The crystal structure of launayite from Taylor Pit, Madoc, Ontario, Canada: crystal chemistry, modulated superstructures, and parent modular structure compared with rouxelite

Abstract

The crystal structure of launayite, ideally Cu2Pb20(Sb,As)26S60 (Z=4) from Taylor Pit, Madoc, Ontario, Canada, has been solved for the first time using the single-crystal X-ray diffraction (SCXRD) method. The mineral is composed of distinct superstructures that can be derived from the same parent structure. The structure of the main component is monoclinic and has been solved in the space group P2/a, with cell parameters a=42.6466(14), b=8.0381(2), c=34.3957(10) Å, β=64.684(2)°, and V=10658.4(6)Å3 from an untwined crystal. The asymmetric unit of launayite contains 48 cation sites and 60 sulfur sites. Final refinement resulted in an R1 value of 0.0955 for 11 741 unique reflections. The structural formula obtained from SCXRD study is Cu2Pb20.330Sb23.024As2.689S60, Z=4, in agreement with the formula Cu2.078Ag0.059Tl0.057Pb20.404Sb22.830As2.772S59.80 from microprobe analysis. The structure of launayite can be viewed both as a boxwork structure and as a rod-based structure. The modular description of the launayite structure reveals a very close relationship with the structure of rouxelite: the parent structures of both can be regarded as merotypes. A full comparison of the crystal chemistry and modular description of both structures is presented.