NUFFIELDITE FROM THE MALEEVSKOE MASSIVE SULFIDE DEPOSIT, RUSSIA

A new occurrence of nuffieldite, a rare lead - bismuth - copper sulfos alt, is reported from a pyrite-rich massive sulfide deposit at Maleevs koe, 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, electr um, aikinite, lindstromite, hammarite, cosalite, kobellite, galenobism utite, gudmundite, berthierite and nisbite. Optical properties, X-ray powder-diffraction data and results of electron-microprobe analyses ar e compared with those of nuffieldite from both the type locality and o ther localities. The mineral contains as much as 5.08 wt.% Sb, which s upports the findings of Maurel & Moelo (1990) and Moelo (1989) that Sb is essential for the stabilization of the mineral. Calculated formula e (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) sit es. These correlations may be accounted for by assuming compensation o f the substitution Pb2+ for Bi3+ by vacancies in the S site and neutra l Cu atoms in excess in structural interstices. Excess Cu may be mobil e, as has been established in synthetic tetrahedrite Makovicky & Skinn er 1979). The compositional field of nuffieldite may be expressed by t he formula Cu1-xPb2(PbyBi1-y-zSbz)Bi2S7-y, where x and v may have simi lar values.