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Mineralogical Magazine; December 2005; v. 69; no. 6; p. 1037-1045; DOI: 10.1180/0026461056960306
© 2005 Mineralogical Society of Great Britain and Ireland
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First Italian occurrence of cumengéite from Vesuvius: crystal-structure refinement and revision of the chemical formula

G. Cruciani1, P. Orlandi2, M. Pasero2,* and M. Russo3

1 Dipartimento di Scienze della Terra, Università di Ferrara, Corso Ercole I d’Este 32, I-44100 Ferrara, Italy
2 Dipartimento di Scienze della Terra, Università di Pisa and CNR-IGG, Via S. Maria 53, I-56126 Pisa, Italy
3 Osservatorio Vesuviano, INGV, Via Diocleziano, 328, I-80124 Napoli, Italy

* E-mail: pasero{at}dst.unipi.it

The first Italian occurrence of cumengéite, a rare Pb and Cu hydroxychloride, is described from Vesuvius (Italy). Italian cumengéite 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 cumengéite is presented. The crystal structure of cumengéite has been refined (R = 0.036) using single-crystal XRD data. Cumengéite is tetragonal, space group I4/mmm, a 15.1007(2), c 24.4940(4) Å. In the structure of cumengéite 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 cumengéite from Boléo, Mexico (the type locality), as well as a new structural study carried out on the type material confirmed the new formula.

KEYWORDS: cumengéite, Vesuvius, Italy, crystal structure, crystal-chemical formula






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