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 F_o > 4σF_o, 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) ų. Its formula derived through the single-crystal structure refinement is Cu_9.00Ag_4.73Hg_1.00S₈, with Z = 1. Electron-microprobe analysis gave the formula (Cu_8.69Fe_0.12Zn_0.08Ni_0.07)_Σ8.96Ag_4.63Hg_0.93S₈, close to the ideal formula (Cu_8.70Me²⁺_0.30)Ag_4.70HgS₈, or (Cu_9−xMe²⁺_x)Ag_5−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 $(101¯)$ 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.