Rudabányaite, a new mineral with a [Ag2Hg2]4+ cluster cation from the Rudabánya ore deposit (Hungary)

Rudabányaite was found in cavities of siliceous sphaerosiderite and limonite rocks at the Adolf mine area of the Rudabánya ore deposit (northern-East Hungary). The new mineral forms small crystals up to 0.6 mm and aggregates of a few mm across. Usually they have a xenomorphic shape, only occasionally cubic symmetry is morphologically discernible; the crystal forms {110} and {100} were recognized. The crystals are transparent, have yellowish-orange to brownish-yellow colour and a lemon-yellow streak, the lustre is adamantine. The Mohs’ hardness is 3–4. No cleavage was observed. Rudabányaite is optically isotropic. The density could not be measured due to lack of material; ρ(calc.) = 8.04 g/cm³. Electron-microprobe analyses gave the average composition (in wt%) Ag₂O 29.39, Hg₂O 52.62, As₂O₅ 13.69, Cl 4.62, SO₃ 0.19, O=Cl −1.04, sum 99.47. The empirical formula based on four oxygen atoms is (Ag_2.06Hg_2.05)_Σ=4.11(As_0.97S_0.02)_Σ=0.99O₄Cl_1.06; the idealized formula as derived from chemical analyses and crystal-structure investigation is [Ag₂Hg₂][AsO₄]Cl. The crystal-structure investigation was performed on single-crystal X-ray data; the refinements on F² converged at wR2(F²) = 0.068 and R1(F) = 0.031 for all 972 unique data and 53 variable parameters. Rudabányaite crystallizes in space group $F4¯3c$⁠, a = 17.360(3) Å, V = 5231.8 Å³, Z = 32. The crystal structure is characterised by two crystallographically different [M₄]⁴⁺ cluster cations forming tetrahedra; M = (Ag,Hg) with a ratio Ag:Hg ~ 1:1. There is not any evidence for an order between the Ag and Hg atoms. Small amounts of the M atoms are displaced by ~0.5 Å. Topologically, the barycentres of the [M₄]⁴⁺ clusters and the As atom positions of the crystal structure of rudabányaite form a cubic primitive lattice with a′ = ½a = 8.68 Å; half of the voids are occupied by Cl atoms.