The crystal structure of barstowite, Pb4Cl6(CO3)·H2O, determined on 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 iron slags. Its crystal structure was determined and refined to R = 0.023. Barstowite crystallizes in the monoclinic space group P21/m with a 4.2043(8) Å, b 9.199(2) Å, c 16.663(3) Å, beta 91.82(1). The lead atoms are nine-fold coordinated in form of tricapped trigonal prisms by chlorine and oxygen atoms. The polyhedra, through sharing of faces, edges and corners, define structural layers which are linked together in a three-dimensional 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