The crystal structure of argentojarosite, AgFe3(SO4)2(OH)6

The crystal structure of natural argentojarosite, ideally AgFe3(SO4)2(OH)6, a 7.3398(7), c 16.538(3) Angstroem, V771 .4(2) Angstroem3, space group R3-m, Z = 3, has been refined to an R1 index of 2.8% based on 307 unique reflections measured using MoΚα radiation on an automated four-circle diffractometer. The refined occupancy of the A site suggests that it is fully occupied by Ag, with Ag-O and Ag-OH bond distances of 2.963 and 2.721 Angstroem, respectively, and an AgO6(OH)6 icosahedral volume of 57.71 Angstroem3. Whereas Ag+ in argentojarosite is in 12-fold coordination, for which the ionic radius is projected to be 1.48-1.56 Angstroem, the crystal-structure results suggest that the radius is 1.35-1.36 Angstroem, closer to the value expected for nine-fold coordination. Consequently, the coordination sphere of Ag+ in argentojarosite is perhaps best thought of as intermediate between six- and 12-coordination. Twelve-fold coordination of Ag is not known in any other mineral, and there is a strong possibility that the published bond-valence curves cannot be applied to this coordination of Ag. (author)