Au(Ag)–Sn–Sb–Pb minerals in association with placer gold from Rumoi province of Hokkaido, Japan: a description of two new minerals (rumoiite and shosanbetsuite)

抄録

Au(Ag)–Sn–Sb–Pb minerals occurring in association with gold, rumoiite (AuSn₂), shosanbetsuite (Ag₃Sn), yuanjiangite (AuSn), aurostibite (AuSb₂), and anyuiite (AuPb₂), were found from the Shosanbetsu River (the former three), Shosanbetsu village and the Ainusawa River (the latter two), Haboro town, Rumoi province, Hokkaido, Japan. Rumoiite (IMA No. 2018–161) and shosanbetsuite (IMA No. 2018–162) have been approved as new minerals by the International Mineralogical Association, the Commission on New Minerals, Nomenclature and Classification (IMA–CNMNC) and named after the locality. Both minerals show anhedral shape at less than 5 µm and occur in close association with one another, yuanjiangite, and native lead in spherical aggregates in placer gold. The densities of rumoiite and shosanbetsuite based on their empirical formulae and powder diffraction data were calculated to be 10.1 and 11.1 g/cm³, respectively. The empirical formulae of rumoiite and shosanbetsuite were (Au_0.95Ag_<0.01)_Σ0.96(Sn_1.93Sb_0.08Pb_0.02Bi_0.01)_Σ2.04 (basis of 3 apfu) and (Ag_2.46Au_0.54)_Σ2.99(Sn_0.97Sb_0.01Pb_0.01Bi_0.01)_Σ1.01 (basis of 4 apfu), respectively. Rumoiite is orthorhombic, Pbca, with lattice parameters a = 6.9088(7) Å, b = 7.0135(17) Å, c = 11.7979(19) Å and V = 571.6(2) ų (Z = 8). Shosanbetsuite is orthorhombic, Pmmn, with lattice parameters a = 5.986(8) Å, b = 4.779(3) Å, c = 5.156(6) Å and V = 147.5(3) ų (Z = 2). Rumoiite and shosanbetsuite correspond to the synthetic AuSn₂ and Ag₃Sn phases, respectively. The chemical compositions for aurostibite, anyuiite, yuanjiangite, and native lead, and the unit cell parameters for yuanjiangite and native lead are also reported in this paper. Hydrothermal activity in ultramafic rocks after the formation of gold (electrum) grains may have been involved in the occurrence of Au(Ag)–Sn–Sb–Pb minerals.