• journal_title：The Canadian Mineralogist
• Contributor：Yves Moëlo ; Paolo Orlandi ; Catherine Guillot-Deudon ; Cristian Biagioni ; Werner Paar ; Michel Evain
• Publisher：Mineralogical Association of Canada
• Date：2011-
• Format：text/html
• Language：en
• Identifier：10.3749/canmin.49.2.623
• journal_abbrev：Can Mineral
• issn：0008-4476
• volume：49
• issue：2
• firstpage：623
• section：Articles

The 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), R_air, R_oil (%)]: 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) ų. 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) ų (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(Sb_14.5As_9.5)_∑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.