Crystallographic analysis of modular structures of cupromakopavonite Cu 8 Ag 3 Pb 4 Bi 19 S 38 and heyrovskyite Pb 6 Bi
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RY OF CRYSTAL STRUCTURES
Crystallographic Analysis of Modular Structures of Cupromakopavonite Cu8Ag3Pb4Bi19S38 and Heyrovskyite Pb6Bi2S9 Minerals S. V. Borisov, S. A. Magarill, and N. V. Pervukhina Nikolaev Institute of Inorganic Chemistry, Siberian Branch, Russian Academy of Sciences, Novosibirsk, pr. Akademika Lavrent’eva 8, 630090 Russia e-mail: [email protected] Received January 22, 2015
Abstract—The well-known “modular” concept of the structure of two complex sulfides has been supplemented by crystallographic analysis fixing specific features of ordering atomic sites in crystal structures. Regular ordering of cations (formation of F sublattices with close parameters) and independent, less regular ordering of anions is established. It is shown that local ordering of atoms in contacting modular layers gives rise to a unified sublattice for the structures and forms a long-range order. DOI: 10.1134/S1063774515060048
INTRODUCTION
CRYSTALLOGRAPHIC ANALYSIS
The modern advances in X-ray diffraction analysis, a technique equipped with precise instruments and automatic software packages for control and calculations, resulted in a great amount of interpreted structures and necessity of their systematic analysis. One of the currently developed lines in this field is the modular approach to the analysis of crystal formation, which postulates the assembly of structures from atomic fragments with relatively strong chemical bonds within a fragment [1–6]. Actually, the case in point is the occurrence of crystallization in two stages: (i) the formation of large atomic complexes in the precrystallization phase with elements of local translational and other symmetries and (ii) the assembly of a unified structure from these complexes, containing to some extent these symmetry elements.
Based on the results of crystallographic analysis of sulfides with a short (~4 Å) translation period and a mirror symmetry plane oriented perpendicular to the translation direction (the percentage of these sulfides with a unit cell characterized as lozengelike [10, 11] among known structures is rather high), it was concluded that this combination of symmetry elements can be considered the first stage of atomic-site ordering. Cations form pseudohexagonal sublattices with close unit-cell parameters, as well as anions (however, the scatter of lattice parameters for the latter is somewhat larger) [10]. Under favorable conditions, ordering could result in the symmetry widespread among sulfides of sphalerite ZnS or galena PbS (NaCl) structure types; the symmetry of both compounds is Fm3 m [10]. However, the complex elemental composition impedes the occurrence of additional mirror symmetry planes in other directions. There must be six such planes in the sp. gr. Fm3 m : (220), (202), (022), (220), (202 ), and (022 ). They are known to be oriented perpendicular to diagonals of cubic F-cell faces. The absence of complete set of symmetry elements does not withdraw the question about the formation of pseudocubic F sublattices due to the arrangement of both
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