Ordered distribution of Cu and Ag in the crystal structure of balkanite, Cu9Ag5HgS8

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 Fo>4sFo, 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, b = 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)S8.96Ag4.63Hg0.93S8, 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 ½), 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.