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Bindi, L., Evain, M. & Menchetti, S. (2007) Complex twinning, polytypism and disorder phenomena in the crystal structures of antimonpearceite and arsenpolybasite. Can. Mineral. 45 321–333. 
Added by: Laurent Cournède (2016-03-10 22:02:30)
Type de référence: Article
DOI: 10.2113/gscanmin.45.2.321
Numéro d'identification (ISBN etc.): 0008-4476
Clé BibTeX: Bindi2007d
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Catégories: MIOPS
Mots-clés: antimonpearceite, arsenpolybasite, crystal-structure determination, disorder, minerals, pearceite, phase-transitions, polybasite, polytypism, series, silver, symmetry, Twinning
Créateurs: Bindi, Evain, Menchetti
Collection: Can. Mineral.
Consultations : 4/554
Indice de consultation : 3%
Indice de popularité : 0.75%
The crystal structures of anti monpearceite, arsenpolybasite-222 and arsenpolybasite-221 have been solved and refined from single-crystal X-ray-diffraction datasets. Anti monpearceite crystallizes in the trigonal space-group P (3) over bar m1, with a 7.4805(5), c 11.8836(13) angstrom, V 575.89(g) angstrom(3) and Z = 1. The refinement of the structure leads to R = 0.0352 for 1095 independent observed reflections [I/sigma(I) {>}= 2] and 98 parameters. Arsenpolybasite-222 crystallizes in the monoclinic space-group C2/c, with a 26.036(2), b 15.0319(13), c 24.042(3) angstrom, beta 90.000(13)degrees (pseudohexagonal cell with the a = -root 3b orthohexagonal relation), V 9409.5(15) angstrom(3) and Z = 16. A second-degree twinning by metric merohedry gives rise to an apparent trigonal symmetry, with the unit-cell parameters (hexagonal cell) a 15.0319(13) and c 24.042(3) angstrom. The refinement of the structure leads to R = 0.0716 for 22008 independent observed reflections [I/sigma(I) {>}= 2] and 552 parameters. Arsenpolybasite-221 crystallizes in the space group P321, with a 14.9746(17), c 11.9982(6) angstrom, V2330.0(4) angstrom(3) and Z = 4. The refinement ofthe structure, including a mirror-twin operation (first degree twin, (3) over bar '2/m'1 polychromatic point-group), leads to R = 0.0434 for 4639 independent observed reflections [I sigma(I) {>}= 2] and 184 parameters. Alt the structures consist of the stacking of [(Ag,Cu)(6)(As,Sb)(2)S-7](2-) and [Ag9CuS4](2+) module layers along [001]; (As,Sb) forms isolated (As,Sb)S-3 pyramids typically occurring in sulfosalts, copper links two sulfur atoms in a linear coordination, and silver occupies sites with coordination ranging from quasilinear to almost tetrahedral. The substitution of Cu for Ag in the [(Ag,Cu)(6)(As,Sb)(2)S-7](2-) module layer becomes greater in going from the 111 structure, through the 221, to the 222 structure. The determination of the crystal structure for all the members of the group leads us to consider them as a family of polytypes.
Added by: Laurent Cournède  
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