IMA 2015-073 = heptasartorite; IMA 2015-074 = enneasartorite

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Marco E. Ciriotti
Messaggi: 24615
Iscritto il: ven 25 giu, 2004 11:31
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IMA 2015-073 = heptasartorite; IMA 2015-074 = enneasartorite

Messaggio da Marco E. Ciriotti » gio 28 set, 2017 9:56

Referenza:
▪ Topa, D., Makovicky, E., Stoeger, B., Stanley, C. (2017): Heptasartorite, Tl7Pb22As55S108, enneasartorite, Tl6Pb32As70S140 and hendekasartorite, Tl2Pb48As82S172, three members of the anion-omission series of ‘sartorites’ from the Lengenbach quarry at Binntal, Wallis, Switzerland. European Journal of Mineralogy, 29, 701-712.

Abstract:
Detailed electron-microprobe investigations and crystal-structure determinations established that ‘sartorite’ represents a group of distinct mineral species, each with unique chemistry and crystal structure. These manifest themselves as the 7-, 9- and 11-fold P21/c superstructures of the basic 4.2 Å substructure. Heptasartorite is Tl7Pb22As55S108 [based on 192 atoms per formula unit (apfu), 84Me + 108S] with a = 29.269(2), b = 7.8768(5), c = 20.128(2) Å, β = 102.065(2)° and unit-cell volume V = 4537.8 Å3; enneasartorite is Tl6Pb32As70S140 (based on 248 apfu, 108Me + 140S) with a = 37.612(6), b = 7.8777(12), c = 20.071(3) Å, β = 101.930(2)° and V = 5818.6(15) Å3; hendekasartorite is Tl2Pb48As82S172 (based on 304 apfu, 132Me + 172S) (empirical ΣMe = 132.48) with a = 31.806(5), b = 7.889(12), c = 28.556(4) Å and β = 99.034(2)° with V = 7076.4(15) Å3. Physical and optical properties (grey with metallic lustre, in polished section white with visible bireflectance, red internal reflections; reflectance curves span 28.7–42.5%; Mohs hardness 3–3½) of these phases are very similar so that chemical analysis and/or single-crystal X-ray diffraction is needed to distinguish them. A brief description of complicated AsmSn crank-shaft chains in the walls of double-ribbons which form the As-based slabs of these structures is given. The three new mineral species differ in their structures by 4.2 Å modular increments, not just by cation substitutions. They represent anion-omission derivatives of the ‘ideal’ PbAs2S4 composition with an important role for thallium in charge compensation. The described minerals belong to the late sulfide phases in the Pb–Tl–Ag–As deposit of Lengenbach, Wallis, Switzerland.
Marco E. Ciriotti

«Things are interesting only in so far as they relate themselves to other things»

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Marco E. Ciriotti
Messaggi: 24615
Iscritto il: ven 25 giu, 2004 11:31
Località: via San Pietro, 55 I-10073 Devesi/Cirié TO - Italy
Contatta:

Re: IMA 2015-073 = heptasartorite; IMA 2015-074 = enneasartorite

Messaggio da Marco E. Ciriotti » ven 02 feb, 2018 17:59

Referenza:
▪ Makovicky, E., Topa, D., Stoeger, B. (2018): The crystal structures of heptasartorite, Tl7Pb22As55S108, and enneasartorite, Tl6Pb32As70S140, two members of an anion-omission series of complex sulfosalts from Lengenbach, the Swiss Alps, and comparison with the structures of As–Sb sartorite homologues. European Journal of Mineralogy, 30, 149-164.

Abstract:
Crystal structures of heptasartorite and enneasartorite, two members of the sartorite anion-omission series were refined from the material from the Lengenbach deposit, Swiss Alps. Heptasartorite, Tl7Pb22(As54Sb)Σ55S108, is monoclinic, space group: P21/c (SG#14), a 29.2691(20), b 7.8768(5), c 20.1275(15) Å, β 102.065(2)°, unit-cell volume V = 4537.8(5) Å3. Enneasartorite, Tl6Pb32As70S140, is monoclinic, space group P21/c (SG#14), and lattice parameters a = 37.612(6), b = 7.8777(12), c = 20.071(3) Å, β = 101.930(2)°, and V = 5818.6(15) Å3. Both structures are typical structures of an N = 3 homologue of the sartorite homologous series with zig-zag walls of (Pb, Tl) trigonal coordination prisms separating (primarily) As-based slabs of structure configured to accommodate lone electron pairs of As. The sevenfold superstructure of heptasartorite is based on a sequence of crankshaft chains: on a surface of a fundamental double-ribbon in the As-rich structure slab one can recognize a W-shaped chain with five As and one mixed coordination polyhedron in which As and a large cation combine, a 4-member chain, a pair of As polyhedra, and two isolated As polyhedra. In the here redetermined structure of enneasartorite, one more 4-member chain of As polyhedra has been added, making the environment of the W-shaped chain symmetric. One of the isolated As polyhedra became a mixed-cation site. In both structures, the S site chelated by the As atoms of the W chain becomes vacant, generating the anion-omission derivatives of the ideal PbAs2S4 motif. The chain motifs differ from those in the Sb–As homologues of sartorite, which display only two-fold superstructures.
Marco E. Ciriotti

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