Although falkmanite of the ideal composition proposed by Ramdohr & Odman (1940), Pb 3 Sb 2 S 6 , could not be synthesized even with the help of halide fluxes, a phase with composition similar to boulangerite, (Pb 5 Sb 4 S 11 ) but with dissimilar X-ray diffraction pattern was synthesized by sulfidation of bournonite at 400 and 500° C. This phase has been provisionally referred to as falkmanite in this study. Molar PbS:Sb 2 S 3 ratio in boulangerite varied from 2.41 to 2.47 and that in falkmanite varied from 2.51 to 2.57. Powder X-ray-diffraction pattern for boulangerite furnished an orthorhombic cell with a = 23.443 A, b = 21.183 A and c = 4.505 A, while that for falkmanite gave a monoclinic cell with a = 21.626 A, b = 11.409 A, c = 7.876 A and β = 96.19°. This raises doubts as to the identity of falkmanite as a member of the boulangerite homologous series, as proposed by Mozgova et al. , (1983). At 530° C, Cu-free meneghinite co-existed with falkmanite, signifying that the meneghinite solid solution field in the ternary Cu 2 S-PbS-Sb 2 S 3 touched the PbS-Sb 2 S 3 binary between 500 and 530°C. From 530 to 560° C, the proportion of Cu-free meneghinite gradually increased relative to that of falkmanite implying increased overlap of the meneghinite solid solution field with the PbS-Sb 2 S 3 binary. Falkmanite and boulangerite synthesized at 500° C and heated at 560° C for 2 hours provided similar X-ray-diffraction patterns, which corresponded with that of Cu-free meneghinite.

The minerals boulangerite, falkmanite and Cu-free meneghinite: synthesis, new powder diffraction data and stability relations

We deal with a single-crystal X-ray diffraction study of boulangerite from a plumose sample from the Bottino mine, Apuan Alps, Italy. Chemical composition of the analyzed sample is Pb 4.89 (Sb 4.08 As 0.04 Bi 0.01 ) ∑4.13 (S 10.98 Se 0.02 ) ∑11.00 . The crystal structure was investigated and evaluated in terms of the order–disorder (OD) theory. Boulangerite belongs to the subcategory Ia of OD structures composed of equivalent layers with symmetry P ( n )2 1 m . Two polytypes with maximum degree of order (MDO) are possible: MDO1 with space group P 12 1 / a 1 and unit-cell parameters a 1 ≈ 21.61, b 1 ≈ 23.54, c 1 ≈ 8.05 A, β 1 ≈ 100.7°; MDO2 with space-group P 2 1 / n 11 and unit-cell parameters a 2 ≈ 21.24, b 2 ≈ 23.54, c 2 ≈ 8.05 A, α 2 ≈ 90°. Single-crystal X-ray diffraction patterns (Mo K α, CCD detector) show strong reflections pointing to an orthorhombic substructure (the “family structure” in the OD terminology) and additional weaker reflections that correspond to the polytype MDO 1 . Frequent twinning with (100) as the twin plane was observed. The MDO1 structure was refined on a twinned crystal to R = 0.062 for 8495 reflections with F o > 4σ( F o ). Unit-cell parameters are a 21.554(4), b 23.454(4), c 8.079(2) A, β 100.76(1)°. The structure of boulangerite is composed of rods of SnS archetype six atomic layers thick and three pyramids wide. The central portion of these thick rods is characterized by ribbons of coordination pyramids of Pb and Sb atoms alternating along [001]; the marginal portions contain ribbons of coordination pyramids primarily occupied by Sb. In the refined structure, there are 18 independent cation sites: 10 are pure lead sites, six are pure antimony sites, two mixed positions split into two close sites occupied by Pb and Sb, respectively. On the basis of an OD interpretation, we assess the relationships between monoclinic and orthorhombic structures reported in the literature for boulangerite.

The plumose boulangerite from bottino, apuan alps, italy: crystal structure, od character and twinning

In the literature, many seemingly contradictory structure models of boulangerite, ideally Pb5Sb4S11, and the related mineral falkmanite have been reported. These can be explained by a phase transition between a low-temperature (LT) modification (P21/c, a ≈ 8.07 A, b ≈ 23.5 A, c ≈ 21.6 A, β ≈ 100.7°) and a disordered high-temperature (HT) modification [Pnma, a = 23.530(2) A; b = 4.0385(8) A and c = 21.273(2) A, R1(obs) = 0.039] that occurs above ca. 350–400 °C. The almost completely ordered distribution of lead and antimony cations in the LT modification involves a superstructure; the corresponding reflections are obvious in electron and X-ray diffraction patterns. Partial ordering may render them more or less diffuse. They vanish upon heating above the transition temperature, also for originally ordered natural boulangerite (from Trepca, Kosovo), and appear again after annealing below the phase transition temperature (e.g. at 330 °C). The HT phase is characterized by pronounced cation disorder. Cation ordering can be described by group-subgroup relationships which may also serve for a unified description of various structure models from literature. Starting from the orthorhombic HT phase, a translationengleiche followed by an isomorphous symmetry reduction yields the degrees of freedom required for cation ordering. The non-standard setting A21/d11 is optimal for a unified comparison of all known structure models. Varying degrees of ordering reported in literature and observed for various new samples suggest that the transition is kinetically controlled. These relationships also contribute to the long-lasting discussion about the nomenclature of minerals with the sum formula Pb5Sb4S11 or very similar ones. A precise structure determination of a single crystal (grown by chemical vapor transport) using synchrotron radiation along with detailed electron diffraction investigations and high-resolution electron microscopy confirms the statistically disordered structure model of the quenched high-temperature phase. Bond-valence sums (BVS) and interatomic distances confirm the interpretation of cation split positions as the superposition of the corresponding independent cation positions in the LT phase. Measurements of transport properties indicate very low n-type electrical and thermal conductivities for both modifications. The order-disorder transition significantly affects the Seebeck coefficient.

The Crystal Structures of Pb5Sb4S11 (Boulangerite) – A Phase Transition Explains Seemingly Contradictory Structure Models

Experimental results are presented concerning the synthesis, crystal chemistry and electrical characterization of Pb–Sb–S sulfosalts, especially boulangerite, Pb 5 Sb 4 S 11 . The crystals were grown by sublimation or chemical vapor transport using NH 4 Cl or iodine as transporting agents. Owing to its rod-like structure, boulangerite typically appears as thin needles as well as fibers with diameters down to a few nanometers. Both ordered as well as disordered structures can be produced depending on growth temperature. In addition, depending on the temperature of deposition, Pb 2 Sb 2 S 5 or halogen-bearing sulfosalts such as dadsonite or iodine-substituted pillaite can be prepared as well. By means of thermodynamic modeling, the chemical vapor-transport reaction of boulangerite is described, and an explanation for the formation of the halogen-containing compounds at lower temperatures of deposition is given. Within the homogeneity range of boulangerite Pb 5− x Sb 4+ x S 11+ x /2 with 0.35 ≥ x ≥ −0.09, the electrical conductivity σ reaches values between 10 −8 and 10 −3 Ω −1 m −1 . For Pb 2 Sb 2 S 5 , iodine-substituted pillaite and dadsonite, σ was determined to be 2.0(8) × 10 −4 , 5(2) × 10 −4 and 7(3) × 10 −4 Ω −1 m −1 , respectively.

Crystal chemistry and electrical conductivity of boulangerite, dadsonite and iodine-substituted pillaite grown by chemical vapor transport

Microprobe studies have facilitated recognition of the first documented occurrence of geocronite from Isik Dagi (Ankara, Turkey). While yenerite remains a discredited species along the PbS-Sb2S3 compositional join, our analyses show significant arsenic as did the original analyses for yenerite; thus, this name might be retained for arsenic-bearing varieties of boulangerite.

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S.C. Robinson

Papers

The identity of falkmanite and yenerite with boulangerite