Manuelarossiite, CaPbAlF₇, a new fluoride from the Vesuvius volcano, Italy

Abstract Manuelarossiite, ideally CaPbAlF 7 , is a new fluoride mineral found in a specimen from a fumarole formed subsequent to the 1944 eruption of the Vesuvius volcano, Naples Province, Campania, Italy. It occurs as very rare tabular crystals, up to 0.06 × 0.04 × 0.015 mm, in the vugs of volcanic scoria, associated with cerussite. In the same scoria where manuelarossiite was found, anglesite, artroeite, atacamite, calcioaravaipaite, cerussite, challacolloite, cotunnite, hephaistosite, matlockite, napoliite and susannite were identified. Manuelarossiite is colourless with a white streak and adamantine lustre. It is brittle and has a laminated fracture. Cleavage is perfect on {001}. D calc = 5.095 g cm –3 . The calculated mean refractive index is 1.625. The chemical composition (wt.%, electron microprobe, H 2 O calculated by stoichiometry) is: CaO 13.98, PbO 55.46, Al 2 O 3 12.59, F 29.45, H 2 O 1.68, –O=F –12.40, total 100.76. The empirical formula calculated on the basis of 7 anions is Ca 1.00 Pb 1.00 Al 1.00 [F 6.25 (OH) 0.75 ] Σ7.00 . Raman spectroscopy confirms the limited presence of OH – groups in the mineral. Manuelarossiite is monoclinic, space group C 2/ m , with a = 7.6754(3), b = 7.4443(4), c = 9.2870(5) Å, β = 93.928(5)°, V = 529.39(5) Å 3 and Z = 4. The strongest lines of the powder X-ray diffraction pattern [ d , Å ( I , %) ( hkl )] are: 9.257 (57) (001); 4.537 (72) (111); 3.725 (98) (020); 3.630 (57) (20 $\bar 1$ ); 3.588 (65) (11 $\bar 2$ ); 3.460 (100) (021); 3.422 (63) (112); and 2.673 (65) (220). The crystal structure was refined to R 1 = 0.056 for 849 reflections with F > 4σ( F ). It is characterised by {001} layers formed by CaF 8 polyhedra decorated on both sides by AlF 6 octahedra, in turn connected to the {001} layer through edge-sharing. Successive {001} layers are bonded through Pb atoms. The new mineral honours Dr. Manuela Rossi (b. 1977) from the University of Naples for her contribution to the study of the Vesuvius volcano and its minerals.