Summary: The crystal structure of dundasite from Gonnesa (Italy) was solved by a three-dimensional Patterson function computed using intensity data collected photographically by the Weissenberg method, and refined by successive Fourier maps and least-squares cycles to an R index 0·.066 for 505 independent observed reflections.

The chemical formula resulting from the structural analysis is PbAl₂(CO₃)₂(OH)₄·H₂O. The mineral is orthorhombic, space group Pbnm, with a 9·08 Å, b 16·37 Å, c 5·62 Å. Thedundasite structure consists of a three-dimensional framework of coordination polyhedra around Pb and Al. Pb ions are coordinated by nine oxygen atoms with distances ranging from 2·53 to 2·91 Å. A1 ions show the usual octahedral coordination with average Al-O distance 1.90 Å. C-O mean distances are 1·28 and 1·27 Å for the two non-equivalent CO₃ groups. The water molecules are not coordinated by the cations.