The crystal structure of hitachiite

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Marco E. Ciriotti
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The crystal structure of hitachiite

Messaggio da Marco E. Ciriotti » mar 14 ago, 2018 13:12

▪ Nagase, T., Kuribayashi, T., Nozaki, T., Ishibashi. J-I., Shimada, K., Momma, K., Shimizu, M. (2018): Crystal structure determination of an unknown mineral of tetradymite group from the Hitachi mine, Ibaraki prefecture, Japan. IMA2018 Abstract book. Poster Presentations, Mineralogical crystallography, 467.

An unknown mineral, Pb5Bi2Te2S6, of tetradimite group, has been found from the Fudotaki deposit, the Hitachi mine, Ibaraki prefecture, Japan (LAT. 36˚ 37’ 16.03”, LON.140˚ 36’ 46.50”). The Fudotaki deposit is one of the Hitachi volcanogenic massive sulfide (VMS) deposits in the Hitachi belt and is Cu-Zn-Pb sulfide deposit. The Fudotaki sulfide ores have been metamorphosed to lower epidote-amphibolite facies.
The unknown mineral has the trigonal space group P-3m1 with a = 4.2200(13), c = 27.02(4) Å, V = 416.7(7) Å3, and Z = 1. The seven strongest lines in the X-ray powder diffraction pattern are [d(Å), (I), (hkl)]: 3.541(35)(013), 3.391(59)(013), 3.039(100)(015), 2.177(14)(1010), 2.114(56)(110), 1.793(18)(118) and 1.735(18) (025). Electron microprobe analysis gave Pb content of 52.01; Bi, 23.06; Te, 13.74; Fe, 0.69; Sb, 0.17; S, 9.71; Se, 0.54; total, 100.04 wt.%, corresponding to an empirical formula of (Pb4.75Fe0.23)Σ=4.98Bi2.09Te2.04(Sb0.03S5.73Se0.13)Σ=5.89. The ideal chemical composition of this mineral is Pb5Bi2Te2S6 and could be regarded as (Pb5Bi2)Σ=7(Te2S6)Σ=8, which is corresponding to the 2.DC. group (Valuable M:S) in the Strunz classification. The chemical formula is plotted on the tie-line of Bi2Te2S-PbS, and also those of saddlebackite and aleksite lie on it. The color is silver grey and luster is metallic. The streak is black. On the Mohs’ scale, the hardness is 2–21/2. The calculated density is 7.54 g/cm3 for the empirical chemical formula. Associated sulfide minerals are pyrite and chalcopyrite, with minor amounts of sphalerite and pyrrhotite, and with very minor amounts of galena and marcasite.
Single-crystal X-ray diffraction experiments were conducted with a four-circle diffractometer using synchrotron radiation at the Beam-Line BL-10A in Photon Factory, KEK, Japan. X-ray diffraction intensity data were collected up to 2thetamax = 60º. Twice of unique reflection region were measured. Initial structure model was leaded from Charge Flipping method with SUPERFLIP sofware. All calculations of structure refinement were conducted by SHELXL97 software. Since the scattering factors of Pb and Bi atoms are similar, the site occupancy of each site did not directly determine by the least square calculation. Anisotropic temperature factors for metals were applied. The crystal structure is based on an ABC-type close-packing along c-axis. The structure has five Pb, two Bi, two Te and six S sheets (total 15 layers) and they are stacked as Te-Bi-S-Pb-S-Pb-S-Pb-S-Pb-S-Pb-S-Bi-Te. The c-axis length corresponding to stacking direction is approximately 27 Å.
Marco E. Ciriotti

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