First occurrence of the M2a2b2c polytype of argentopolybasite, [Ag6Sb2S7][Ag10S4]: Structural adjustments in the Cu-free member of the pearceite-polybasite group

The chemistry and the crystal structure of the recently described mineral argentopolybasite are critically discussed based on the study of two new occurrences of the mineral: Gowganda, Timiskaming District, Ontario, Canada and IXL Mine, Silver Mountain mining district, Alpine County, California.The crystal structure of argentopolybasite can be described as the sequence, along the c axis, of two alternating layers: a [Ag6Sb2S7](2-) A layer and a [Ag10S4](2+) B layer. In the B layer there are linearly-coordinated metal positions (B sites), which are usually occupied by copper in all members of the pearceite-polybasite group, resulting in a B-layer composition [Ag9CuS4](2+). In argentopolybasite, however, Ag fills all the metal sites in both A and B layers. By means of a multi-regression analysis on 67 samples of the pearceite-polybasite group, which were studied by electron microprobe and single-crystal X-ray diffraction, the effect of Ag, Sb and Se on the B sites of the B layer was modelled. Although the nomenclature rules for these minerals are based on chemical data only, we think this approach is useful to evaluate the goodness of the refinement of the structure (Ag/Cu disorder) and thus fundamental to discriminate different members of the pearceite-polybasite group.