Kermesite, Sb₂S₂O: crystal structure revision and order–disorder interpretation

The structure of kermesite, Sb₂S₂O, was refined and interpreted as an order–disorder (OD) structure of layers. The maximum-degree-of-order (MDO) polytypes are derived, twinning explained, and desymmetrization discussed and evaluated. The interpretation requires the use of a non-standard pseudo-monoclinic octuple unit cell with extra centering points. Its relation to the primitive triclinic cell is displayed in the picture.