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

The crystal structures of 222- and 221-polybasite [(Ag,Cu)(16)(Sb,As)(2)S(11)] 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)(6)Sb(2)S(7) and Ag(9)CuS(4) module layers in which Sb forms isolated SbS(3) 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 --> Cu substitution in the (Ag,Cu)(6)Sb(2)S(7) 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 Ag(15)CuSb(2)S(11) 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(bar)3m1.