Crystal-chemical study of ecdemite from Harstigen, a new natural member of the layered lead oxyhalides group

Ecdemite from Harstigen, Värmland, Central Sweden, was studied through Raman and FTIR spectroscopy, electronmicroprobe techniques, synchrotron powder and single-crystal diffraction. The ideal mineral formula proposed by Palache, Pb6Cl4As2O7, is confirmed. In contrast to previous suggestions, however, the present study demonstrates that the mineral is monoclinic, with space-group type P21 and unit-cell parameters a = 10.828(4) Å, b = 10.826(2) Å, c = 6.970(1) Å, b = 113.26(2). The crystal-structural study of ecdemite shows that the mineral can be considered as made up of regularly alternating Cl layers and litharge-like defective layers, also hosting As cations, with the representative formula {Pb2[Pb4O](AsO3)2}4+, in a 1:1 ratio. These features indicate that ecdemite can be considered as a new member of the layered lead oxyhalides group.