NUFFIELDITE FROM THE MALEEVSKOE MASSIVE SULFIDE DEPOSIT, RUSSIA

A new occurrence of nuffieldite, a rare lead - bismuth - copper sulfosalt, is reported from a pyrite-rich massive sulfide deposit at Maleevskoe, Rudny Altai, Russia. The mineral occurs as platy crystals (0.3 x 0.08 mm) and as aggregrates of anhedral grains up to 0.6 mm within a (Cu,Zn)-rich zone. Associated minerals are hessite, tetradymite, electrum, aikinite, lindstromite, hammarite, cosalite, kobellite, galenobismutite, gudmundite, berthierite and nisbite. Optical properties, X-ray powder-diffraction data and results of electron-microprobe analyses are compared with those of nuffieldite from both the type locality and other localities. The mineral contains as much as 5.08 wt.% Sb, which supports the findings of Maurel & Moelo (1990) and Moelo (1989) that Sb is essential for the stabilization of the mineral. Calculated formulae (based on an ideal formula with 13 atoms) show an excess in Cu and a deficiency in S, which are correlated with Pb in mixed (Pb,Bi,Sb) sites. These correlations may be accounted for by assuming compensation of the substitution Pb2+ for Bi3+ by vacancies in the S site and neutral Cu atoms in excess in structural interstices. Excess Cu may be mobile, as has been established in synthetic tetrahedrite Makovicky & Skinner 1979). The compositional field of nuffieldite may be expressed by the formula Cu1-xPb2(PbyBi1-y-zSbz)Bi2S7-y, where x and v may have similar values.