Vicanite-(Ce): a new Ca-Th-REE borosilicate from the Vico volcanic district (Latium, Italy)

Vicanite-(Ce), ideally (Ca, REE, Th) As-15(5+) (As0.53+Na0.5Fe3+ Si-6 B-4 O-40 F-7, was found inside a volcanic ejectum collected from a pyroclastic formation of the Vice volcanic complex at Tre Croci, Vetralla (north of Rome), Viterbo province, Italy. The host rock is composed of K-feldspar (high sanidine), plagioclase, clinopyroxene plus minor quantities of magnetite and clinoamphibole. Vicanite-(Ce) occurs in small myarolitic cavities in association with zircon, thorite, thorian uraninite, betafite, thorian hellandite, titanite, antimonian asbecasite, apatite, stillwellite-(Ce), and other unidentified phases. It is found as rare euhedral crystals up to 0.3 mm across, yellowish green in colour; uniaxial (-), epsilon 1.722(2), omega 1.757(2), non pleochroic. The streak is white, and the mineral shows no fluorescence under UV radiation. The crystal chemical formula calculated from electron microprobe analyses, on the basis of 47(O+F) and B fixed to 4 atoms, is: (Ca-8.03 Ce-2.32 La-1.95 Th-1.82 Pr-0.44 Nd-0.28 U-0.19)(Sigma 15.06) (As-0.86(5+) P-0.14)(Sigma 1.00) (As-0.34(3+) Na-0.12)(Sigma 0.46) (Fe0.573+Ti0.05Al0.08)(Sigma 0.67) Si6.07B4 (O-36.57 F-10.43)(Sigma 47). Density is higher than 4.2. The mineral is trigonal and crystallizes in the non-centrosymmetric R3m space group. The refined unit-cell parameters are: a = 10.795(1), and c = 27.336(4) Angstrom, Z = 3 (hexagonal cell). The eight strongest X-ray diffraction lines are [d in Angstrom (I) (hkl)]: 7.70 (50) (012), 4.42 (50) (202), 3.13 (50) (214), 2.993 (100) (027), 2.950 (70) (303), 2.698 (50) (220), 1.839 (50) (3,0,12), 1.802 (50) (2,0,14). The main feature of the structure is the presence of a new polyanion (Si-3 B-3 O-18)(15-). The cell dimensions of vicanite-(Ce) are very close to those of okanoganite-(Y), but the chemical composition is rather different as no Y, Gd, Dy, Sm and Er are present in vicanite-(Ce). The name is for the Vican volcanic complex where the mineral was found. Holotype is deposited in Museo di Mineralogia, Universita di Roma ''La Sapienza'' (MMUR).