茨城県日立市日立鉱山から発見された新鉱物・日立鉱

抄録

Hitachiite (IMA2018-027), ideally Pb₅Bi₂Te₂S₆, has been discovered from the Hitachi mine, Ibaraki Prefecture, Japan. Hitachiite commonly occurs as small size of crystals (∼ 30-50 μm) with euhedral hexagonal plate form within pyrite crystals, and co-exists with pyrite, chalcopyrite, sphalerite, pyrrhotite, and galena. Hitachiite shows a metallic luster and black streak, and its macroscopic color is silver grey. Hitachiite has a Mohs' hardness of 2½-3, and its calculated density is 7.54 g/cm³. The discovered hitachiite has trigonal symmetry with a = 4.2200(13) Å, c = 27.02(4) Å. Its space group is P3m1. Hitachiite has a layer-type structure based on ABC-type closed packing of each single element atomic sheet stacked along the c-axis. The stacking sequence is -Te-Bi-S-Pb-S-Pb-S-Pb-S-Pb-S-Pb-S-Bi-Te- (15 layers). It is possible that hitachiite would be forming a homologous series expressed as Bi₂Te₂S・nPbS with tetradymite (n = 0), aleksite (n = 1), and suddlebackite (n = 2).