Acta Crystallographica Section B

Structural Science

Volume 62, Part 3 (June 2006)

research papers

Acta Cryst. (2006). B62, 447-456 [ doi:10.1107/S010876810600975X ]

Structural complexity in minerals: twinning, polytypism and disorder in the crystal structure of polybasite, (Ag,Cu)₁₆(Sb,As)₂S₁₁

M. Evain, L. Bindi and S. Menchetti

Abstract: The crystal structures of 222- and 221-polybasite [(Ag,Cu)₁₆(Sb,As)₂S₁₁] crystals have been solved and refined by means of X-ray diffraction data (collected at 100 and 120 K, respectively) from twinned crystals. Both structures consist of the stacking of [(Ag,Cu)₆Sb₂S₇]^2- and [Ag₉CuS₄]²⁺ module layers in which Sb forms isolated SbS₃ pyramids typically occurring in sulfosalts; copper links two S atoms in a linear coordination and silver occupies sites with coordination ranging from quasi-linear to almost tetrahedral. An Ag [rightwards arrow] Cu substitution in the [(Ag,Cu)₆Sb₂S₇]^2- module layer is observed in both structures, the substitution amount being larger in the 221- than in the 222-polybasite. A pattern of the possible mechanism regulating the type of unit cell that is stabilized is proposed: starting from the hypothetical stoichiometric and fully ordered Ag₁₅CuSb₂S₁₁ 222-polybasite structure, with a low C2/c monoclinic symmetry and a large 222 supercell, the disorder introduced by the substitution of Cu for Ag increases the symmetry with a cell reduction along the c axis yielding the 221 supercell and a trigonal crystal system. A further increase of the substitution gives rise to a folding of the cell along the a and b axes and the 111-pearceite structure, space group $P\bar3m1$ .

Keywords: polybasite; crystal X-ray structure determination; twin; polytype; disorder.


	Structure factor file (CIF format) (321.4 kbytes) [ doi:10.1107/S010876810600975X/lc5045polybasite221sup2.hkl ] Contains datablock polybasite221

	Structure factor file (CIF format) (2054.1 kbytes) [ doi:10.1107/S010876810600975X/lc5045polybasite222sup3.hkl ] Contains datablock polybasite222

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