REFINEMENT OF THE CRYSTAL-STRUCTURE OF NUFFIELDITE, PB2CU1.4(PB0.4BI0.4SB0.2)BI2S7 - STRUCTURAL RELATIONSHIPS AND GENESIS OF COMPLEX LEAD SULFOSALT STRUCTURES

A crystal-structure refinement of nuffieldite from Les Houches (France ) indicates the composition Pb2Cu1.37(Pb0.37Bi0.39Sb0.24)Bi2S7. There are significant differences relative to the originally reported struct ure, primarily concerning the distribution of heavy atoms (Pb, Bi and Sb) and of Cu. There are two distinct Cu sites: Cu(1) (full occupancy) shows a pronounced eccentric tetrahedral coordination, whereas the pa rtly occupied Cn(2) site has a (sub)regular tetrahedral coordination. Using the general crystal-chemical principles developed by Makovicky ( 1993) for complex lead sulfosalts, nuffieldite is compared with aikini te, the meneghinite homologues, berthierite and galenobismutite. The r ole of Cu in linking adjacent rod-layers across the incommensurate int erface in the nuffieldite structure is illustrated by bond-valence cal culations. The hierarchy of structural organization (from 0-D to 3-D) in complex lead sulfosalts takes into account bond valence at correspo nding structural levels, and is an effective way to understand the dyn amics of crystallization of such structures.