The crystal structure of ramdohrite, Pb_5.9Fe_0.1Mn_0.1In_0.1Cd_0.2Ag_2.8Sb_10.8S₂₄: A new refinement

Abstract

The crystal structure of ramdohrite, Pb_5.9Fe_0.1Mn_0.1In_0.1Cd_0.2Ag_2.8Sb_10.8S₂₄, from the Chocaya mine, Potosí, Bolivia, determined by Makovicky and Mumme from film data in 1983, was refined from single-crystal diffractometer data to the R value 0.060, based on 5230 reflections [I > 2σ(I)] from a twinned crystal. Lattice parameters are a = 8.7348(3), b = 13.0543(4), c = 19.3117(6) Å, and β = 90.179(2)°, space group P2₁/n.

Two bicapped trigonal prismatic sites of lead bridge and unite adjacent (311)_PbS slabs. These slabs contain five distinct coordination pyramids of Sb with trapezoidal cross sections, a mixed and disordered Sb-Ag-Cd-(Pb) site, refined as 0.39 Sb + 0.61 Ag, a pure Ag site with a very open, irregular tetrahedral coordination, and an octahedral site occupied by Pb. The (311)_PbS slabs contain large lone electron pair micelles formed by four distinct antimony sites in alternation with small such micelles formed by a single Sb site. The geometric arrangement of these slabs is not based on crankshaft chains of short, strong Me-S bonds but on a chess-board arrangement of (predominantly) Sb pairs that share two common S atoms via short bonds. Relationships to, and differenced from, fizelyite and uchucchacuaite are described and discussed.