The crystal structure of barstowite, Pb4Cl6(CO3)center dot H2O, determinedon crystals from Etruscan slags and from a Late-Hellenistic shipwreck

Crystals of the rare lead mineral barstowite, Pb4Cl6(CO3) . H2O, were found on a lead object from a Late-Hellenistic shipwreck and within Etruscan iro n slags. Its crystal structure was determined and refined to R = 0.023. Bar stowite crystallizes in the monoclinic space group P2(1)/m with a = 4.2013( 8) A, b = 9.199(2) A, c = 16.663(3) A, beta = 91.82(1)degrees. The lead ato ms are nine-fold coordinated in form of tricapped trigonal prisms by chlori ne and oxygen atoms. The polyhedra, through sharing of faces. edges and cor ners, define structural layers which are linked together in a three-dimensi onal framework. Oxygen atoms belonging to the water molecules form hydrogen bonds with oxygens of the carbonate groups. The relationship of barstowite with cotunnite and phosgenite is shortly discussed.