 8.3965(5), b 27.9540(4), c 43.8840(13) Å, β 90.061(12)°, V = 10300(3) Å3. The main powder-diffraction lines [d in Å(I)(hkl)] are: 3.62(100)(075, 0112, 2̅3̅4, 234), 3.42(45) (2̅4̅4, 244), 3.35(95)(0113), 3.23(65)(078), 3.01(45)(239, 2̅3̅9), 2.945(85)(088), 2.885(80), 1.916(45). The name parasterryite reflects its close structural relationship with sterryite. Sterryite is more abundant than parasterryite. Electron-microprobe analyses of two samples gave: 1) Sb-rich sterryite (wt.%): Cu 1.27(1), Ag 2.19(11), Hg 0.55(5), Tl 0.56(8), Pb 44.76(25), Bi 0.26(7), Sb 24.71(11), As 5.33(9), S 20.47(11), total 100.10(39), and 2) Sb-poor sterryite: Cu 0.73(4), Ag 4.12(13), Pb 44.58(16), Sb 20.84(22), As 8.27(12), S 20.92(7), total 99.47(15). The unit cell of Sb-rich sterryite is: a 8.1891(1), b 28.5294(13), c 42.98(2) Å, β 94.896(8)°, V = 10005(5) Å3 (space group P21/n). It presents a powder diagram close to that of sterryite from Madoc, Ontario, the type locality. The crystal structures of sterryite and parasterryite (not described here) are very similar, and represent a limiting case of homeotypy; they are also expanded homologues of owyheeite. The structural formula of parasterryite is based on 48 cations and 58 S, ideally Ag4Pb20(Sb14.5As9.5)∑24S58 (Z = 4). The structural formula of sterryite is based on 47 cations and 56 S, with an (As-As)4+ dimer [d(As-As) = 2.64 Å] and a specific Cu site, ideally Cu(Ag,Cu)3Pb19(Sb,As)22(As-As)S56 (Z = 4). The presence of the As-As dimer in sterryite indicates its formation with a lower f(S2) than parasterryite, which may also explain the formation of some rare Pb-Sb-As sulfosalts at Madoc