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Density functional theory (DFT) is used to obtain structural information of seven members of the tetradymite homologous series: Bi2Te3 (tellurobismuthite), BiTe (tsumoite), Bi4Te3 (pilsenite), Bi5Te3, Bi2Te, Bi7Te3 (hedleyite), and Bi8Te3. We use the formula S(Bi2kTe3)·L[Bi2(k+1)Te3] as a working model (k = 1–4) where S and L are short and long modules in the structures. The relaxed structures show an increase in the a parameter and decrease in the interlayer distance (dsub) from Bi2Te3 (2.029 Å) to Bi8Te3 (1.975 Å). DFT-derived formation energy for each phase indicates that they are all thermodynamically stable. Scanning transmission electron microscopy (STEM) simulations for each of the relaxed structures show an excellent match with atom models. Simulated electron diffractions and reflection modulation along c* are concordant with published data, where they exist, and with the theory underpinning mixed-layer compounds. Two modulation vectors, q = γ· c sub ∗ (γ = 1.800–1.640) and qF = γF· d sub ∗ (γF = 0.200–0.091), describe the distribution of reflections and their intensity variation along d sub ∗ = 1/dsub. The γF parameter reinforces the concept of Bi2kTe3 and Bi2(k+1)Te3 blocks in the double module structures, and γ relates to dsub variation. Our model describing the relationship between γ and dsub allows prediction of dsub beyond the compositional range considered in this study, showing that phases with k >5 have values dsub within the analytical range of interlayer distance in bismuth. This, in turn, allows us to constrain the tetradymite homologous series between γ values of 1.800 (Bi2Te3) and 1.588 (Bi14Te3). Phase compositions with higher Bi/Te should be considered as disordered alloys of bismuth. These results have implications for mineral systematics and classification as they underpin predictive models for all intermediate structures in the group and can be equally applied to other mixed-layer series. Our structural models will also assist in understanding variation in the thermoelectric and topological insulating properties of new compounds in the broader tetradymite group and can support experimental work targeting a refined phase diagram for the system Bi-Te. [ABSTRACT FROM AUTHOR]

An enrichment in nickel (Ni) or platinum group elements (PGE) is seldom observed in ores of the iron oxide–copper–gold (IOCG) type. This phenomenon is, however, known from a few deposits and prospects in the Carajás Mineral Province, Brazil. The Ni-PGE enrichment is explained, in part, by the spatial association of the IOCG-type ores with altered mafic-ultramafic lithologies, as well as by reworking and remobilization of pre-existing Ni and PGE during multiple mineralization and tectonothermal events across the Archean-Proterozoic. One such example of this mineralization is the Jatobá deposit in the southern copper belt of the Carajás Domain. This is the first detailed study of the Ni and PGE mineralization at Jatobá, with implications for understanding ore genesis. Petrographic and compositional study of sulfides shows that pyrite is the main Ni carrier, followed by pyrrhotite and exsolved pentlandite. Measurable concentrations of palladium (Pd) and platinum (Pt), albeit never more than a few ppm, are noted in pyrite. More importantly, however, the trace mineral signature of the Jatobá deposit features several platinum group minerals (PGM), including merenskyite, naldrettite, sudburyite, kotulskite, sperrylite, and borovskite. These PGM occur as sub-10 µm-sized grains that are largely restricted to fractures and grain boundaries in pyrite. All Pd minerals reported contain mobile elements such as Te, Bi, and Sb and are associated with rare earth- and U-minerals. This conspicuous mineralogy, differences in sulfide chemistry between the magnetite-hosted ore and stringer mineralization without magnetite, and microstructural control point to a genetic model for the sulfide mineralization at Jatobá and its relative enrichment in Ni and PGE. Observations support two alternative scenarios for ore genesis. In the first, an initial precipitation of disseminated or semi-massive Ni-PGE-bearing sulfides took place within the mafic rock pile, possibly in a VHMS-like setting. Later partial dissolution and remobilization of this pre-existing mineralization by mineralizing fluids of IOCG-type, possibly during the retrograde stage of a syn-deformational metamorphic event, led to their re-concentration within magnetite along structural conduits. The superposition of IOCG-style mineralization onto a pre-existing assemblage resulted in the observed replacement and overprinting in which PGE combined with components of the IOCG fluids like Sb, Bi, and Te. An alternative model involves leaching, by the IOCG-type fluids, of Ni and PGE from komatiites within the sequence or from ultramafic rocks in the basement. The discovery of PGM in Jatobá emphasizes the potential for additional discoveries of Ni-PGE-enriched ores elsewhere in the Carajás Domain and in analogous settings elsewhere. [ABSTRACT FROM AUTHOR]

Density functional theory methods are applied to crystal structures and stabilities of phases from the aleksite homologous series, PbnBi4Te4Sn+2 (n = homologue number). The seven phases investigated correspond to n = 0 (tetradymite), 2 (aleksite‐21R and ‐42R), 4 (saddlebackite‐9H and ‐18H), 6 (unnamed Pb6Bi4Te4S8), 8 (unnamed Pb8Bi4Te4S10), 10 (hitachiite) and 12 (unnamed Pb12Bi4Te4S14). These seven phases correspond to nine single‐ or double‐module structures, each comprising an odd number of atom layers, 5, 7, (5.9), 9, (7.11), 11, 13, 15 and 17, expressed by the formula: S(MpXp+1)·L(Mp+1Xp+2), where M = Pb,Bi and X = Te,S, p ≥ 2, and S and L = number of short and long modules, respectively. Relaxed structures show a and c values within 1.5% of experimental data; a and the interlayer distance dsub decrease with increasing PbS content. Variable Pb—S bond lengths contrast with constant Pb—S bond lengths in galena. All phases are n‐fold superstructures of a rhombohedral subcell with c/3 = dsub*. Electron diffraction patterns show two brightest reflections at the centre of dsub*, described by the modulation vector qF = (i/N) · dsub*, i = S+L. A second modulation vector, q = γ · csub*, shows a decrease in γ, from 1.8 to 1.588, across the n = 0 to n = 12 interval. The linear relationship between γ and dsub allows the prediction of any theoretical phases beyond the studied compositional range. The upper PbS‐rich limit of the series is postulated as n = 398 (Pb398Bi4Te4S400), a phase with dsub (1.726 Å) identical to that of trigonal PbS within experimental error. The aleksite series is a prime example of mixed layer compounds built with accretional homology principles. [ABSTRACT FROM AUTHOR]