Lead-antimony sulfosalts from Tuscany (Italy) : II - Crystal structure of scainiite, Pb₁₄Sb₃₀S₅₄O₅, an expanded monoclinic derivative of Ba₁₂Bi₂₄S₄₈ hexagonal sub-type (zinkenite group)

The crystal structure of scainiite, Pb₁₄Sb₃₀S₅₄O₅, has been solved from single-crystal X-ray data and refined to an R index of 8.9 % based on 3703 unique reflections. The structure is monoclinic C2/m, with cell parameters a 51.996(8), b 8.148(1), c 24.311(4) Å, β 104.09(1)°, V9990(10) ų, and with Z = 4. The structure analysis was essential in determining the position of oxygen atoms in a special double-ribbon, Sb₈(Pb,Sb)₂S₁₀O₅, directly related to the structure of kermesite, Sb₂S₂O. The general organization of the structure can be described according to the general principles developed by Makovicky (1993) for complex sulfides related to the PbS and SnS archetypes. This structure falls into the group of “pure rod structures” (zinkenite group), and is an expanded homologue of cyclic hexagonal Ba₁₂Bi₂₄S₄₈. The expansion of the parent structure is obtained by shearing along a zig-zag surface, that creates lozenge shaped spaces in place of the original hexagonal channels, and filling these spaces by the kermesite-type fragments.