Melonite from Kingash and Kuskanak, Eastern Sayans, Russia, and the extent of Bi-for-Te substitution in melonite and synthetic Ni(Te,Bi) (sub 2-x)

We describe occurrences of palladoan melonite in intimate intergrowths with cobaltite-gersdorffite from the Neoproterozoic dunite-wehrlite-gabbro complexes of Kingash and Kuskanak, Eastern Sayans, Russia. The observed compositional trends of melonite are consistent with the overall variations examined on the basis of numerous literature sources. The levels of Bi in NiTe (sub 2) are normally limited to < or =0.25 Bi atoms per formula unit (apfu), under natural conditions. Greater levels (< or =0.5 Bi apfu) are associated with the (Pd + Pt) enrichment in the palladoan varieties. The telluride-sulfarsenide intergrowths probably formed at Kingash and Kuskanak late in the crystallization history of the ore zones, from microdroplets of residual melt rich in semimetals (Te, Bi, As) and noble metals (Pd and Ag), below the solidus of the enclosing gabbroic rocks and within a narrow range of temperatures (500-550 degrees C). On the basis of our observations made on specimens of melonite and synthetic Ni(Te,Bi) (sub 2-x) (x = 0.6), we infer that the limit of incorporation of Bi into a melonite-type phase is < or =0.5 Bi apfu.