Contribution to the crystal chemistry of lead-antimony sulfosalts: Systematic Pb-versus-Sb crossed substitution in the plagionite homologous series, Pb2N-1(Pb1-xSbx)2(Sb1-xPbx)2Sb6S13+2N

The plagionite homologous series contains four well-defined members with the general formula Pb1+2NSb8S13+2N: fülöppite (N = 1), plagionite (N = 2), heteromorphite (N = 3), and semseyite (N = 4). The crystal structure of several natural and synthetic samples of fülöppite, plagionite, and semseyite have been refined through single-crystal X-ray diffraction, confirming the systematic Pb-versus-Sb crossed substitution observed previously in semseyite and fülöppite. This crossed substitution takes place mainly in two adjacent cation sites in the middle of the constitutive SnS-type layer. The substitution coefficient x appears variable, even for a given species, with the highest values observed in synthetic fülöppite samples. The developed structural formula of the plagionite homologues can be given as Pb2N-1(Pb1-xSbx)2(Sb1-xPbx)2Sb6S13+2N. In the studied samples, x varies between ∼ 0.10 and 0.40. In the ribbons within the SnS-type layer, (Pb=Sb) mixing can be considered the result of the combination, in a variable ratio, of two cation sequences, i.e. (Sb-Sb-Sb)-Pb-Sb-(..), major in plagionite and semseyite, and (Sb-Sb-Sb)-Sb-Pb-(..), major in fülöppite and, probably, in heteromorphite. The published crystal structure of synthetic "Pb-free fülöppite"is revised according to this approach. It would correspond to a Na derivative, with a proposed structural formula of (Na0.5Sb0.5)(Na0.2Sb0.8)2(Na0.3Sb0.7)2Sb6S15, ideally Na1.5Sb9.5S15. In fülöppite, increasing x induces a flattening of the unit cell along c, with a slight volume decrease. Such a general Pb-versus-Sb crossed substitution would attenuate steric distortions in the middle of the SnS-type layer of the plagionite homologous series. Crystallization kinetics seem the main physical factor that controls such an isochemical substitution.