Structure refinement of natural robinsonite, Pb4Sb6S13: cation distribution and modular description

Abstract

Crystal structure of robinsonite from the locality Male Zelezne, Nizke Tatry Mountains, Slovakia, has been refined using single-crystal data (MoKa X-ray diffraction, CCD area detector) to the conventional R-factor: R 1 = 0.045. Robinsonite crystallizes in the space group 12/m, with a = 23.642(4), b = 3.9761(6), c = 24.420(4) A, β = 93.808(3)° and Z = 4. X-ray diffraction and chemical analysis resulted in comparable formulae, Pb 4 . 0 3 Sb 5 . 9 7 S 1 3 and Pb 3 . 9 3 Sb 5 . 7 7 S 1 3 , respectively, which are close to the ideal formula Pb 4 Sb 6 S 1 3 . Robinsonite is a member of a large family of rod-based sulphosalts, composed of rods of SnS (or TII) archetype. In robinsonite, two types of rods, respectively 2 and 3 Sb coordination pyramids broad, combine. Their arrangement results in two types of rod-layers, denoted according to their inter-rod connections as Type 1 and Type 4, respectively. In the former case, a tightly bonded double-layer continues from one rod into the other. In the latter, the rods are interconnected by the smallest possible fragment of SnS-like configuration that is rotated 90° in respect to the SnS-like (TIl-like) arrangement in the rod interiors. The mixed cation sites are the main characteristic of the robinsonite structure. The present study is the second one performed on natural robinsonite and the first in which all mixed positions are successfully refined. The mixed sites are concentrated in slabs parallel to [010]. The composition of these slabs can be explained by the necessity to match dimensions of the structure motifs across a noncommensurate interspace. Comparison with monoclinic boulangerite shows many similarities in distribution of mixed cation sites.