Mineralogical Magazine; April 2004; v. 68; no. 2;
p. 279-300; DOI: 10.1180/0026461046820187
© 2004 Mineralogical Society of Great Britain and Ireland
Micron- to nano-scale intergrowths among members of the cuprobismutite series and pad
raite: HRTEM and microanalytical evidence
C. L. Ciobanu1,*,
A. Pring2,3,4 and
N. J. Cook1
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
* E-mail: cristiana.ciobanu{at}ngu.no
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ABSTRACT
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Coherent intergrowths, at the lattice scale, between cuprobismutite (N = 2) and structurally related pad
raite along both major axes (15 Å and 17 Å repeats) of the two minerals are reported within skarn from Ocna de Fier, Romania. The structural subunit, DTD, 3 layers of padraite, is involved at interfaces of the two minerals along the 15 Å repeat, as well as in transposition of 1 padraite unit to 2 cuprobismutite units along the 17 Å repeat in slip defects. Lattice images obtained by HRTEM across intervals of 200–400 nm show short- to long-range stacking sequences of cuprobismutite and padraite ribbons. Such nanoscale slabs mimic µm-scale intergrowths observed in back-scattered electron images at three orders of magnitude greater. These slabs are compositionally equivalent to intermediaries in the cuprobismutite-padraite range encountered during microanalysis. Hodrushite (N = 1.5) is identified in the µm-scale intergrowths, but its absence in the lattice images indicates that, in this case, formation of polysomes between structurally related phases is favoured instead of stacking disorder among cuprobismutite homologues. The tendency for short-range ordering and semi-periodic occurrence of polysomes suggests they are the result of an oscillatory chemical signal with periodicity varying from one to three repeats of 15 Å, rather than simple ‘accidents’ or irregular structural defects. Lead distribution along the polysomes is modelled as an output signal modulated by the periodicity of stacking sequences, with Pb carried within the D units of padraite. This type of modulator acts as a patterning operator activated by chemical waves with amplitudes that encompass the chemical difference between the minerals. Conversion of the padraite structural subunit DTD to the C unit of cuprobismutite, conserving interval width, emphasizes that polysomatic modularity also assists interference of chemical signals with opposite amplitudes. Observed coarsening of lattice-scale intergrowths up to the µm-scale implies coupling between diffusion-controlled structural modulation, and rhythmic precipitation at the skarn front during crystallization.
KEYWORDS: cuprobismutite, padraite, HRTEM, stacking disorder, polysomatism, Ocna de Fier, Romania
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Introduction
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AS analytical methods have evolved in the past decades, a comprehensive systematic classification of modular structures has been established covering a broad spectrum of mineral groups, e.g. silicates, sulphosalts and other complex sulphides (e.g. Merlino, 1997, and references therein). Members of most sulphosalt series, and the bismuth sulphosalts (Bi-ss) in particular, are derived from archetypal modules (e.g. PbS, SnS) and form coherent accretional (polysomatic) series that may be related one to another in a hierarchical manner (e.g. Makovicky, 1997aa). According to Makovicky (1981), homology/polysomatism in a broader sense within Pb-Bi-sulphosalts is realized by building operators that allow combinations between octahedral (111)PbS-like layers (‘H’ layers) and pseudotetragonal (100)PbS-like layers (‘T’ layers).
At odds with such highly constrained and predictable crystal-chemical variation (e.g. rigorous substitution lines within the PbS-Bi2S3- Ag2S compositional triangle for the lillianite and pavonite homologous series; e.g. Makovicky and Karup-Møller, 1977; Makovicky, 1979), are the extensive compositional fields reported for many natural Bi-ss specimens, and also the broad solid-solution ranges obtained in experimental studies (e.g. Springer, 1971; Mariolacos, 2002). The extent of, and reasons for, non-stoichiometry among Bi-ss (e.g. Mozgova, 1985) remains an open question. Non-stoichiometric compositions are often dismissed as being due to analytical errors or fine microscopic-scale intergrowths that were overlooked. Examination of apparently homogenous samples, by HRTEM revealed lattice-scale disordered intergrowths among members within the two most common Bi-ss series: bismuthinite derivatives in the aikinite-krupkaite range (Pring and Hyde, 1987; Pring, 1989) and lillianite homologues (Pring et al., 1999).
Synthetic work in the PbS-Bi2S3-(Ag2S) system, combined with HRTEM, has enabled the documentation of defect structures and stacking disorder within an accretional series (i.e. the lillianite series; Tilley and Wright, 1982; Prodan et al., 1982; Skowron and Tilley, 1986, 1990). Similarly, annealing experiments on synthetic hammarite (Cu2Pb2Bi4S9) have shown that the state of cation ordering and formation of polysome strips progresses with cooling (Pring, 1995). Together, these studies illustrate that stacking disorder rather than simple substitutional solid solution is currently a more favoured mechanism to explain compositional fields within both of these series. Moreover such nanoscale intergrowths can occur between members of different but closely structurally related series, for example between cosalite and lillianite homologues (Pring and Etschmann, 2002).
In this paper we report on disordered intergrowths among members of the cuprobismutite series and the related species, padraite. Even though cuprobismutite homologues and/or padraite have been mentioned from a total of 13 localities, there remains considerable ambiguity regarding the identity of these minerals, the extent of solid-solution ranges or substitution mechanisms. In our material from skarn ore from Ocna de Fier, southwest Romania (Cook and Ciobanu, 2003), the presence of intergrowths has been identified at scales differing by three orders of magnitude (from micron- to nanoscale). Such an ‘intergrowths-upon-intergrowths’ assemblage represents the first such documented example among Bi-sulphosalts, and closely mirrors that known for biopyriboles and humite groups (Veblen et al., 1977; Veblen and Buseck, 1979; White and Hyde, 1982a,b). We address the question as to whether this type of coarsening of lattice-scale intergrowths up to the µm-scale is related to the diffusion-controlled patterning phenomena that often occurs in skarn systems (Ciobanu and Cook, 2000, 2004).
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The cuprobismutite series and related padraite
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The cuprobismutite homologous series has the general structural formula: Cu8Me4(N–1)+2(quasi)octahedral Bi8square pyramidal S4N+16 (Makovicky, 1989) and includes three recognized minerals and two synthetic compounds (Table 1
). Even though the cuprobismutite series is not part of the Pb-Bi sulphosalt group sensu stricto, the structures can be considered in terms of the periodic intergrowth of (331)PbS-like octahedral layers with layers of metals in tricapped trigonal-prismatic coordination (Makovicky, 1989). Each member contains a layer with Cu atoms linked with chains of Bi2S4, the so-called ‘C’ layer, common to all members. These layers alternate with a second type of layer where Cu atoms are linked to ribbons of Bi with octahedral and square-pyramidal coordination. Incremental width variation of this second layer, achieved by the addition of more square-pyramids of Bi accounts for accretional homology between members with integer N number within the series (Kodra et al., 1970; Ozawa and Nowacki, 1975; Mariolacos et al., 1975). Makovicky (1997b) describes poly-somatism within the cuprobismutite series as coherent intergrowths of incremental octahedral slabs with trigonal prismatic slabs (Table 1, Fig. 1a); and denoted the species by: N1, N2 = (1,1), (1,2), (2,2). Mumme (1986) described the structurally related phase, padraite, (Cu5.5Ag1.1Pb1.2Bi11S22; Mumme and Zák, 1985) and although Pb-bearing, padraite has unit-cell dimensions closely resembling those of hodrushite, a member of the cuprobismutite series (Kup
ik and Makovicky, 1968). The structural refinement for padraite was based upon a slightly different chemistry: an ideal formula of Cu6AgPb2Bi11S22, assuming a ratio of Me:S equal to 20:22, rather than 19:22 derived from the empirical formula. The formula was recalculated by Mumme (1986) to give Cu5.9Ag1.3Pb1.6Bi11.2S22, thus matching both the charge balance and structural data.
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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.
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Mumme (1986) stressed common 17.5 Å repeats for all phases within the cuprobismutite series and in padraite. This 17.5 Å repeat is given by two CuBiS10 lozenges arranged en echelon at the margins of the C layer (with pairs of Bi2S7 and CuS4 groups in the middle). These lozenges consist of pairs of BiS6 octahedra attached to a square pyramidal Cu; seen in each of the structures under consideration (Fig. 1b
). Mumme (1986) discussed the layer realized by insular octahedra (H configuration) which links the C layers in hodrushite and Cu4Bi5S10. We will denote this as (H). Differences among individual members of the cuprobismutite series are achieved by the presence (or absence) of another layer, the ‘D’ layer, a one octahedron (H configuration) wide strip that follows lozenges en echelon parallel to (331)PbS. A further difference is the distinct stacking combinations between C and D layers (Fig. 1, Table 1). At the inner part of the double C layers, as found in Cu4Bi4S10 and hodrushite, arrays of insular H octahedra [C(H)C] are present. In such an approach, the structure of Cu4Bi5S10 is a sequence of C(H)C(H) layers (with no D layer); cuprobismutite consists of alternating DCDCD layers, whereas hodrushite is a sequence of (H)CDC(H)CD layers.
The D layer, parallel to (331)PbS is also in the padraite structure, and this layer incorporates the Pb atoms. The D layer also incorporates Cu atoms, which are not part of the C layer (Fig. 1b). The Pb atoms have distorted octahedral coordination that can be considered as sheared pseudo H-layers [(111)PbS]. There are two modules of 2D layers, even though each of them has a variable 1–2 octahedra thickness (Fig. 1b). These are separated by a T layer, which has a (100)PbS-like structure and contains Bi4S12 strips with adjacent trigonally co-ordinated Cu. These Bi4S12 strips are linked along the c axis by pairs of AgS4 tetrahedra. Therefore, along the c axis the D module is either one octahedron thick at the T contact, or two octahedra thick along the trigonal-tetrahedral match. The difference between hodrushite and padraite can thus be defined in a polysomatic manner (Fig. 1b), on the basis of the layer stacking sequence; padraite is TDDCDDTDDC. Therefore, padraite has a similar repeat to cuprobismutite (DCD), whereas hodrushite includes the double C repeat ‘C-C’ linked by D layers; i.e. CCDCCDCC (Table 1).
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Sample description
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The material used in this study comes from Paulus Mine, in the northern part of the 76.5 Ma Fe-(Cu)-(Zn-Pb) skarn at Ocna de Fier, southwest Romania (Ciobanu et al., 2002a). The locality is known for the occurrence of bismuth sulphosalts, and especially for specimens consisting of fine intergrowths, e.g. co-type samples of ‘rezbanyite’ (Zák et al.; 1992). In this new Paulus occurrence (the mineral collector Constantin Gruescu discovered this occurrence during the 1980s), dense pockets of Bi-ss form swarms within an area several metres in width in massive magnetite ore on the 206 m mining level. Intergrowths are abundant and have varied morphologies. The intergrowths span the bismuthinite derivative series and occur with makovickyite and galenobismutite (Ciobanu and Cook, 2000). Cook and Ciobanu (2003) reported compositional and textural data for cuprobismutite, padraite and occasional hodrushite from an individual pocket, several cm in size, from the same occurrence.
As seen in Table 1, the empirical formulae for padraite from Ocna de Fier, calculated for 42 a.p.f.u., is close to the ideal Cu7(Ag,Pb)2Bi11S22, thus differing from TL padraite (ideal Cu6AgPb2Bi11S2) with respect to the Cu:Ag:Pb ratio. The Me:S ratio, however, is close to 20:22, the ratio used for the structural refinement. A similar formula for padraite, close to ideal Cu7(Cu,Pb)2Bi11S22, was obtained for material from another locality (Swartberg, South Africa; Table 1; Ciobanu et al., 2002b). This allows us to speculate that the number of Pb atoms in padraite might be fixed at 2 (at 42 a.p.f.u.), while Ag can be absent. Variation in the Cu:Ag:Pb ratio can be written as a substitution mechanism Ag(Cu) + Bi > 2Pb. Further variation of the Cu:Bi ratio, beyond a fulfilment of Pb positions, would imply structural modifications and indicate a possible padraite series (Ciobanu et al., 2002b). However, a structural refinement of Ag-free padraite is required to substantiate this hypothesis.
Cuprobismutite, the most abundant phase in the material, tends to develop thin prisms (5–10 µm) with pyramidal termination (Fig. 2a). It also occurs as narrow bands within a matrix of finely intergrown, and apparently homogeneous ‘patches’ having an intermediate composition between padraite and cuprobismutite (Fig. 2b). However, more common are aggregates of cuprobismutite homologues with padraite (CBP; Fig. 2c), 100–200 µm in diameter, with angles of 90–120° between laths randomly distributed within a coarse-grained matrix of oversubstituted bismuthinite (BD10 – the BD index represents the aikinite number calculated after Makovicky and Makovicky (1978)). Minor hodrushite is occasionally included in the aggregates (Fig. 2d). The stepwise arrangement of laths within some of the orthogonal CBP aggregates, with lath-width reducing towards the interior of the aggregates, suggests similarities with patterns developed during skeletal growth. Cuprobismutite has a strong idiomorphic tendency where formed against laths dominated by padraite compositions (Fig. 2e). Lamellae within CBP aggregates mainly consist of alternating cuprobismutite and padraite (Fig. 2f).
Makovickyite is almost always present in small quantities between lamellae of CBP. In contrast to the entire occurrence in Paulus, in this particular suite of samples, the compositional range of the bismuthinite derivatives is restricted to a limited range between gladite and bismuthinite (BD33- BD2–3). Krupkaite and coarse gladite-krupakaite intergrowths are also observed in minor amounts. In reflected light, equilibrium crystal boundaries with 120° triple joint points are commonly observed between BD10 grains.
Micron-scale intergrowths
There are well-defined compositional variations between certain minerals in individual aggregates. Even though hodrushite is part of these aggregates, intermediate composition intervals are in the cuprobismutite-padraite range rather than the cuprobismutite-hodrushite range. The orthogonal CBP aggregates (Fig. 2c) are formed by laths 40–60 µm wide, which consist of alternating sequences of cuprobismutite (Cbs) and either padraite (Pad) or mixed (‘Mix’) domains with intermediate compositions between the two minerals.
Two such sequences are shown in Fig. 2e and f, across laths of 55–60 µm within CBP. In both cases, an interval 5 µm wide represents the stepwise variation of Pb between 1 wt.% (Cbs) and 6–7 wt.% (Pad). This interval is also seen as the smallest width of individual cuprobismutite needles (Fig. 2a). The ‘Mix’ interval is found instead of either one or the other mineral in a regular Cbs.Pad.Cbs.Pad sequence (Fig. 2e), or as a shoulder between intervals of Cbs and Pad (Fig. 2f). If the differences in wt.% Pb between two consecutive intervals are plotted (dPb; Fig. 2g,h), we obtain, in both cases, a bimodal variance across the entire sequences. The first sequence is more regular, with dPb = 3 wt.%, whereas the second sequence combines two steps of dPb = 3 wt.% and dPb = 6 wt.%, respectively. This bimodal inversion of the Pb gradient across individual laths can be considered as the most contrasting chemical signal controlling the appearance of µm-scale intergrowths within CBP.
Similarly, the bismuthinite derivatives in the sample also display a wide range of µm-scale intergrowths. Here, the observed lamellae are even smaller than in cuprobismutite, not exceeding 1–2 µm in width. Commonly, fields of fine pencil-like intergrowths of BD32-17 are nested within rectangular aggregates of CBP. Between the two phases at the ends of the compositional range, i.e. BD10 and BD32, there is a four-fold difference in Pb (3 vs. 12 wt.%), coupled with a three-to four-fold difference in Cu (1.2 vs. 4.3 wt.%). However, an intermediate composition, BD17, may correspond to the finest lamellae within the intergrowths fields rather than gladite (BD32). Such a composition would give appreciably smaller ratios (a factor of two) for Pb (3 vs. 6 wt.%) and (Cu 1.2 vs. 2 wt.%) compared to BD10. Wider lamellae of gladite are meshed across these fields, or outline the margin of makovickyite. Gladite also occurs as larger grains, 50–100 µm in diameter, enveloped by halos consisting of lamellar intergrowths dissipating within the matrix. Rarely, at the inner part of such gladite grains, cores of krupkaite composition are observed that dissipate into gladite. Surprisingly, makovickyite is homogenous (Cu, Ag and Pb = 5, 5 and 2 wt.%) in the assemblage, even though this phase is known for its abundant basket-weave intergrowths (e.g. Zák et al., 1994).
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Methodology
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Electron microscopy
Several fragments of the Bi-ss material were taken from the same sample that provided the polished blocks (GS) analysed previously (Cook and Ciobanu, 2003). The fragments were ground under acetone in an agate mortar and the resultant suspension was dispersed on Cu grids coated with holey-carbon support films. The grids were analysed using a 200 kV Philips CM200 electron microscope fitted with a standard side-entry goniometer (360°), objective lens with Cs = 2.00 mm and a W filament. This configuration gives a point-to-point resolution of 2.8 Å. The crystal fragments seen over the holes in the film were tilted into the [010] zone. Lattice images and diffraction patterns were taken at 250,000xmagnification using exposure times of 2–4 s. A series of image simulations was performed by the conventional multi-slice method, using local programs based on the routines by G.R. Anstis and T.B. Williams (pers. comm.) in order to establish the criteria for image interpretation.
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Results
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In Fig. 3a, we show an electron diffraction pattern for cuprobismutite down to [010], corresponding to the lattice images in Figs 3b and 4b. The pattern shows a net, 1/9.75 and 1/7.75 Å –1 in size, at an angle of 100°, however the absences due to the space group C2/m are such that only rows with h = 2n are present. The streaking along the main rows in the diffraction pattern, and the weak streaked lines of intensity between the main h0l rows are both due to the disordered intergrowth of padraite.
Individual or paired satellite reflections visible in the diffraction pattern correspond to the padraite cell. The relationship between the diffraction patterns is acbs || cpad and apad is otated 6° from ccbs. The streaking in the diffraction pattern is due to lattice-scale disorder seen in the images (Figs 3b and 4b). These lattice images show thicker and brighter rows due to padraite within cuprobismutite (Figs 3b and 4b). Interpretation of the images is verified by computer simulation of padraite and cuprobismutite (down to [010]; Fig. 4b, insets). Differences between the appearance of the padraite strips in Figs 3b and 4b are due to the variable thickness of the crystals; at the left side of Fig. 4b several rows of padraite appear as in Fig. 3b.
Nano-scale intergrowths
The lattice images (Figs 3b and 4b) represent intervals of 370 and 270 nm, respectively, with intergrowths on the scale of 10 to 100 Å, i.e. three orders of magnitude smaller than the 5 µm interval observed for the variation of Pb and Ag across CBP laths by microanalysis. Even though intergrowths <~1 µm are beyond microanalytical resolution limits of scanning electron microscope and microprobe techniques, they can nevertheless be distinguished in back-scattered electron images of mixed material with compositions intermediate between cuprobismutite and padraite, especially at the boundary between two different compositional domains (e.g. Fig. 2b,e). Alternatively, a compositional transition can be seen in larger domains (~20–30 µm wide), when the Pb/Ag bimodal inversion takes place across a couple of strongly contrasting, 5 µm wide lamellae, i.e. Fig. 3a, Table 2. We assume that both styles of transition observed within domains of intermediate composition can include lattice-scale disorder, as these are compositionally equivalent to intermediates in the cuprobismutite–padraite range.
The lattice image in the lower part of Fig. 3b shows an area of intergrowths 370 nm wide, in which 
of the unit cells are padraite (38 units) within 
cuprobismutite (130 units). The padraite units are scattered through the crystal in what appears to be a random manner. However, single isolated units of padraite are rare; instead regular alternation of padraite and cuprobismutite units is common, often in groups of 3 or 4 repeats. Sometimes, the units of padraite are separated by two or even three cuprobismutite units. This suggests that ordered intergrowths of padraite and cuprobismutite might occur and even be stable as long repeat homologues. Such intergrowth homologues would have the general form PadNCbsPad, and the stacking of individual slabs can be written as TDDCDDT/(DCD)1–3/TDDCDDT.
In order to achieve such a layer stacking, there are two possible ways to coherently intergrowth the layer sequences of padraite and cuprobismutite. This first is through common D layers (Fig. 5a). The second is linking through common C layers (Fig. 5b,).
We draw attention to the fact that DCD sequences (in italics in the preceding formalism) that formalize cuprobismutite units are also found in the middle part of padraite. Even though there are differences introduced by the peculiar 2D configuration in padraite, their similarity permits a coherent stacking sequence. Therefore the lattice image can be interpreted as a regular matrix of DCDC..., with insertion of coupled DTD—DTD groups at intervals that vary as DCD, DCDCD, DCDCDCD within a 4xPadNCbsPad slab.
In contrast, the lattice image in Fig. 4b shows an alternation of wider slabs of well-ordered padraite and cuprobismutite (50–100 nm wide). Such a combination approximates the style of compositional variation shown in Fig. 4a, at scales differing by an order of magnitude.
The sequence shown in Fig. 4b shows three distinct intervals as follows: disordered padraite, ordered cuprobismutite and disordered cuprobismutite (~140 nm, 100 nm and 30 nm wide, respectively). The disorder in padraite is seen at both ends of the interval as insertion of cuprobismutite rows in repeats of 1–3 unit cells resembling similar PadNCbsPad modules as in the previous image. However, ordered intergrowth sequences of padraite and cuprobismutite (PadNCbsPad modules) are absent and the inter-growths are more irregular. Nevertheless, a similarity to the previous case is seen in the partial short-range order of the modules at the left end of the interval, even though they here include repeats of PadNCbsPadPadNCbsPad(NCbs = 1–3), which doubles the inner padraite row in the sequence. Repeats that share the inner padraite (T layer) unit are seen further to the right, at the end of padraite to the boundary against ordered cuprobismutite.
The layer stacking for such a disordered padraite sequence (2Pad/2Cbs/2Pad) reads as: TDDCDDTDDCDDTDDCDCDDTDDCD DTDDCDDT. It consists of 6 unit cells combined with a ratio of 2:2 and 10 repeats of 3-layer modules (DCD, DTD, CDC) of ~15 Å wide each. We point to the fact that a repeat of CDC layers corresponding to a unit of hodrushite encompassed in a group of two cuprobismutite units within such modules, even though the image resolution does not allow us to discriminate between a D layer and a row of insular H modules. There is, therefore, a degree of uncertainty in this assumption.
The equivalent 10 repeats, (of the same 3-layer modules) sequence found in the disordered cuprobismutite with 1:2 ratio (Pad/2Cbs/Pad/2Cbs/Pad) reads: TDDCDDTDDCDCDDTD DCDDTDDCDCDDTDDCDDT and encompasses 7 instead of 6 unit cells. In both of the sequences considered, the lattice disorder can be seen as regular insertion of a DTD type of module (with TDPad/DCbs and DCbs/DTPad at Pad/Cbs joints) in a matrix where DCD and CDC modules alternates by rhythms of 2:1 (padraite) or 1:1 (cuprobismutite), respectively. Bearing in mind that the Pb atoms are included in the D layer adjacent to T, recognition of such DTD modules as markers for the state of disorder in the cuprobismutite-padraite range has implications for defining the smallest intervals that can be associated with Pb distribution during diffusion-controlled crystallization, growth or reaction.
Using average compositions for cuprobismutite and padraite at Ocna de Fier, we can calculate the compositions for the various slabs shown in Figs 3b and 4b (Table 3). Even though Slab 2 has Pb contents (2.76 wt.%) that are 1 wt.% lower than Pb values obtained across the profile in Fig. 4a (3.6–5.8 wt.%; Table 2), the rest of the short- or long-period slabs have calculated Pb values in a comparable range (3.05–5.36 wt.%; Table 3). Similarly, the Ag values obtained for mixed points along the profile in Fig. 4a (1.06–1.73 wt.%) are comparable to the calculated values for the slabs (1.32–2.06 wt.%). Therefore, we further stress that the mixed material in the intergrowths at the µm-scale is represented by comparable compositions at the lattice scale.
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TABLE 3. Calculated compositions for various strips and short/long-range slabs using mean compositions for Cbs and Pad (Table 1).
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Jumps between sequences of cuprobismutite-padraite intergrowths
In Fig. 6, we show a slip in the stacking sequence of a disordered intergrowth of padraite ribbons in cuprobismutite. The slip defect is ~25 nm wide, consisting of seventeen 15 Å repeats. A continuous ribbon of padraite defines the left boundary of the defect. The entire intergrowth lies in the centre of a well-ordered cuprobismutite (320 nm). The stacking sequence of units below the slip defect (from the left is: Cbs/Pad/4Cbs/Pad/2Cbs/Pad/4Cbs, and above: Cbs/Pad/Cbs/Pad/3Cbs/Pad/2Cbs/2Pad. On either side of the defect, the cuprobismutite lattice is regular and coherent, with minimal strain. If we ignore the first row of padraite from the left, the layer sequences for the two chains with Pad/2Cbs ‘slips’ between each other within the intergrowth area are: Slab 1 [3Cbs/Pad/2Cbs/Pad/4Cbs] (lower area on figure) DCDCDCDDTDDCDDTDDCDCDDTDDCDDTD DCDCDCDCDC and
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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.
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Slab 2 [Pad/3Cbs/Pad/2Cbs/2Pad] (upper area on figure)
DTDDCDDTDDCDCDCDDTDDCDDTDDCDCD DTDDCDDTDDCDDTD.
The lattice image suggests that there is a single type of ‘switch’ between the two slabs, with one Pad transposed into two Cbs units. Using the structures of cuprobismutite (Ozawa and Nowacki, 1975) and padraite (Mumme, 1986), we can model such a transformation (Fig. 7a) by slicing along the following chains of atoms: (S 19 Bi 7 S 22 ) D 
T -(S 11 Bi 3 S 10 Bi 2 S9 ) TD –(S9Bi1S8-S14)DC-(S20Bi8S19)DT in padraite and (S 3 Bi 4 S 6 S 5 Bi 3 S 2 ) C –(S2 Bi 1 S 2 ) D -(S2Bi3S5S5)C–(S3Bi2S1)D in cuprobismutite. In terms of layers, a DTD sequence in padraite is transposed into a C layer in cuprobismutite (Fig. 7b). The boundary between the 2 slabs is curved, with maximum amplitude of aCbs.
Replacing the groups DTD with C, the two slabs including the first padraite row between Cbs units, become:
(1): DCDCDCDCDCD/CDCDCDCDCDCDCDCDCDCDCDCDCDCDCD/C and
(2): DCDCDCDCDCD/CDCDCDCDCDCDCDCDCDCDCDCDCDCDCDC.
By using the polytypic approach of Mumme (1986), we obtained similar DCDC... codes for the two chains. However, it should be mentioned that the C...C insertion in a regular DC matrix is a chemical marker for the 2 Pb atoms present in padraite. These slip defects where padraite ribbons can be intergrown coherently with two cuprobismutite ribbons, emphasizes the close structural relationships and modular nature of these structures.
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Discussion
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The data show that coherent intergrowths, at the lattice scale, between cuprobismutite and padraite are possible along both major axes (the 15 Å and 17 Å repeat) of the two minerals. The same structural subunit, DTD, three layers of the padraite structure, is involved in Pad/Cbs interfaces along the 15 Å repeat, as well as coherent intergrowth between Pad/2Cbs along the 17 Å repeat in slip defects.
A schematic diagram of twenty 15 Å units (30 nm), encompassing the 4 Pad(NCbs)1-3Pad slabs, is shown in Fig. 8. If we consider the intergrowths through the D layer along the 15 Å axis, the sequences represent 3 nm units of 7 layers each. If the presence of Pb is denoted by an output signal ‘1’, and the absence of Pb by ‘0’, then the insertion of 2 Pb in each 2D module of one padraite unit is denoted as 101 in terms of periodic signal. The stacking of the two different mineral ribbons allow different forms of the chemical modulation signal to be expressed. The width of the cuprobismutite slabs can have periodicities of between x3 and x11 layers Pad(NCbs)1-3Pad. Slab 1 (characteristic for padraite-rich regions) modulates 101[3gaps]101 [11gaps]101[3gaps] 101[11gaps], whereas slab 2 (seen in more cuprobismutite-rich regions) is a regular structural modulator (for the signal 101[11gaps]101[11gaps]). By taking the two other observed NCbs (NCbs = 1,3), for example, within a padraite slab (Fig. 8; slabs 3 and 4), we obtain the signals 101[3gaps]101[9gaps]101 [3gaps]101, and 101[3gaps]101[13gaps]101 [3gaps]101. Such variants of the chosen structural modulator, seen as long-period polysomes, can readily be interpreted as ‘accelerators’ or ‘decelerators’ of a variable input chemical signal. Nevertheless, the short-range polysome, containing two Cbs units positioned between Pad units (i.e. ‘Modulator 2’), is the most common in the images, either in Cbs or Pad strips Their long - range polysomes [4xModulator 2] are most predictable; calculated compositions are given in Table 3.
Across the low-magnification strip in Fig. 3b, we see a tendency towards a banding induced by variable combinations of Pad.(NCbs)1-3.Pad modules. We can therefore interpret the lattice image as a series of irregular bands ~30 nm wide, comprising a more-or-less regular sequence of a modular 101 signal with periodic 11 gaps. To the left of the image, a group of eight PadCbsPad repeats, the presence of only one Cbs in the middle suggests that serial accelerators can occasionally occur within intervals an order of magnitude greater than the modular banding (only one within 370 nm). A comparable type of banding, realized by insertion of structural modulators of both types 1 and 2, can be seen in Fig. 4b. We can also see that the modulators are inserted at both boundaries of regularly stacked cuprobismutite, ~100 nm wide; further evidence for a banding tendency.
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Genetic implications
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The chemical oscillations are encoded in the ratios between Pad-Cbs ribbons along the stacking sequence, with period varying from one-to-three repeats of 15 Å (Fig. 8). The Pb distribution can be seen as representing an output signal that is modulated by the periodicity of stacking sequences. The Pb is carried in the D units of padraite. In this way, the intergrowth of padraite ribbons in the cuprobismutite matrix represents a digital type of nanoscale structural modulator for the Pb distribution. This type of polysomatic modulator acts as a patterning operator and can be activated by chemical waves with amplitudes that encompass the chemical difference between the two distinct minerals. On the other hand, conversion of the padraite structural subunit DTDPad to the C unit of cuprobismutite, CCbs, along the 17 Å repeats, conserving the width of the interval as in Fig. 6, emphasizes that polysomatic modularity can also assist interference of chemical signals with opposite amplitudes (Fig. 9). The DTDPad/CCbs ‘switchers’ modulate the ‘jumps’ across the two different Pad-Cbs slabs at interference nodes between two chemical waves in opposite phase.
Further evidence for chemical constraints in activating the structural modulators are the µm-scale, regular, alternating intervals with compositions in the padraite-cuprobismutite range. As noted above, the Pb gradient across a single lath in the CBP aggregates has a bimodal variation with jumps of ~3 wt.% amplitude across an interval of 5 µm. An inverse variation for Ag is coupled to the Pb variation. However, the difference in wt.% Ag between the two phases under discussion is rather small (some 1–2 wt.%), in comparison with that of Pb. The assumption that a certain minimum gradient in chemical signal is necessary to activate structural modulation is confirmed by the absence of hodrushite units in the stacking sequences, despite the fact that this mineral is occasionally present in the CBP aggregates. Hodrushite stability is controlled by the substitution ratios of Fe (Kupik and Makovick
, 1968; Kodra et al., 1970). Iron may therefore represent a similar type of chemical variable for structural modularity in the cuprobismutite series. Such smaller chemical differences may nevertheless attract structural modulations in a different assemblage. As stressed, previously in our material the intermediate compositions in the ‘Mix’ intervals considered at the µm-scale, are observed so far only in the cuprobismutite–padrange.
Even though diffusion persists in sulphides to very low temperatures (e.g. Pring et al., 1999), a diffusion-controlled crystallization process may be able to steadily lock in an intermediary compound in a modular series, if the free energy difference between the many possible stacking sequences were small. We believe that in these sulphosalt systems, the differences in free energy which stabilize various stacking sequences are small. Such preservation of intermediary compounds has been documented previously for minerals in the sartorite group (e.g. Pring, 2001) and also for the lillianite homologues (Pring et al., 1999).
In our material, we have illustrated the type of µm-scale intergrowths that are reproduced at the nanoscale as irregular slabs of Pad-Cbs. Indeed, the tendency for short-range ordering of stacking sequences and their semiperiodic occurrence in the lattice images indicates that they are the result of an oscillatory chemical signal rather than simple ‘accidents’ or irregular defects. Mean Pb and Ag compositions (in wt.%) for various short/long-period polysomes, stable over the padraite-cuprobismutite range represent similar intermediate values to those obtained for the ‘Mix’ intervals 5–10 µm wide in the CBP (Fig. 2e,f). The Pb values representing the polysomes cluster around 4 wt.% Pb, irrespective of the number of ribbons they encompass (Fig. 10).
Coarsening of banding from the lattice- to µm-scale can be seen in the Pad-Cbs range. Such phenomena have been reported in other polysomatic series (e.g. biopyriboles; Veblen et al., 1977). However, a patterning operator is required to enhance the coupling between chemical signal and structural modulation over scales differing by three orders of magnitude. One of the readily found patterning operators in a skarn environment is Liesegang banding (e.g. Ortoleva, 1994), involving an adjustment between diffusion rates and structural modulation seen in the lattice coupled to rhythmic precipitation.
The deposit at Ocna de Fier is known for its abundant rhythmic textures involving magnetite (von Cotta, 1864; Ciobanu and Cook, 2004), many of which can be interpreted as Liesegang phenomena (e.g. Kissling, 1967). The types of modular accelerators and decelerators for a chemical signal seen in Fig. 8, as well as the interference switchers in Fig. 9, are good indicators that Liesegang banding might be a suitable mechanism to explain the intergrowth-upon-intergrowth packages in our material. However, we consider that the data presented here are only a preliminary step towards a numerical model that could substantiate this hypothesis.
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Acknowledgements
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Funding from the South Australian Museum is gratefully acknowledged for two of us (CLC, NJC) to visit Adelaide and conduct the micro-analytical work. The senior author also gratefully acknowledges a NATO post-doctoral fellowship, focusing on the geological significance of sulphosalts, tellurides and selenides in a range of Au-bearing deposits. The assistance of Peter Self and the rest of the CEMMSA staff, Adelaide, with electron microscope operation are gratefully acknowledged. Discussions with Emil Makovicky and Dan Topa have helped us to formulate some of the ideas expressed in this paper. Constantin Gruescu is thanked sincerely for making the sample material available to us. The editor of this special issue, Chris Stanley, and an anonymous reviewer are acknowledged for their comments that helped to improve this manuscript. Lastly, we dedicate this work to the memory of Alan Criddle, with respect to his extraordinary contributions to ore mineralogy.
Dedicated to the memory of Dr A. J. Criddle, Natural History Museum, London, who died in May 2002
[Manuscript received 2 February 2003:
revised 29 September 2003]
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References
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ABSTRACT
Introduction
