Abstract

The progress of ordering of Cu+ and Pb2+ in synthetic hammarite, Cu2Pb2Bi4S9, has been monitored by the form and sharpness of satellite reflections in electron diffraction patterns and by high-resolution transmission electron microscopy (HRTEM). The initial state of the synthetic hammarite is that of the parent bismuthinite cell, with a disordered distribution of Bi and Pb on the M2 sites and Cu disordered over the available tetrahedral sites with two-thirds occupancy. Strong superlattice reflections, corresponding to a supercell of the bismuthinite parent with a 3a period, were observed in samples annealed at 175 and 225 °C for 24 months or longer, and HRTEM images show crystals with domains of a well-ordered 3a supercell phase. In addition to the 3a supercell, superlattices with a 5a period were also observed in some crystal fragments, indicating chemical variation within the annealed products. HRTEM was used to monitor Cu ordering and revealed that the distribution of Cu and Pb in the cell is not that of hammarite but of a lower symmetry supercell consisting of the intergrowth of one aikinite cell (two aikinite units) with two krupkaite cells (four krupkaite units). Fully ordered natural hammarite has a 3a supercell of the bismuthite parent, but with the two aikinite units and four krupkaite units ordered into a centrosymmetric structure in Pbnm. The lattice images show steps in ordering; cations order first on a local scale to create end-member polysome strips, then the polysome strips order to form simple supercells, and finally, with longer annealing time, the Pb and Cu are reordered into the higher symmetry hammarite cell. The electron diffraction and high-resolution lattice image data suggest that the ordering of Cu+ and Pb2+ in these minerals is coupled and that Pb2+ does not order before Cu+. The results of these annealing experiments indicate that in nature intermediate compositions in the bismuthinite-aikinite series consist of compositionally well-ordered polysome strips, which, depending on the initial composition and the cooling rate, may remain as disordered intergrowths (stacking disorder), form ordered intergrowths, or fully order into the supercell minerals.

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