Fettelite, [Ag6As2S7][Ag10HgAs2S8] from Chañarcillo, Chile: Crystal structure, pseudosymmetry, twinning, and revised chemical formula

The crystal structure of the rare mineral fettelite was solved using intensity data collected from a twinned crystal from Chañarcillo, Copiapó Province, Chile. This study revealed that, in spite of the strong hexagonal pseudosymmetry, the structure is monoclinic (space group C2) with a = 26.0388(10), b = 15.0651(8), c = 15.5361(8) Å, β = 90.48(1)°, and V = 6094.2(5) ų. The refinement of an anisotropic model led to an R index of 0.0656 for 7143 observed reflections [I > 2σ(I)] and 0.0759 for all 17 447 independent reflections. Fettelite is intimately twinned with six twin domains. The structure consists of the stacking of two module layers along [001]: an A module layer with composition [Ag₆As₂S₇]²⁻ and a B module layer with composition [Ag₁₀HgAs₂S₈]²⁺. The As atoms form isolated AsS₃ pyramids typical of sulfosalts, Hg links two sulfur atoms in linear coordination, and Ag occupies sites with coordination ranging from quasi linear to almost tetrahedral. The A module layer found for fettelite is identical to that described for the minerals belonging to the pearceite-polybasite group. On the basis of information gained from this characterization the crystal chemical formula was revised according to the structural results, yielding [Ag₆As₂S₇][Ag₁₀HgAs₂S₈] (Z = 8).