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 structure, 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 partly 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 aikinite, the meneghinite homologues, berthierite and galenobismutite. The role of Cu in linking adjacent rod-layers across the incommensurate interface in the nuffieldite structure is illustrated by bond-valence calculations. The hierarchy of structural organization (from 0-D to 3-D) in complex lead sulfosalts takes into account bond valence at corresponding structural levels, and is an effective way to understand the dynamics of crystallization of such structures.