Minerals within the Pd-Ni-As system: Crystal structures

F. Laufek¹, A. Vymazalová¹, D.A. Chareev², T.L. Grokhovskaya³, V.V. Kozlov³, J. Plášil⁴

¹Czech Geological Survey, Geologická 6, Prague 5, Czech Republic

²Institute of Experimental Mineralogy, RAS, Chernogolovka, Moscow,Russia

³Institute of Geology of Ore Deposits, Petrology, Mineralogy and Geochemistry RAS, Moscow, Russia

⁴Institute of Physics ASCR, v.v.i., Na Slovance 2, 128 21 Prague 8, Czech Republic

 frantisek.laufek@geology.cz

There are three ternary phases in the Pd-Ni-As system described as minerals, nipalarsite Ni₈Pd₃As₄, menshikovite Pd₃Ni₂As₃ and majakite, PdNiAs. Majakite and menshikovite were described as new minerals by Genkin [1] and Barkov [2], respectively. Their crystal structures have been hitherto unknown.  Nipalarsite was described together with its crystal structure determination by Grokhovskaya et al. [3]. Majakite was found in intergrowths with other platinum minerals in chalcopyrite and thalnakhite ores of the Mayak mine (Talnakh deposit), menshikovite was discovered in mafic-ultramafic layered complexes Lukkulaisvaara and Chiney, Russia. A fragment of menshikovite extracted from a sample from Lukkulaisvaara intrusion, Russia, was used for a structure analysis of this mineral. As the natural majakite proved to be unsuitable for a structural analysis, crystal structure analysis was carried out on a synthetic analogue PdNiAs.

The synthetic analogues of minerals in the system and phases on a Pd₂As-Ni₂As join were prepared using the Kullerud´s evacuated silica-glass tube method. Pure elements were used as starting materials for synthesis. The evacuated tube with charges were heated at 400 °C for several weeks. In order to study the extent of the (Pd,Ni)_Σ2As solid solution, selected experiments at the Pd₂As-Ni₂As join were prepared at 450, 500, 520 and 540°C. The experimental products were rapidly quenched in cold water and analysed by powder or single crystal X-ray diffraction and electron microprobe analysis.  

The performed experiments revealed three structurally different phases (solid solutions) along the Pd₂As-Ni₂As join system at 450°C: α-Pd₂As (Cmc2₁), β-(Pd,Ni)_Σ2As (P-62m) and Pd_1-xNi_1+xAs (Pnma).  The low-temperature orthorhombic phase α-Pd₂As transforms at 484°C to a hexagonal phase and belongs to the to the high-temperature β-(Pd,Ni)_Σ2As solid solution.

The phase PdNiAs is at 450°C part of the Pd_1-xNi_1+xAs solid solution showing Pnma symmetry. Its crystal structure contains a mackinawite-like blocks of edge sharing [NiAs₄] tetrahedra parallel to (001). Palladium shows unusual five-fold coordination resembling a tetragonal pyramid by As atoms. The coordination of Pd is further completed by close contacts with Ni and Pd atoms. Palladium atoms are located in voids between blocks of [NiAs₄] tetrahedra. A phase transition from low-temperature orthorhombic phase to the high-temperature hexagonal phase was observed. The hexagonal phase PdNiAs was also described by Evstigneeva [4]. Menshikovite Pd₃Ni₂As₃ crystal structure contains deformed [NiAs₄] tetrahedra. Each [NiAs₄] tetrahedra shares one edge with one adjacent tetrahedra along the a-axis and two opposing edges with adjacent tetrahedra along the c-axis forming chains of edge-shared [NiAs₄] tetrahedra running in 001 direction. Nickel atoms have three close contacts with adjacent Ni atoms across the shared tetrahedral edges. Palladium atoms show trigonal bipyramidal coordination by As atoms.

1. A. D. Genkin, T.L. Evstigneeva, N.V. Troneva, L.N. Valsov, Int. Geol. Rev., 20, (1976), 96.  

2. A. Y. Barkov, R.F. Martin, Y.A. Pakhomovsky, N.D. Tolstykh, A.P. Krivenko, Can Mineral., 40, (2002), 679.

3. T. L. Grokhovskaya, O.V. Karimova, A. Vymazalová, F. Laufek, D.A. Chaarev, E.V. Kovalchuk, L.O. Magazina, V. A. Rassulov. Min. Mag., 83, (2019), 837.

4. T. Evstigneeva, Y. Kabalov, J. Schneider. Material Science Forum, 321-324, (2000), 700.

This research was supported by the by the Strategic Research Plan of the Czech Geological Survey (DKRVO/ČGS 2023–2027).