FIG. 1. The I4/mmm sub-cell of asisite, a = 3.893 Å and c = 22.803 Å. This type of sub-cell is shared with parkinsonite. Structure refinement indicates that ~75% of the Pb(2) sites are occupied. Pb(1) forms a Pb[O4Cl4] square antiprism; Pb(2) forms a PbO4 array and is not bonded to Cl.
FIG. 2. (a) A typical hk0 electron diffraction pattern of asisite with the reciprocal sub-cell shown. (b) A schematic of the geometrical relationship between the reciprocal sub- and super-cells of asisite superimposed upon the hk0 electron diffraction pattern.
FIG. 3. A schematic of the relationship between super-cell and sub-cell (shaded) superimposed upon a PbO sheet. Solid circles are Pb sites that lie above and below the sheet of oxygen atoms (open circles). Asisite has a 26-cation-site motif [13 Pb(1) + 13 Pb(2)] within the sheet and there are two Si atoms (not shown explicitly) per super-sheet. The super-sheet repeat is 14.06 Å.
FIG. 4. Comparison of the local environments of the tetrahedral cations in symesite (SO4), kombatite (VO4) and that proposed for asisite (SiO4). Numbers refer to bond valences.
