Contribution to the crystal chemistry of lead-antimony sulfosalts: systematic Pb-versus-Sb crossed substitution in the plagionite homologous series, Pb 2 N − 1 (Pb 1 − x Sb x ) 2 (Sb 1 − x Pb x ) 2 Sb 6 S 13+2 N

Abstract. The plagionite homologous series contains four well-defined members with the general formula Pb 1+2N Sb 8 S 13+2N : fuloppite ( N=1 ), plagionite ( N=2 ), heteromorphite ( N=3 ), and semseyite ( N=4 ). The crystal structure of several natural and synthetic samples of fuloppite, 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 fuloppite. 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 fuloppite samples. The developed structural formula of the plagionite homologues can be given as Pb 2N−1 (Pb 1−x Sb x)2 (Sb 1−x Pb x)2 Sb 6 S 13+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 fuloppite and, probably, in heteromorphite. The published crystal structure of synthetic “Pb-free fuloppite” is revised according to this approach. It would correspond to a Na derivative, with a proposed structural formula of (Na 0.5 Sb 0.5) (Na 0.2 Sb 0.8)2 (Na 0.3 Sb 0.7)2 Sb 6 S 15 , ideally Na 1.5 Sb 9.5 S 15 . In fuloppite, 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.