Neyite from the Yugo-Konevo Deposit (Southern Urals, Russia) and Its Crystal Structure

Neyite Ag2Cu6Pb25Bi26S68 was identified in the wastes of the Yugo-Konevo tungsten deposit (Chelyabinsk oblast, Southern Urals) for a first time in Russia. This rare sulfosalt occurs as prismatic crystals up to 3 × 0.5 mm in fluorite–muscovite–quartz veins and is associated with aikinite, tetradymite, and pyrite. Reflectance spectra and coefficients of neyite are published for the first time. The electron microprobe composition of neyite (wt %, mean of seven analyses): Ag 1.75, Cu 3.59, Pb 35.01, Cd 0.12, Bi 42.59, S16.54, total 99.60. The empirical formula based on S = 68 apfu is Ag2.14Cu7.45Pb22.27Cd0.14Bi26.86S68. The crystal structure of neyite from Yugo-Konevo was refined based on the monocrystal, R1 = 3.43%, wR2 = 7.22%. The mineral is monoclinic, space group С2/m, a = 37.3900(6), b = 4.05500(10), c = 43.5821(7) Å, β = 108.740(2)°, V = 6257.5(2) Å3, Z = 2. The structure of neyite is based upon three different modules of archetypal structure of galena. (111)PbS blocks make up octahedral “columns” extended along the b direction, which alternate with (100)PbS blocks in the a direction. The resulting layers are alternated with (922)PbS layers along c. The main structural feature of neyite from Yugo-Konevo deposit that distinguishes it from previously described neyite from the type locality and cuproneyite is the disorder of the Me16 position and the presence of the additional Cu5 position. These features, along with analysis of the bond valence sums, suggest the presence of chains of CuS4 tetrahedra alternating with chains of BiS6 octahedra.