Original paper

Structural relationships between sicherite, marrite, freieslebenite, and diaphorite - analysis based on anionic nets and polyhedral characteristics

Berlepsch, Peter; Makovicky, Emil; Armbruster, Thomas

Abstract

The crystal structures of sicherite TlAg2(As,Sb)3S6, freieslebenite PbAgSbS3, marrite PbAgAsS3, and diaphorite Pb2Ag3Sb3S8 all can be understood as superstructures of the PbS archetype with cations forming distorted MeS6 (Me = Tl, Ag, As, Sb, Pb) octahedra. Structural relationships between the four minerals are discussed on the basis of anionic nets and polyhedral characteristics. In order to deal with more reliable crystallographic data, a refinement of the crystal structure of marrite has been performed: space group P21/c, a = 5.9970(7), b = 12.6168(21), c = 7.2633(16)Å, β = 91.23(1)° (R1 = 2.93% for 1176 unique reflections with I > 2σI). The (100)PbS anionic nets of underlying archetype, used for the structural comparison, consist of two different network building units: the lozenge and the trapezoid. Variations in the mutual arrangements of lozenges and trapezoids lead to the different structures treated in this study. These structures reflect both different chemical compositions and space demand as well as a satisfactory bond valence balance for sulphur. Calculated geometrical values related to the coordination polyhedra of the four structures include the polyhedral volumes (Vp), the sphere volumes Vs of the spheres least-squares fitted to the ligands of individual coordination polyhedra, the volume eccentricity (ECCv), the volume sphericity (SPHv), and the volume distortion ν.

Keywords

sicheritemarritefreieslebenitediaphoritestructure refinement