Contribution to the crystallography of Ag-Pd chalcogenides

F. Laufek^1,4, A. Vymazalová¹, M. Drábek¹, D. A. Chareev ², A. V. Kristavchuk ^{2, 3} J. Drahokoupil⁴,

¹Czech Geological Survey, Geologická 6, 152 00 Praha 5, Czech Republic

² Institute of Experimental Mineralogy RAS, 142 432 Chernogolovka, Russia

³ University of Dubna, Universitetskaja 19, 141 980 Dubna, Russia

⁴Institute of Physics of the AS CR, v.v.i., Na Slovance 2, 182 21, Praha 8, Czech Republic

frantisek.laufek@geology.cz

The chalcogenides of Ag and Pd occur in nature as minerals or poorly characterized phases in various geological environments. The Ag-Pd tellurides can be found in association with mafic or ultramafic igneous rocks in complexes like Sudbury (Canada), Norilsk (Russia) or Salt Chuck intrusion (USA) among many other deposits [1]. The Ag-Pd selenide Ag₂Pd₃Se₄ (known as a mineral chrisstanleite) was observed e.g. from carbonate veins in Torquay (England), from Tilkerode in eastern Harz Mountains (Germany) and in uranium mineralization in Předbořice (Czech Republic) [1]. The Ag-Pd sulphides were reported from the Duluth Complex [2] and Norilsk (Russia) [3]. In order to understand the formation of these minerals and phases at natural conditions and predict possible new minerals, the systems Ag-Pd-S/Se/Te were experimentally investigated at selected temperatures. The evacuated silica glass tube technique was used for the synthesis of all phases. The experimental products were investigated by means of optical and electron microscopy, electron microanalysis, powder and single-crystal X-ray diffraction techniques.

Up to date, twelve Ag-Pd chalcogenides (Ag₄Pd₃Te₄, Pd₁₄Ag₂Te₉, (Pd,Ag)₃Te, (Pd,Ag)₈Te₃, (Pd,Ag)₄Te; Ag₂Pd₃Se₄ AgPd₃Se, (Ag,Pd)₂₂Se₆, Ag₇Pd₇₃Se_20; Ag₂PdS, Ag₂Pd₃S, Ag₃Pd₁₃S₄) have been synthesised and most of them have been structurally characterized. One new mineral (lukkulaisvaaraite - Pd₁₄Ag₂Te₉) has been also described. The research revealed strong different coordination preferences of palladium and silver in these compounds. While the palladium atoms prefer the square planar or octahedral coordination, the silver atoms show larger variability in their coordination environments. The character of crystal structures also varies from the three-dimensional framework structures to the phases with a layered structure. Compounds with strong metallic behaviour were also observed. The number of direct Pd-Pd, Ag-Ag and Pd-Ag contacts is a function of the metal:chalcogen ratio. The limits of Ag for Pd substitution were extensively studied at various temperatures. In the Ag-Pd-Te and Ag-Pd-Se systems, the Ag for Pd substitution occurs in compounds with high metal : Se(Te) ratio. The maximal range of this substitution (i.e. 34 - 56 wt. % of Ag) was detected for the (Ag,Pd)_Σ22Se₆ solid solution. Relevant analogical structures containing Cu and alkali metals will be discussed.

1. L. J. Cabri: The geology, geochemistry, mineralogy and benefication of the platinum-group elements.

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2. M. J. Severson and S. A. Hauck, S.A. Natural Resources Research Institute, University of Minnesota, Duluth, (2003) Technical Report, NRRI/TR-2003/37, 2010.  

3. S. F. Sluzhenikin, Russian J. General Chem. 81(6), 1288, 2010.