Occurrence and crystal-structure of U-rich betafite from Le Carcarelle (Vico volcanic complex).

Here we report a new occurrence of betafite, a pyrochlore-group mineral, at Le Carcarelle, S. Martino al Cimino community, Vico volcanic complex. The mineral occurs into myarolitic cavities of a foid-bearing syenitic ejectum enclosed within the pyroclastic formation known as "ignimbrite C" (Locardi, 1965), belonging to the main effusive phase of Vico. The host rock is composed of K-feldspar, biotite, augitic clinopyroxene and minor sodalite. Accessory minerals include titanite, magnetite, vonsenite, baddeleyite and a REE-bearing silico-phosphate close in composition to cheralite (an intermediate U-REE silico-phosphate solid-solution phase in the brabantite-huttonite-monazite triangle). The pyrochlore-group mineral occurs as very rare euhedral elongated crystals up to 100 µm in length and 20 µm across. It evidently formed from late-stage hydrothermal fluids, since it is characteristically found as a late-formed mineral associated with an intergrowth of heavy-minerals, whose the crystallisation sequence is baddeleyite ? uraninite ? pyrochlore ? cheralite. Coating the betafite is a thin crust of a F-bearing aluminium silicate (possibly topaz). Textural SEM observations suggest, at least for "cheralite", homogeneous nucleation from a supersaturated solution, and point to a significant circulation of mineralising phosphate-bearing fluids enriched in heavy and incompatible elements such as actinides, Zr, Nb and REEs during the very-late volcanic stages, as already suggested by Della Ventura et al. (1999) and Bellatreccia et al. (2002). Detailed EMP analyses gave the following simplified crystal-chemical formula: (Ca1.24Na0.17U0.49REE0.03)?=1.93 (Ti1.05Nb0.76Zr0.14Fe0.04Ta0.01)?=2.00 O6(O,OH), which, according to the classification of Hogarth (1977), is betafite Compared to the other occurrences reported in the mineralogical literature, betafite from Le Carcarelle is very enriched in U and depleted in Th. Non-metamict pyrochlores with comparable U concentrations are extremely rare. To our knowledge only one sample, actually poorer in U (up to 26.4 wt%) and richer in Nb and Ta, has been described by Hogarth & Horne (1989) in a radioactive tuff at Ndale (Uganda); however, no structural data have been reported for this mineral. No evidence of metamictisation was found in the sample from Le Carcarelle, this feature being related to the young age of formation (20-15 ky); accordingly, Pb was not detected in WDS analyses. Betafite from Le Carcarelle is cubic F d-3m, with edge = 10.2622(5) Å, and V = 1080.7(1) Å3. Good-quality X-ray data were collected on a crystal with size 0.20 x 0.03 x 0.03 mm in the ? range 2-35°, yielding R(int) = 1.5%, and the crystal structure was refined to wR2 = 13%, R1 = 4.5% for all the independent reflections. The scattering values refined at the various sites are in close agreement with those calculated from the unit-formula by EMP analysis. Bellatreccia, F., Della Ventura, G., Williams, T.C., Lumpkin, G.R., Smith, K., and Colella, M. Eur. J. Miner., 14, 809-820 (2002). Della Ventura, G., Williams, T.C., Cabella, R., Oberti, R., Caprilli, E., and Bellatreccia, F. Eur. J. Miner., 11, 843-854 (1999). Hogarth, D.D. Am. Mineral., 62, 403-410 (1977) Hogarth, D.D., and Horne, J.E.T. Min. Mag., 53, 257-262 (1989). Locardi, E. Locardi, E. Atti Soc. Toscana Sc. Naturali, 45, 55?173. (1965)