Structural complexity in minerals: twinning, polytypism and disorder in the crystal structure of polybasite,...

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)](2-) and [Ag(9)CuS(4)](2+) 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)](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 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.