Structural and compositional modulations in complex sulphide minerals

Full text: The development of structural classification systems in which complex structures are reduced to a set of simple fundamental blocks and a set of building or assembly instructions has done much to emphasise and simplify the relationships between minerals within a group. Our understanding of the chemistry, structure and paragenesis of the minerals has benefited by such systematic treatments; this is particularly so for the complex sulphides that can be related to PbS and SnS parent structures. High-resolution transmission electron microscopy (HRTEM) studies of on a number of mineral systems, notably the sartorite, and lillianite groups, have revealed a range of different forms of structural and compositional modulation. The sartorite group of Pb-As sulphides have modular structures based on the intergrowth N 4 and N = 3 units, with N = 4 units dominated the more Pb rich minerals. The more Pb rich members of the group, with a mean N number between 4 and 3.75, are perfectly ordered with no evidence of polytypism or polysomatic disorder. The compositional region between liveingite (N =3.75) and baumhauerite (N =3.5) is dominated by the occurrence of long period homologues with 10 and 12 units in the structural repeat. Disordered intergrowths of N = 4 and N = 3 units occur in this compositional region. Baumhauerite (N = 3.5) exhibits extensive polytypism, both ordered and disordered and this is thought to be associated with periodic inversions of the Pb/As. Sartorite (N = 3) exhibits a complex modulated structure that is associated with the inherent mismatch of As and Pb polyhedra within the structural modules. Pb-Bi sulphides of lillianite group have structures composed of alternating slabs of galena-like (PbS) structure; the boundaries of which are parallel to (311) galena. The slabs are connected via trigonal coordination prisms [PbS6+2], and are related to adjacent slabs by a mirror along (010)lillianite. The homologue number, N, gives the number of metal sites in the galena like slabs. Recent examinations of lillianite group minerals from Greenland has revealed both perfectly ordered members as well as extensive regions of disordered intergrowth between N = 4 and N = 7. An Axial Next-Nearest-Neighbour Ising model has been used to follow the fluctuations in the average homologue number across several thousand slabs and these fluctuations appear to be a form of long range compositional modulation or zoning. Copyright (2002) Australian Society for Electron Microscopy Inc