Refinement of the Crystal Structure of Argentotetrahedrite: A New S(3) Site in Octahedral Cluster Ag, Cu6

Using electron microprobe analysis and X-ray diffraction (XRD), we studied crystal fragments of four minerals of the fahlore group from the Mangazeyskoye Ag–Pb–Zn deposit (Yakutia, Russia) with 3. 7–5. 8 atoms per formula units (apfu) Ag and 13–12 apfu S. Both methods revealed a negative correlation between Ag and S contents in the fahlore chemical composition. The crystal structures of four Ag-bearing fahlores are refined using XRD. The parameters of their unit cells (a0, Å) in group I 43m (cubic) are as follows: 10. 58829 for argentotetrahedrite- (Fe) (Ag3. 68Cu2. 32) Σ6 (Cu4. 04Fe1. 73Zn0. 27Σ2. 00) Σ6. 04 (Sb3. 94As0. 03) Σ3. 97S12S1. 01; 10. 53378 for argentotetrahedrite- (Zn) (Ag3. 82Cu2. 18) Σ6 (Cu4. 09Zn1. 71Fe0. 28Σ1. 99) Σ6. 08 (Sb3. 82As0. 10) Σ3. 92S12. 74; 10. 51544 for kenoargentotetrahedrite- (Fe) (Ag4. 34Cu1. 66) Σ6 (Cu4. 10Zn0. 96Fe1. 03Σ1. 98) Σ6. 08 (Sb3. 88As0. 03) Σ3. 92S12. 47; and 10. 48411 for kenoargentotetrahedrite- (Zn) (Ag5. 83Cu0. 17) Σ6 (Cu3. 96Zn1. 98Fe0. 04Σ2. 02) Σ5. 98 (Sb3. 99As0. 02) Σ4. 01S12S0. 04. The XRD studies of the fahlore crystal structure with a high resolution (2θmax = 82°) allowed us to refine the positions of Ag and Cu atoms in site M (2) and to find for the first time a new sulfur site S (3). А high Ag content (3. 7–5. 8 apfu) shifts S from site S (2) in the center of the octahedron by 0. 42–0. 44 Å to site S (3), which is located on a 3m symmetry element. Quantum chemical modeling showed that the sequential substitution of Cu atoms by Ag atoms in octahedral cluster SCu64+ gradually shifts the S atom from the center of the octahedron that illustrates the transition from site S (2) in the pure Cu cluster to the refined site S (3) in cases of mixed clusters S (Ag, Cu) 6. The unit cell parameter depends on the size of the Ag, Cu6 octahedral cluster in the fahlore crystal structure, which is compressed with an increasing Ag content due to a decrease in the Ag1–Ag1, Cu1–S (3), and Ag1–S (3) distances.