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1 Museo di Storia Naturale, Sezione di Mineralogia, Università degli Studi di Firenze, via La Pira 4, I–50121 Firenze, Italy
2 Laboratoire de Chimie des Solides, I.M.N., UMR C6502 CNRS – Université de Nantes, 2, rue de la Houssinière, BP 32229, F–44322 Nantes Cedex 3, France
3 Dipartimento di Scienze della Terra, Università degli Studi di Firenze, via La Pira 4, I–50121 Firenze, Italy
E-mail address: lbindi{at}geo.unifi.it
The crystal structures of antimonpearceite, arsenpolybasite-222 and arsenpolybasite-221 have been solved and refined from single-crystal X-ray-diffraction datasets. Antimonpearceite crystallizes in the trigonal space-group P
m1, with a 7.4805(5), c 11.8836(13) Å, V 575.89(8) Å3 and Z = 1. The refinement of the structure leads to R = 0.0352 for 1095 independent observed reflections [I/
(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) Å, ß 90.000(13)° (pseudohexagonal cell with the a = 
3b orthohexagonal relation), V 9409.5(15) Å3 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) Å. The refinement of the structure leads to R = 0.0716 for 22008 independent observed reflections [I/(I) 2] and 552 parameters. Arsenpolybasite-221 crystallizes in the space group P321, with a 14.9746(17), c 11.9982(6) Å, V 2330.0(4) Å3 and Z = 4. The refinement of the structure, including a mirror-twin operation (first-degree twin, '2/m'1 polychromatic point-group), leads to R = 0.0434 for 4639 independent observed reflections [I/(I) 2] and 184 parameters. All the structures consist of the stacking of [(Ag,Cu)6(As,Sb)2S7]2– and [Ag9CuS4]2+ module layers along [001]; (As,Sb) forms isolated (As,Sb)S3 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)2S7]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.
Keywords: antimonpearceite, arsenpolybasite, polytypism, disorder, twinning, crystal-structure determination.
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