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Micron- to nano-scale intergrowths among members of the cuprobismutite series and padraite: HRTEM and microanalytical evidence

¹ Geological Survey of Norway, N-7491 Trondheim, Norway
² South Australian Museum, North Terrace, Adelaide, South Australia 5000, Australia
³ Department of Geology and Geophysics, University of Adelaide, North Terrace, Adelaide, South Australia 5005, Australia
⁴ School of Chemistry, Physics and Earth Sciences, The Flinders University of South Australia, GPO Box 2100 Adelaide, South Australia 5001, Australia

View larger version (72K): [in a new window]   FIG. 1. The cuprobismutite series and related padraite, according to (a) Makovicky (1989) and (b) Mumme (1986). Padraite shares the common Cu8Bi8S16 layer with members of the cuprobismutite series. In the padraite structure (b, right), the D module has variable thickness along the c axis: either one octahedron thick at the T contact, or two octahedra thick along the trigonal-tetrahedral match. In each drawing, atoms are, in decreasing size, S, Bi and Cu. In the drawings for padraite, the order is Pb, S, Bi, Ag and Cu.

View larger version (115K): [in a new window]   FIG. 2. BSE images showing cuprobismutite homologues and padraite. (a) Needles (5-10 µm) of cuprobismutite (Cbs) with pyramidal termination. The matrix consists of over-substituted bismuthinite (BD10) with fields of fine intergrowths. The average composition within such fields is in the range between gladite and pekoite (BD32-16). (b) Area with mixed composition between padraite (Pad) and Cbs. Several narrow bands with Cbs composition are seen in the middle part. (c) Orthogonal aggregates of cuprobismutite-padraite (CBP) in a matix of BD10. Note the stepwise arrangement of laths, with lath-width reducing towards the interior of the aggregate. Makovickyite (Mak) and gladite (Gl) are seen as darker shades than the matrix between the laths. (d) Hodrushite (Hod) and Cbs in a lath dominated by Pad. (e and f) Details of laths within cuprobismutite-padraite aggregates (CBP) showing Pb (wt. %) variance across sequences of alternating cuprobismutite (noted as ‘C’), padraite (‘P’) and mixed material in the Cbs-Pad range (‘M’). In (e), the lower marginal needle of Cbs crosscuts the boundary of makovickyite (Mak) towards BD10. The points mark the line of the profile. (g and h) Diagrams showing the bimodal variance of Pb (in wt.%) across the sequences in (e) and (f), plotted as difference between two adjacent intervals of 5–10 µm each.

View larger version (115K): [in a new window]   FIG. 3. (a) Electron diffraction pattern of cuprobismutite (Cbs) down [010]. Note slight streaking along the a* axis, due to intergrowths with padraite (Pad). Satellite reflections corresponding to padraite are arrowed. (b) High resolution transmission electron microscopy image of cuprobismutite with strips of padraite (arrowed). The intergrowths are along the c axis of cuprobismutite and the a axis of padraite (17 Å repeat axis). The white box marks the sequence: 5–2Cbs.Pad (Slab 2) as characteristic for disordered Cbs (see text). The image corresponds to the black box indicated on the low magnification strip at the bottom of the figure. Here we see a tendency towards a banding induced by variable combinations of Pad.(NCbs)1-3.Pad modules. The modules are shown as white (Pad) and grey (Cbs) bands at the top of the strip.

View larger version (134K): [in a new window]   FIG. 4. (a) BSE image showing micron-scale intergrowths between cuprobismutite (Cbs) and padraite (Pad). Needles of Cbs within the lath with Cbs dominant composition are arrowed. Variation of Pb and Ag across an energy dispersive line scan is also shown. Bd10: oversubstituted bismuthinite. The points 1–7 represent the location for the analyses in Table 2. (b) Lattice image showing styles of coherent intergrowth between padraite (Pad) and cuprobismutite (Cbs); irregular strips of Cbs within a domain of Pad (to the left). At the top of the image the intergrowths sequence is shown as white (Pad) and grey (Cbs) bands. The sequences discussed in the text as Slab 1, 3 and 4 are marked in black boxes. Insets show details of Pad and Cbs with corresponding computer simulations down [010], at 900 Å defocus.

View larger version (51K): [in a new window]   FIG. 5. Coherent intergrowths along the 15 Å repeat axis between slabs of padraite and cuprobismutite through common D layers (a) or through common C layers (b). The layers and atoms are according to Fig. 1b.

View larger version (155K): [in a new window]   FIG. 6. A slip defect linking two different sequences of cuprobismutite (Cbs) and padraite (Pad) cells. One of the transpositions between one Pad and two Cbs along the 17 Å repeat axis is marked as white box (a). The position of this slip within ordered Cbs is shown in the low-magnification strip at the top of the image.

View larger version (60K): [in a new window]   FIG. 7. Transposition along the 17 Å repeat axis of one Pad into two Cbs, as indicated by the lattice image shown in Fig. 5. (a) The correspondent chains of atoms in Pad and Cbs are shown using the structures of cuprobismutite (Ozawa and Nowacki, 1975) and padraite (Mumme, 1986). (b) Match between the DTDPad module and CCbs across the slicing shown in (a). The boundary between the two slabs is curved with maximum amplitude of 3 aCbs. Lc refers to CuBiS10 lozenges attached to the C layer (see Fig. 1).

View larger version (139K): [in a new window]   FIG. 8. Schematic diagram of slabs of coherent intergrowth between Pad and Cbs along the 15 Å repeat interpreted from the lattice images in Figs 3b and 4b. Such ‘structural modulators’ can be seen as long-period polysomes obtained in response to periodic chemical oscillations and local minima in free energy. The corresponding output signals realized by ordering of Pb in such polysomes are shown at the top of the figure in terms of 1 for the presence of 2Pb in D layer and 0 (gap) for the absence of Pb. These structural modulations are often thought of as being the result of long-range variation of strain in the lattice. Legend for layers: white = T; middle grey = DPad at the contact to Cbs; black = D; light grey = C.

View larger version (83K): [in a new window]   FIG. 9. Schematic representation of DTDPad/CCbs ‘switchers’ modulating the structural ‘jumps’ along the 25 nm wide slip in the stacking sequence of a disordered intergrowth of padribbons in cuprobismutite shown in Fig. 5. The legend for layers is the same as in Fig. 8. The dashed line represents the equivalent ‘C’ layer for each DTDPad in the slabs. With thin dashed lines are suggested the chemical modulation as ripples with amplitude varying from one to three repeats of 15 Å along the free energy surface.

View larger version (18K): [in a new window]   FIG. 10. Means of Pb and Ag composition (wt.%) for various short- and long-period polysomes stable within the range padraite-cuprobismutite (discussed in the text and Table 3), as identified from the lattice images in Figs 3b, 4b and 8. They represent similar intermediate values to those obtained for the ‘Mix’ intervals of 5–10 µm width in the CBP (Figs 2e,f and 4a; Table 2). The Pb values representing the polysomes cluster around 4 wt.% Pb, irrespective of the number of ribbons they encompass.