We report data on the composition and crystal structure of the most Ag-rich (15.63 apfu) natural polybasite yet discovered. It shows the -M2a2b2c polytype. The crystal studied was found in a sample (mineralogical collection of the Royal Ontario Museum) from Gowganda, Timiskaming District, Ontario, Canada. Electron microprobe analysis yields the formula [Ag6(Sb1.78As0.18)∑=1.96S7] [Ag9(Ag0.63Cu0.43)∑=1.06S4]. Lattice parameters are a = 26.2625(4), b = 15.1623(5), c = 24.1061(6) Å, β = 90.045(5)°, V = 9599.0(4) Å3. The structure was refined in the space group C2/c to R = 0.0581 using 7725 observed reflections [I > 2σ(I)]. The refinement shows that one of the three structural positions of the B module layer usually occupied by Cu is dominated by Ag. Crystal-chemical characteristics are compared with published data on the other members of the pearceite-polybasite group. Some remarks concerning nomenclature are also given.

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