Lillianite Homologous Series Research Articles

Structural rationale for twinning in galena, and a possible relation with the lillianite homologous series – Revision 1

Structural rationale for twinning in galena, and a possible relation with the lillianite homologous series – Revision 1

Lazerckerite, Ag3.75Pb4.5(Sb7.75Bi4)S24, from Kutná Hora, Czech Republic: a new Sb–Bi member of the andorite branch of the lillianite homologous series

Abstract. Lazerckerite, ideally Ag3.75Pb4.5(Sb7.75Bi4)S24, is a new mineral species found in medieval mine dumps of the historic Ag–Pb–Zn Kutná Hora ore district, Czech Republic. The mineral is associated with other Sb–Bi lillianite homologues (terrywallaceite, gustavite, holubite) and Ag,Bi-bearing galena, most frequently as grain aggregates and replacement rims of earlier Ag–Pb–Bi minerals, growing together in aggregates of up to 0.6×0.3 mm. Lazerckerite is opaque, is steel grey in colour, and has a metallic lustre; the calculated density is 5.920 g cm−3. In reflected light lazerckerite is greyish white, and bireflectance and pleochroism are weak with grey tints. Anisotropy is weak to moderate with grey to bluish-grey rotation tints. Internal reflections are not observed. Electron microprobe analyses yielded the empirical formula, based on 44 apfu, (Ag3.61Cu0.04)Σ3.65(Pb4.55Fe0.01Cd0.01)Σ4.57(Sb7.87Bi3.75)Σ11.62(S24.15Se0.01)Σ24.16. Its unit cell parameters are a=13.2083(9), b=19.4595(8), c=8.4048(13), β=90.032(7)°, V=2160.3(4) Å3, space group P21/c, and Z=2. The structure of lazerckerite contains two Pb sites (Pb1 and Pb2) in bicapped trigonal prismatic coordination, 8 independent octahedral sites, and 13 distinct sulfur positions. Four of the octahedral sites are mixed (Sb,Bi) and (Bi,Sb) sites, one is a mixed (Ag,Bi) site, and one is a mixed (Sb,Pb) site. The new mineral belongs to the andorite branch of the lillianite homologous series with N=4 and is a new addition to the group of Sb–Bi mixed members of the series. Lazerckerite is defined as a lillianite homologue with the three following requirements: N=4, L (Ag++ (Bi3+, Sb3+) ↔ 2 Pb2+ substitution) ≈ 90 %–95 %, and approximately one-third of atom percentage of antimony is replaced by bismuth (Bi/(Bi+Sb) ≈ 0.30–0.38). The new mineral has been approved by the Commission on New Minerals, Nomenclature and Classification (CNMNC) of the International Mineralogical Association (IMA 2022-113) and named after Lazarus Ercker, the supreme Royal Bergmeister of the Kingdom of Bohemia and the Master of Prague Mint.

Holubite, Ag3Pb6(Sb8Bi3)Σ11S24, from Kutná Hora, Czech Republic, a new member of the andorite branch of the lillianite homologous series

AbstractA new mineral species, holubite, ideally Ag3Pb6(Sb8Bi3)Σ11S24, has been found at Kutná Hora ore district, Czech Republic. The mineral is associated with other lillianite homologues (gustavite, terrywallaceite, vikingite and treasurite) most frequently as grain aggregates and replacement rims of earlier Ag–Pb–Bi minerals, growing together in aggregates up to 200 × 50 μm. It typically occurs in a close association with Ag,Bi-bearing galena and terrywallaceite. Holubite is opaque, steel-grey in colour and has a metallic lustre, the calculated density is 5.899 g/cm3. In reflected light holubite is greyish white and bireflectance and pleochroism are weak with grey tints. Anisotropy is weak to medium with grey to bluish-grey rotation tints. Internal reflections were not observed. Electron microprobe analyses yielded an empirical formula, based on 44 atoms per formula unit (apfu) of (Ag3.03Cu0.03)Σ3.06(Pb6.19Fe0.02Cd0.01)Σ6.22(Sb7.71Bi2.90)Σ10.61S24.12. Its unit-cell parameters are: a = 19.374(4), b = 13.201(3), c = 8.651(2) Å, β = 90.112(18)°, V = 2212.5(9) Å3, space group P21/n and Z = 2. Holubite is a new member of the andorite branch of the lillianite homologous series with N = 4. The structure of holubite contains two Pb sites with a trigonal prismatic coordination, eight distinct octahedral sites, of which one is a mixed (Bi,Ag) site and one is a mixed (Sb,Pb) site, and twelve anion sites. Holubite is defined as a lillianite homologue with the three following requirements: N = 4, L% [Ag+ + (Bi3+,Sb3+) ↔ 2 Pb2+ substitution] ≈ 70% and approximately one quarter to one third at.% of antimony is replaced by bismuth [Bi/(Bi+Sb) ≈ 0.26–34]. The new mineral has been approved by the Commission on New Minerals, Nomenclature and Classification of the International Mineralogical Association (IMA2022-112) and named after Milan Holub, a key Czech geologist and specialist in the Kutná Hora ore district.

Study on andorite-series minerals from Meleg Hill, Velence Mts., Hungary

During the last decades, different sulphosalts were observed in several samples that were investigated from the Meleg hill, Velence Mts. The investigated sample of this study was collected from the less studied north-eastern, stibnite-rich part of the hydrothermal breccia at the Meleg Hill. Based on the results of electron microprobe analysis, we found that the sample contained two sulphosalt minerals of the andorite series that have not been described from Hungary before. The andorite series is a subgroup of the lillianite homologous series. The individual minerals, andorite VI and roshchinite, within the andorite series were identified on the basis of their chemical composition and the andorite substitution percentage (L%). Andorite VI has Ag1.06Cu0.04Pb0.80Sb2.49Bi0.22As0.3S6 average chemical formula (normalized to 6 sulphur atoms) and L%=102.65–109.84, whereas roshchinite has Ag17.23Cu0.53Pb10.4Hg0.04Zn0.04Fe0.02Sb39.73Bi5.52As5.98S96 average chemical formula (normalized to 96 sulphur atoms) and L% = 119.52–123.48. Our results suggest that the formerly used comparative ternary diagrams of the system (Ag2S + Cu2S)–(Sb2S3 + Bi2S3 + As2S3)–(PbS + HgS + FeS + ZnS + CdS) is not adequate to differentiate the andorite-series minerals from each other, due to the highly variable element substitution of Me+, Me2+ and Me3+ cations. The andorite substitution percentage (L%) and the andorite homologue order value (N) are always necessary to calculate to distinguish the mineral species.

Terrywallaceite from the Kochkar Au Deposit, South Urals: First Find in Russia

Terrywallaceite, AgPb(Sb,Bi)3S6, a rare sulfosalt of the lillianite homologous series, was found in a sample from the Kochkar Au deposit (Chelyabinsk oblast). It occurs as anhedral grains up to 0.4 × 0.3 mm and is associated with Sb-bearing bismuthinite, tetrahedrite-(Fe), arsenopyrite and minerals of the tintinaite–kobellite series. Its average empirical formula based on eight analyses is as follows: (Ag0.97Cu0.07)Σ1.04(Pb0.94Cd0.02)Σ0.96(Bi1.84Sb1.17)Σ3.01(S5.99Se0.01)Σ6.00.The monoclinic unit cell dimensions are a = 6.992(2), b = 19.328(4), c = 8.381(10) Å, β = 107.68(7)°, V = 1079(1)ų. This is the first reliable find of the mineral in Russia. Figures 3. Tables 3. References 24. Key words: terrywallaceite, Kochkar Au deposit, South Urals, first find of mineral in Russia, lillianite homologous series.

Thermoelectric Material SnPb2Bi2S6: The 4,4L Member of Lillianite Homologous Series with Low Lattice Thermal Conductivity.

Although the binary sulfides Bi2S3, PbS, and SnS have attracted extensive interest as thermoelectric materials, no quaternary sulfides containing Sn/Pb/Bi/S elements have been reported. Herein, we report the synthesis of a new quaternary sulfide, SnPb2Bi2S6, which crystallizes in the orthorhombic space group Pnma with unit cell parameters of a = 20.5458(12) Å, b = 4.0925(4) Å, c = 13.3219(10) Å. SnPb2Bi2S6 has a lillianite-type crystal structure consisting of two alternately aligned NaCl-type structural motifs separated by a mirror plane of PbS7 monocapped trigonal prisms. In the lillianite homologous series, SnPb2Bi2S6 can be classified as 4,4L, where the superscripted numbers indicate the maximum numbers of edge-sharing octahedra in the two adjacent NaCl-shaped slabs along the diagonal direction. The obtained SnPb2Bi2S6 phase exhibited good thermal stability up to 1000 K and n-type degenerate semiconducting behavior, with a power factor of 3.7 μW cm-1 K-2 at 773 K. Notably, this compound exhibited a very low lattice thermal conductivity of 0.69-0.92 W m-1 K-1 at 300-1000 K. Theoretical calculations revealed that the low thermal conductivity is caused by the complex crystal structure and the related elastic properties of a low Debye temperature, low phonon velocity, and large Grüneisen parameters. A reasonable figure of merit (ZT) of ∼0.3 was obtained at 770 K.

The definition and crystal structure of clino-oscarkempffite, Ag15Pb6Sb21Bi18S72

Clino-oscarkempffite, Ag 1.27 Pb 0.49 Sb 1.68 Bi 1.54 S 6.01 , idealized as Ag 1.25 Pb 0.5 Sb 1.75 Bi 1.5 S 6 ( Z = 12), from the Colorada vein, Animas mine, Chocaya Province, Department of Potosi, Sur Chichas, Bolivia, is an N = 4 member of the lillianite homologous series, oversubstituted according to the Ag + (Bi, Sb) ↔ 2Pb substitution to 122.8% in comparison to the formula of gustavite, AgPbBi 3 S 6 . Optical properties of clino-oscarkempffite are very similar to those of oscarkempffite and other Bi (Bi–Sb) lillianite homologues. Reflectance values in air vary between 47.5 and 35.2%. Hardness was evaluated to 3–3½, based on VHN 50 range 189–208, mean 200 kg mm −2 . Density was calculated to 6.04 g cm −3 . Minerals associated with clino-oscarkempffite are aramayoite, oscarkempffite, arsenopyrite, ferrokesterite, freibergite, miargyrite, pyrite and quartz. Clino-oscarkempffite is monoclinic, lattice parameters a 39.811(25) A, b 19.280(12) A, c 8.278(5) A, β 96.195(9)°, V 6316.7 A 3 , space group P 2 1 / c . It is a three-fold superstructure of the lillianite aristotype, with a doubled 4 A axis and, uniquely, a tripled ∼13.3 A a parameter. In the crystal structure, configured as a general N = 4 lillianite homologue, the central octahedra of the PbS-like slabs are mixed (Bi,Sb), (Sb,Bi) and Sb sites in alternation along the 8.3 A direction, whereas in the marginal coordination octahedra (Sb,Bi) positions alternate with distorted tetrahedral coordinations of Ag. The bicapped trigonal coordination prisms on meeting planes of two octahedral slabs are occupied by mixed (Pb,Bi) sites. Cation occupation schemes result in blocks with monoclinic overall symmetry which alternate along [1 0 0] with blocks with orthorhombic overall symmetry. The order–disorder implications of this division are discussed in the paper.

Staročeskéite, Ag0.70Pb1.60(Bi1.35Sb1.35)Σ2.70S6, from Kutná Hora, Czech Republic, a new member of the lillianite homologous series

ABSTRACTA new mineral species, staročeskéite, ideally Ag0.70Pb1.60(Bi1.35Sb1.35)Σ2.70S6, has been found at Kutná Hora ore district, Czech Republic. The mineral occurs in the late-stage Bi-mineralization associated with other lillianite homologues (gustavite, terrywallaceite, vikingite, treasurite, eskimoite and Bi-rich andorite-group minerals) and other bismuth sulfosalts (izoklakeite, cosalite and Bi-rich jamesonite) in quartz gangue. The mineral occurs as lath shaped crystals or anhedral grains up to 80 µm × 70 µm, growing together in aggregates up to 200 µm × 150 µm across. Staročeskéite is steel-grey in colour and has a metallic lustre, the calculated density is 6.185 g/cm3. In reflected light staročeskéite is greyish white; bireflectance and pleochroism are weak with greyish tints. Anisotropy is weak to medium with grey to bluish grey rotation tints. Internal reflections were not observed. The empirical formula based on electron probe microanalyses and calculated on 11 apfu is: (Ag0.68Cu0.01)Σ0.69(Pb1.56Fe0.01Cd0.01)Σ1.58(Bi1.32Sb1.37)Σ2.69(S6.04Se0.01)Σ6.05. The ideal formula is Ag0.70Pb1.60(Bi1.35Sb1.35)Σ2.70S6, which requires Ag 7.22, Pb 31.70, Bi 26.97, Sb 15.72 and S 18.39 wt.%, total 100.00 wt.%. Staročeskéite is a member of the lillianite homologous series withN= 4. Unlike gustavite and terrywallaceite, staročeskéite, similarly to lillianite, is orthorhombic, space groupCmcm, witha= 4.2539(8),b= 13.3094(8),c= 19.625(1) Å,V= 1111.1(2) Å3andZ= 4. The structure of staročeskéite contains four sulfur sites and three metal sites: one pure Pb site and two mixed sites,M1 (0.52Bi + 0.356Ag + 0.124Sb) andM2 (0.601Sb + 0.259Pb + 0.14Bi). The mineral is characterized by the Bi:Sb ratio 1:1 (Bi/(Bi + Sb) = 0.50) and the Ag++ Bi3+, Sb3+↔ 2 Pb2+substitution (L%) equal to 70%. Thus the mineral lies between two series of the lillianite structures withN= 4, between the lillianite–gustavite series and the andorite series.

Oyonite, Ag3Mn2Pb4Sb7As4S24, a New Member of the Lillianite Homologous Series from the Uchucchacua Base-Metal Deposit, Oyon District, Peru

The new mineral species oyonite, ideally Ag3Mn2Pb4Sb7As4S24, has been discovered in the Uchucchacua base-metal deposit, Oyon district, Catajambo, Lima Department, Peru, as very rare black metallic subhedral to anhedral crystals, up to 100 μm in length, associated with orpiment, tennantite/tetrahedrite, menchettiite, and other unnamed minerals of the system Pb-Ag-Sb-Mn-As-S, in calcite matrix. Its Vickers hardness (VHN100) is 137 kg/mm2 (range 132–147). In reflected light, oyonite is weakly to moderately bireflectant and weakly pleochroic from dark grey to a dark green. Internal reflections are absent. Reflectance values for the four COM wavelengths [Rmin, Rmax (%) (λ in nm)] are: 33.9, 40.2 (471.1); 32.5, 38.9 (548.3), 31.6, 38.0 (586.6); and 29.8, 36.5 (652.3). Electron microprobe analysis gave (in wt %, average of 5 spot analyses): Cu 0.76 (2), Ag 8.39 (10), Mn 3.02 (7), Pb 24.70 (25), As 9.54 (12), Sb 28.87 (21), S 24.30 (18), total 99.58 (23). Based on 20 cations per formula unit, the chemical formula of oyonite is Cu0.38Ag2.48Mn1.75Pb3.79Sb7.55As4.05S24.12. The main diffraction lines are (d in Å, hkl and relative intensity): 3.34 (−312; 40), 3.29 (−520; 100), 2.920 (−132; 40), 2.821 (−232; 70), 2.045 (004; 50). The crystal structure study revealed oyonite to be monoclinic, space group P21/n, with unit-cell parameters a = 19.1806 (18), b = 12.7755 (14), c = 8.1789 (10) Å, β = 90.471 (11)°, V = 2004.1 (4) Å3, Z = 2. The crystal structure was refined to a final R1 = 0.032 for 6272 independent reflections. Oyonite belongs to the Sb-rich members of the andorite homeotypic sub-series within the lillianite homologous series. The name oyonite is after the Oyon district, Lima Department, Peru, the district where the type locality (Uchucchacua mine) is located.

New metal chalcogenides found in MnN-1(Gd2-xInx)SN+2 (N = 3, 4, 5): syntheses, structures, and magnetic properties.

Three new metal chalcogenides have been identified in MnN-1(Gd2-xInx)SN+2 with N = 3, 4, and 5 via a flux-growth synthesis. All compounds crystallize in the same space group of orthorhombic Cmcm with cell constants: Mn2GdInS5 (1), a = 3.789(1) Å, b = 12.411(1) Å, and c = 15.489(1) Å; Mn3Gd2S6 (2), a = 3.778(1) Å, b = 12.505(2) Å, and c = 19.114(2) Å; Mn4Gd2S7 (3), a = 3.769(1) Å, b = 12.466(2) Å, and c = 22.289(3) Å. Compounds 1-3 form a homologous series through the modulation of the MnS unit, whose structures represent a complete system of the corresponding lillianites (N1,N2L) of 3,3L, 4,4L and 5,5L. The gradually wider slabs formed in the series result in a monotonic increase along the c dimensions from 1 to 3. Crystal 3 is the first to achieve a predicted structure of 5,5L. Mn2GdInS5 (1) displays a weak antiferromagnetic (AFM) ordering at 10 K and the Weiss constant (θ) of -0.76 K. Mn2Gd1.5In0.5S5 (1a), an isostructure of 1, shifts the AFM transition temperature to 12 K and possesses a slightly larger θ constant of -6.06 K. Mn4Gd2S7 (3), featuring the thickest slabs in this series, shows a significant antiferromagnetic behavior beginning at a high temperature of 70 K and has a largest θ constant of -40.25 K. A small amount of impurity α-Gd2S3 with an AFM transition temperature around 4 K was characterized in sample 3, which does not interfere with the magnetic ordering of 3 at much higher temperatures. These magnetic chalcogenides display band gaps of 1.66 eV for 1, 1.75 eV for 1a, and 1.44 eV for 3.

Crystal Structure and Thermoelectric Properties of the 7,7L Lillianite Homologue Pb6Bi2Se9.

Pb6Bi2Se9, the selenium analogue of heyrovsyite, crystallizes in the orthorhombic space group Cmcm (#63) with a = 4.257(1) Å, b = 14.105(3) Å, and c = 32.412(7) Å at 300 K. Its crystal structure consists of two NaCl-type layers, A and B, with equal thickness, N1 = N2 = 7, where N is the number of edge-sharing [Pb/Bi]Se6 octahedra along the central diagonal. In the crystal structure, adjacent layers are arranged along the c-axis such that bridging bicapped trigonal prisms, PbSe8, are located on a pseudomirror plane parallel to (001). Therefore, Pb6Bi2Se9 corresponds to a 7,7L member of the lillianite homologous series. Electronic transport measurements indicate that the compound is a heavily doped narrow band gap n-type semiconductor, with electrical conductivity and thermopower values of 350 S/cm and -53 μV/K at 300 K. Interestingly, the compound exhibits a moderately low thermal conductivity, ∼1.1 W/mK, in the whole temperature range, owing to its complex crystal structure, which enables strong phonon scattering at the twin boundaries between adjacent NaCl-type layers A and B. The dimensionless figure of merit, ZT, increases with temperature to 0.25 at 673 K.

Lillianites and andorites: new life for the oldest homologous series of sulfosalts

AbstractThe current state of the lillianite homologous series is presented, with its two branches − the lillianite branch of the predominantly Pb-Bi-Ag sulfosalts and the andorite branch of predominantly Pb-Sb-Ag sulfosalts. Both the natural and synthetic members are discussed, especially from the structural and compositional point of view and the related, chemically distinct and structurally more complicated members of the series are described. A number of new published, or hitherto unpublished observations is given, together with fairly exhaustive tables of data. Relationships between the complex structures of different ‘andorite’ species and principal structural features of ‘oversubstituted’ As-Sb and Bi-Sb species are discussed. Contrary to many studies of this subject, synthetic phases form an integral part of the paper. It is concluded that research in this long-known homologous series still supplies new interesting phases, especially in the fields of synthetic products and in its andorite branch.

The crystal structure of jasrouxite, a Pb-Ag-As-Sb member of the lillianite homologous series

Jasrouxite, Ag 16 Pb 4 (Sb 25 As 15 ) ∑40 S 72 , is triclinic, space group P- 1, lattice parameters a = 8.2917(5), b = 19.101(1), c = 19.487(1) A, α = 89.731(1)°, β = 83.446(1)°, and γ = 89.944(1)°. Unit-cell volume is V = 3066.1(3) A 3 , Z = 1 for the title formula. It is the N =4 member of the lillianite homologous series of modular structures created by unit-cell twinning of (311) slabs of the PbS archetype. The structure contains 30 independent cation sites, from which 12 are mixed sites, and 36 independent sulphur sites ( i.e . six times the sites of the lillianite-like subcell). Only the c parameter copies that of lillianite, a is doubled and b is about 3/2 of a diagonal to (001) of lillianite. In agreement with the “2Pb→ Ag + (Sb,As) oversubstitution” against ideal PbAgSb 3 S 6 , the trigonal prismatic sites on composition planes of twinning are occupied by two Pb-Sb rows and one Sb-Sb row, and the PbS-like slabs contain excess number of Ag sites. Unlike lillianite, the alternating (311) PbS slabs are non-equivalent and each of them has two types of differently occupied diagonal planes of atoms, always present in a 2:1 ratio. This results in triclinic symmetry with only small distortions from monoclinic metrics. In both slab types lone electron pairs of As and Sb congregate in large micelles with elliptic cross-section. Among lillianite homologues, jasrouxite exhibits hitherto unseen complications of cation ordering, resulting from the presence of two distinct metalloids in the structure, oversubstitution by (Ag + M 3+ ), and highly expressed lone electron pair activity of trivalent cations.

Jasrouxite, a new Pb-Ag-As-Sb member of the lillianite homologous series from Jas Roux, Hautes-Alpes, France

The thallium-rich sulfosalt deposit of Jas Roux, situated in the Pelvoux Massif (Hautes-Alpesdepartement, France), occurs in a Triassic sedimentary series. Jasrouxite belongs to the early lead-containing stages of the Tl-As-Sb period of mineralization. It occurs in a silicified gangue, along with smithite and late realgar. Jarouxite forms dark gray anhedral grains with metallic luster, up to several millimeters in size, as well as aggregates of grains which range to more than 10 mm in diameter. From their surface, grains of jasrouxite are embayed by a three-phase myrmekite aggregate composed of stibnite, boscardinite and smithite. The mineral is brittle, with irregular fracture; no cleavage or parting was observed. In reflected light, color is off-white. Bireflectance is weak, in off-white tones; anisotropy is distinct. Rare straight twin lamellae occur in otherwise untwinned crystals. The empirical formula, on the basis of 132 apfu derived from the crystal-structure determination, is Cu0.79Ag14.64Pb4.10Tl0.05As15.37Sb24.87S72.18. Calculated density is

LiPbSb3S6: A Semiconducting Sulfosalt with Very Low Thermal Conductivity

The new semiconductor LiPbSb3S6 crystallizes in the space group P21/c. The structure is a member of the lillianite homologous series and is composed of layers of PbS archetype Sb/Li-S separated by trigonal-prismatic-coordinated Pb/Li. Electronic band structure calculations indicate an indirect band gap, with direct gaps lying very close in energy. LiPbSb3S6 has one of the lowest thermal conductivities seen in a crystalline material, ∼0.24 W m(-1) K(-1) at room temperature, and a high resistivity, ∼4 × 10(9) Ω·cm, and exhibits strong light absorption with a nearly direct band gap of 1.6 eV.

Structure refinement of Ag-free heyrovskyite from Vulcano (Aeolian Islands, Italy)

The first single-crystal structure refinement of Ag- and Cu-free heyrovskýite was performed in this study. Crystals investigated were sampled from the high-temperature fumaroles of La Fossa crater of Vulcano, Aeolian Islands, Italy. Electron microprobe analyses gave the average chemical formula (Pb 5.86 Cd 0.03 ) Σ5.89 Bi 2.04 (S 8.52 Se 0.53 Cl 0.03 ) Σ9.08 , which is very close to the ideal composition of heyrovskýite, Pb 6 Bi 2 S 9 . Lattice parameters are a = 13.7498(4), b = 31.5053(8), c = 4.1475(1) Å, V = 1796.7(1) Å 3 , space group Bbmm. The structure refinement converges to R = 4.17% for 1312 reflections with F o > 4σ(F o ). In Ag-free heyrovskýite from Vulcano, as well as in the synthetic Pb 6 Bi 2 S 9 , the trigonal prismatic coordinated position Me1, as well as the octahedrally coordinated position Me3 are occupied only by Pb. Me2, also octahedrally coordinated, is dominated by Pb, whereas the octahedra situated at the edges of the octahedral layers (Me4 and Me5) are centered around mixed (Pb,Bi) positions, with almost equal occupancy. The octahedrally coordinated site Me3 was found to incorporate vacancies (□), created by the substitution 3Pb 2+ → 2Bi 3+ +□, which allows for the observed deviations from the ideal composition, Pb 6 Bi 2 S 9 . Selenium is preferentially ordered at the fivefold-coordinated anionic sites. Taking into account vacancies, as well as Se for S substitutions the structural formula of Ag-free heyrovskýite from Vulcano is Pb 5.82 Bi 2.12 □ 0.06 S 8.70 Se 0.30 . Comparison with the Ag-bearing heyrovskýite structures shows that during the 2 Pb → Ag(Cu)+Bi substitution the increased content of Bi is incorporated preferentially in the Me5 site until 2/3 Bi occupancy and thereafter in the two central octahedrally coordinated sites (Me2 and Me3). Silver occupies exclusively marginal octahedrally coordinated Me4 site like in the other members of the lillianite homologous series. The observed crystal chemical characteristics of the Ag-free heyrovskýite are in accordance with a model suggested by Callegari and Boiocchi, which describes the monoclinic form, aschamalmite, as an ordered polymorph of Pb 6 Bi 2 S 9 , and heyrovskýite as a fully disordered polymorph of the same compound. Ag incorporation is expected to increase the Pb/Bi disorder and to stabilize the orthorhombic heyrovskýite form.

Transmission electron microscopy investigation of Ag-free lillianite and heyrovskyite from Vulcano, Aeolian Islands, Italy

We present a transmission electron microscopy (TEM) investigation of lillianite (Pb3Bi2S6) and heyrovskýite (Pb 6 Bi 2 S 9 ), from Vulcano, Aeolian Islands, Italy. The minerals investigated represent the only naturally occurring Ag- and Cu-free sulfosalts in the lillianite homologous series (LHS). Three methods (crushing, ion-milling, and ultramicrotomy) were used to prepare TEM specimens. Selected area electron diffraction (SAED) patterns and high-resolution TEM (HRTEM) images indicate wellordered crystals with only minor stacking faults and, more rarely, nanoscale intergrowths of lillianite and heyrovskýite. The latter were sometimes found to form an incommensurate structural modulation with an angle of ~29° relative to b* in the (hk0) plane and a wavelength of ~75 Å. This represents the first observation of such incommensurate modulations in heyrovskýite. Although considerable evidence points toward an artifact induced by the sample preparation technique (i.e., ion-milling), the possibility that the incommensurate modulation could be a primary feature of heyrovskýite itself cannot be completely ruled out. The modulation could derive from an ordering process of Pb and Bi cations over Me4 and Me5 sites within the PbS-like layer or from ordering of vacancies, naturally present or induced by Bi 2 S 3 sublimation during ion-milling

Crystal structure of natural orthorhombic Ag0.71Pb1.52Bi1.32Sb1.45S6, a lillianite homologue with N = 4; comparison with gustavite

The crystal structure of natural orthorhombic Ag0.71Pb1.52Bi1.32Sb1.45S6, a lillianite homologue with N = 4, from hydrothermal veins at Kutna Hora in central Bohemia, Czech Republic, has been refined from single-crystal X-ray data to final R w = 5.08 %. The symmetry is orthorhombic, space group Cmcm , with a = 4.2539(8), b = 13.3094(8), c = 19.625(1) A, V = 1111.1(2) A3, Z = 4. On the basis of electron-microprobe analysis, the formula is Ag0.69Pb1.56(Bi1.32Sb1.37)∑=2.69(S6.04Se0.01)∑6.05 with N chem = 3.94, Bi/(Bi + Sb) = 0.49 and a gustavite substitution percentage L% = 70.5, which characterize the phase as a natural orthorhombic Sb–Bi 4,4L member of the lillianite homologous series. The structure contains seven independent positions: three metal sites M 1, M 2 and M 3 and four sulphur positions. The crystal structure consists of blocks of diagonal chains of four octahedra M 1 (Bi/Ag/Sb) – M 2 (Sb/Pb/Bi) – M 2 – M 1 separated by Pb atoms in bicapped trigonal prismatic coordination of the site M 3. The marginal octahedron of the chain of four octahedra is formed by one mixed site M 1 containing 52 % Bi, 35.6 % Ag and 12.4 % Sb while the central M 2 octahedron is a mixed site containing 60.1 % Sb, 25.9 % Pb and 14 % Bi. The mixed population of the M 1 site results in orthorhombic symmetry, space group Cmcm , with a 4 A a -repeat while in natural gustavite, the alternating occupation of the M 1 site by Ag and Bi atoms (which are thus two sites M 1a and M 1b) reduces the symmetry to monoclinic, space group P 21/ c , with an 8 Arepeat. The structure is similar to that of Ag-free lillianite.

THE CRYSTAL STRUCTURE OF A SYNTHETIC ORTHORHOMBIC N = 8 MEMBER OF THE LILLIANITE HOMOLOGOUS SERIES

The new compound, ideally Ag 2.25 Pb 2.5 Bi 4.25 S 10 , has been synthesized at 850°C by reacting simple sulfides of Bi, Pb and Ag. It crystallizes in the orthorhombic system, space group Cmcm , with a 4.084(1), b 13.453(4), c 33.932(9) A, V 1864.4(9) A 3 and Z = 4. The crystal structure has been refined to R 1 = 3.66% for 1132 unique [ F o > 4σ( F o )] reflections collected on a Bruker single-crystal diffractometer with a CCD detector and monochromated Mo K α X-radiation. The structure contains one Pb site with a trigonal prismatic coordination, positioned on the reflection planes of the space group, four distinct octahedral sites, of which one is a mixed (Pb,Ag) site and two are (Bi,Ag) sites, and six anion sites. The new compound is the member number N 1,2 = 8,8 of the lillianite homologous series. It is the homologue of lillianite richest in silver known to date. The present synthesis and investigation of the lillianite homologue 8,8 L allow us to contribute a new structural interpretation for the silver-rich portions of the system Pb–Bi–Ag–S. The existence of Ag-rich phases with N = 8 is caused by a viability of ordered slabs of Ag–Bi–Pb octahedra in the homologues that have favorable Ag:Bi:Pb proportions.

Synthesis and characterization of quaternary selenides Sn 2Pb 5Bi 4Se 13 and Sn 8.65Pb 0.35Bi 4Se 15

Quaternary selenides Sn 2Pb 5Bi 4Se 13 and Sn 8.65Pb 0.35Bi 4Se 15 were synthesized from the elements in sealed silica tubes; their crystal structures were determined by single-crystal and powder X-ray diffraction. Both compounds crystallize in monoclinic space group C2/ m (No.12), with lattice parameters of Sn 2Pb 5Bi 4Se 13: a = 14.001(6) Å, b = 4.234(2) Å, c = 23.471(8) Å, V = 1376.2(1) Å 3, R 1/ wR 2 = 0.0584/0.1477, and GOF = 1.023; Sn 8.65Pb 0.35Bi 4Se 15: a = 13.872(3) Å, b = 4.2021(8) (4) Å, c = 26.855(5) Å, V = 1557.1(5) Å 3, R 1/ wR 2 = 0.0506/0.1227, and GOF = 1.425. These compounds exhibit tropochemical cell-twinning of NaCl-type structures with lillianite homologous series L(4, 5) and L(4, 7) for Sn 2Pb 5Bi 4Se 13 and Sn 8.65Pb 0.35Bi 4Se 15, respectively. Measurements of electrical conductivity indicate that these materials are semiconductors with narrow band gaps; Sn 2Pb 5Bi 4Se 13 is n-type, whereas Sn 8.65Pb 0.35Bi 4Se 15 is a p-type semiconductor with Seebeck coefficients −80(5) and 178(7) μV/K at 300 K, respectively.