First Italian occurrence of cumengéite from Vesuvius: crystal structure refinement and revision of the chemical formula

The first Italian occurrence of cumengeite, a rare Pb and Cu hydroxychloride, is described from Vesuvius (Italy). Italian cumengeite has a volcanic origin (it occurs in the fumarole), which is unique for that mineral. The mineralogy of Vesuvius' fumarole is outlined briefly. A powder X-ray diffraction (XRD) pattern of cumengeite is presented. The crystal structure of cumengeite has been refined (R = 0.036) using single-crystal XRD data. Cumengeite is tetragonal, space group I4/mmm, a 15.1007(2), c 24.4940(4) Å. In the structure of cumengeite there are five independent Pb atoms, which are linked to 6-9 anions in five different ways. The structural study revealed the presence of two independent and previously unobserved water molecules, and led to the revision of the accepted chemical formula for the mineral: the new formula is Cu20Pb21Cl42(OH)40.6H2O. Analytical data taken from the literature for cumengeite from Boleo, Mexico (the type locality), as well as a new structural study carried out on the type material confirmed the new formula