The crystal structure of jasrouxite, a Pb-Ag-As-Sb member of the lillianite homologous series
Makovicky, Emil; Topa, Dan
European Journal of Mineralogy Volume 26 Number 1 (2014), p. 145 - 155
published: Feb 1, 2014
ArtNo. ESP147052601012, Price: 29.00 €
Jasrouxite, Ag16Pb4(Sb25As15)Σ40S72, is triclinic, space group P-1, lattice parameters a = 8.2917(5), b = 19.101(1), c = 19.487(1) Å, α = 89.731(1)°, β = 83.446(1)°, and γ = 89.944(1)°. Unit-cell volume is V = 3066.1(3) Å3, Z = 1 for the title formula. It is the N = 4 member of the lillianite homologous series of modular structures created by unit-cell twinning of (311) slabs of the PbS archetype. The structure contains 30 independent cation sites, from which 12 are mixed sites, and 36 independent sulphur sites (i.e. six times the sites of the lillianite-like subcell). Only the c parameter copies that of lillianite, a is doubled and b is about 3/2 of a diagonal to (001) of lillianite. In agreement with the "2Pb → Ag + (Sb, As) oversubstitution" against ideal PbAgSb3S6, the trigonal prismatic sites on composition planes of twinning are occupied by two Pb-Sb rows and one Sb-Sb row, and the PbS-like slabs contain excess number of Ag sites. Unlike lillianite, the alternating (311)PbS slabs are non-equivalent and each of them has two types of differently occupied diagonal planes of atoms, always present in a 2:1 ratio. This results in triclinic symmetry with only small distortions from monoclinic metrics. In both slab types lone electron pairs of As and Sb congregate in large micelles with elliptic cross-section. Among lillianite homologues, jasrouxite exhibits hitherto unseen complications of cation ordering, resulting from the presence of two distinct metalloids in the structure, oversubstitution by (Ag + M3+), and highly expressed lone electron pair activity of trivalent cations.