MS15-P02 Synthesis and crystallographic study of laflammeite (Pd3Pb2S2) and thalhammerite (Pd9Ag2Bi2S4)Mineral laflammeite (Pd3Pb2S2) was firstly described by Barkov et al. (2002) from the Kirakkajuppura platinum-deposit, Penikat layered complex, Finland. Barkov et al. (2002) provided chemical and physical characterisation of this mineral, however its detailed crystal structural analysis has been lacking. Thalhammerite (Pd9Ag2Bi2S4) was discovered in millerite-pyrite-chalcopyrite vein-disseminated ore from the Komsomolsky mine in the Talnakh deposit, Russia (Sluzhenikin and Mokhov, 2015). Crystal structures of both minerals and relevant crystal-chemical implications will be presented.
Laflammeite occurs as subhedral platelets up to 150 μm, however the crystals are finely twinned and consequently unsuitable for a direct crystal structure study (Barkov et al. 2002). Thalhammerite occurs as tiny inclusions (from few μm up to about 40-50 μm) in sulphide ore where it forms intergrowns with other Pd-bearing minerals. Therefore, both minerals were synthetized by silica glass tube technique by heating at 400 ºC from stoichiometric mixture of elements. The prepared synthetic analogues of laflammeite and thalhammerite were used for a crystal structure study. The structural identity between natural and synthetic materials was subsequently confirmed by an electron-backscattered diffraction.
Laflammeite, Pd3Pb2S2, crystalizes in Pmmn space group (a = 5.78, b = 8.18, c = 5.96 Å) and Z = 2. Its crystal structure show many similarities with structures of shandite (Ni3Pb2S2, Rm), parkerite (Ni3Bi2S2, C2/m) and vymazálovaite (Pd3Bi2S2, I213). All these minerals show a common structure motive: a pseudocubic subcell of the CsCl-type composed of Bi(Pb) and S atoms. A half of available octahedral voids is occupied by Ni(Pd) atoms. The distribution of Ni(Pd) atoms (i.e. the ordering scheme) determines the structure type (Weihrich et al. 2007). Laflammeite can be considered as antiperovskites superstructures.
Thalhammerite, Pd9Ag2Bi2S4 shows I4/mmm symmetry (a = 8.02, c = 9.15 Å) and Z = 2. Its unique crystal structure is based on a three-dimensional framework which consists of two types of blocks of polyhedra that interpenetrate and support each other. The first type consists of corner-sharing [PdS4] and [PdBi2S2] squares. The second is formed by flattened tetrahedra [PdBi2S2]. Silver atoms occupy channels running along the c direction. Thalhammerite crystal structure merges metallic building blocks with structure motives typical for polar sulphides.
Barkov, Y.A. et al. (2002). Can Mineral 40, 671–678.
Sluzhenikin, S.F. & Mokhov, A.V. (2015). Mineralium Deposita 50, 465-492.
Weihrich, R. et al. (2007). Prog Solid State Chem 35, 309-327.
Keywords: crystal structure, new minerals, sulphides