Structural investigations of Bi-based sulphosalts

Sulphosalts, a specific group of complex sulphides of geological, mineralogical and potential material-science importance, were not a frequent object of investigation, like some families of oxides compounds of equal complexity. This is in contrast to potential benefit in technologies like photovoltaics and superconductibility. The applicant investigated various natural sulphosalts and found many new minerals as well as some new relations between structural types. This work will be continued to reach a better understanding of natural sulphosalt groups. X-ray micro-analysis and single-crystal X-ray diffraction methods will be used to investigate the complex Bi- based sulphosalt phases from available samples, in order to (1) fully characterize some new potential minerals, (2) to complete the structural characterization and classification of some other sulphosalt minerals in need of it, and (3) to improve the structural description of some not well defined ones. Structure investigations on individual phases will be followed by crystal-chemical calculation and their assessment, modular interpretation, comparative studies and attempts of generalization of the observed structural features. All these results will contribute to a better understanding of: chemical ranges, mechanisms of substitution, modular description, coordination polyhedra of Bi and Pb, and crystal-chemistry of the Bi-based homologous series of lillianite, pavonite, cuprobismutite, meneghinite, junoite and galenobismutite.

This is a mineralogical study concerned with a series of complex bismuth sulphosalts, which aims at establishing the detailed structural and chemical features of these compounds. X-ray micro-analysis and single-crystal X-ray diffraction methods were involved to investigate the natural mineral samples within this study. Crystal structure investigations on individual phases were followed by crystal-chemical calculation and assessment, modular interpretation, comparative studies and attempts to generalize the observed structural features. Two newly described minerals - cupromakovickyite and cupromakopavonite - belong to the pavonite homologous series. Compared to makovickyite - which is the (Cu,Pb)-poor counterpart - cupromakovickyite displays a doubled unit cell due to the asymmetry of the thinner layers of the structure which is determined by tetrahedrally co- ordinated copper sites and by the peculiar partitioning of Pb-Bi; this is the basic building mechanism of the entire range of the copper-rich members of individual homologues. Cupromakopavonite represents the first natural occurrence of a pavonite homologue containing a regular intergrowth of N=4 and N=5 homologues. The asymmetry of the thin layers in cupromakopavonite is the same as in other (Cu,Pb)-rich members of the pavonite series. Similarly, a certain module in the newly deciphered crystal structure of berryite, may be related to the pavonite series, as it combines modules belonging to the N=5 and N=6 pavonite homologues. Our study establishes berryite as monoclinic, but elaborates on the potential of this mineral to have also an orthorhombic modification. The crystal structure of the new mineral angelaite has many features in common with that of galenobismutite. A new and remarkable substitution mechanism has been identified in angelaite, by which two Bi atoms are replaced by two Ag and by four Cu atoms, respectively. A new and improved refinement of the paderaite crystal structure reveals the strong relationship of this mineral with the cuprobismutite family. Thus, the paderaite structure represents a 1:1 association between kupcikite and a new complex module. Such a relation leads to new possibilities of finding intergrowths among paderaite and other members of the cuprobismutite family. All the results obtained contribute to a better view on the complicated crystal chemistry and physical properties of mineral sulphosalts, with potential benefits for the technology of photovoltaic and superconductive materials, as well as for adding a certain by-product value to common ores hosting sulphosalt minerals. Our study confirms that changes in chemistry are always connected with changes in crystal structure which are amenable to modular principles of homologous series, and gives new insights on the mechanisms of element substitution in such natural compounds.