Leguernite, Bi (sub 12.67) O (sub 14) (SO (sub 4) ) (sub 5) ; a new Bi oxysulfate from the fumarole deposit of La Fossa Crater, Vulcano, Aeolian Islands, Italy

Leguernite, ideally Bi (sub 12.67) O (sub 14) (SO (sub 4) ) (sub 5) , is a new mineral found in high-temperature fumarolic assemblages at La Fossa crater, Vulcano, Aeolian Islands, Italy. It occurs as aggregates of needle-shaped crystals associated strictly with anglesite, baliczunicite and an unknown Bi sulfate. Leguernite is colourless to white, transparent, non-fluorescent, has a sub-adamantine lustre and a white streak. Electron microprobe data led to the chemical formula (on the basis of 34 anions p.f.u.) (Bi (sub 12.40) Pb (sub 0.15) ) (sub Sigma =12.55) S (sub 5.08) O (sub 34) . The calculated density is 7.375 g cm (super -3) . A Raman spectrum collected on a single crystal of leguernite confirmed the anhydrous nature of the mineral.Leguernite is monoclinic, space group P2, with a = 11.2486(11), b = 5.6568(6), c = 11.9139(10) Aa, beta = 99.177(7) degrees , V = 748.39(12) Aa (super 3) and Z = 1. The crystal structure is built up of Bi-O blocks of a fluorite-like structure with Bi (sub 12) O (sub 14) composition separated by a single sulfate ion along [100] and by Bi(SO (sub 4) ) (sub 4) (super 5-) groups along [101]. It can also be described as composed of (001) layers with composition [Bi (sub 12) O (sub 14) (SO (sub 4) ) (super 6+) ] (sub n) alternating with layers of composition [Bi(SO (sub 4) (sub 4) ] (sub n) (super 5-) along [001]. Leguernite shows significant similarities with the synthetic Bi (sub 14) O (sub 16) (SO (sub 4) ) (sub 5) compound.The eight strongest reflections in the powder X-ray diffraction data [d in Aa (I) (hkl)] are: 3.220 (100) (013), 3.100 (95) (3I11), 2.83 (30) (020), 2.931 (25) (302), 2.502 (25) (3I04), 2.035 (20) (322), 1.875 (20) (3I24) and 5.040 (15) (110).The name is in honour of Francois "Fanfan" Le Guern (1942-2011), who was a very active volcanologist and specialist in volcanic gases and sublimates. Both the mineral and the mineral name have been approved by the IMA-CNMNC (2013-051).