Ordered distribution of Cu and Ag in the crystal structure of balkanite, Cu9 Ag5 HgS8

The crystal structure of balkanite from the San Giovanni mine, Sardinia, Italy, has been solved by X-ray single-crystal diffraction on the basis of 883 reflections with Fogt;4σFo, with a final R1 = 0.055. Balkanite is monoclinic, space group P2/m, with unit-cell parameters a = 9.5539(11), b = 3.9150(4), c = 10.6424(12) Å, β = 90.047(9)°, V = 398.06(8)Å3. Its formula derived through the single-crystal structure refinement is Cu9.00Ag4.73Hg1.00S8, with Z = 1. Electron-microprobe analysis gave the formula (Cu8.69Fe0.12Zn0.08Ni0.07)σ8.96Ag4.63Hg0.93S8, close to the ideal formula (Cu8.70Me2+0.30)Ag4.70HgS8, or (Cu9-xMe2+x)Ag5-xHgS8, with x = 0.3. Although balkanite is not a layered compound, its crystal structure can be described as formed by the alternation, along a, of Cu-pure and Ag-rich layers. Additionally, it can also be described as based on two different alternating (010) atomic layers with all atoms on special positions (at y = 0 and), having chemical composition [Cu4Ag3HgS4]+ and [Cu5Ag2S4]-, or as formed by the alternation of two (10-1) polyhedral layers, having composition [Cu7Ag3S4]2+ and [Cu2Ag2HgS4]2-. Hg is linearly coordinated, Ag assumes a tetrahedral or a distorted octahedral coordination, whereas Cu displays a linear or planar triangular coordination. A critical comparison with the related mineral danielsite, ideally (Cu,Ag)14HgS8, is also presented