Ordered distribution of Cu and Ag in the crystal structure of balkanite, Cu9 Ag5 HgS8
Biagioni, Cristian; Bindi, Luca
European Journal of Mineralogy Volume 29 Number 2 (2017), p. 279 - 285
published: Apr 1, 2017
ArtNo. ESP147052902016, Price: 29.00 €
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 >4 σF o, with a final R 1 = 0.055. Balkanite is monoclinic, space group P 2/ 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 stucture refinement is Cu9.00 Ag4.73 Hg1.00 S8, with Z = 1. Electron-microprobe analysis gave the formula (Cu8.69 Fe0.12 Zn0.08 Ni0.07)Σ8.96 Ag4.63 Hg0.93 S8, close to the ideal formula (Cu8.70 Me 2+ 0.30)Ag4.70HgS 8, or (Cu9 - x Me2+ x )Ag5 - x HgS8, 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 [Cu5 Ag2 S4]-, or as formed by the alternation of two (10 1) polyhedral layers, having composition [Cu7 Ag3 S4]2+ and [Cu2 Ag2 HgS4]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)14 HgS8, is also presented.