Non-metamict betafite from Le Carcarelle (Vico volcanic complex, Italy): occurrence and crystal structure.

Non-me t a m i c t b e t a f i t e , a pyrochlore-group mi n e r a l with g e n e r a l formula A2mB2 XO6 Y(O,OH,F)1n·pH2O and 2 Ti > Nb+Ta and U > 20% at the A site, has been found at Le Carcarelle, (Latium, Italy). It occurs within miarolitic cavities of a foid-bearing syenitic ejectum enclosed within the pyroclastic formation known as ''ignimbrite C'', which belongs to the main effusive phase of the Vico volcanic complex. The host rock is composed of K-feldspar, biotite, augitic clinopyroxene, magnetite and minor sodalite. Electron microprobe analyses gave the following crystalchemical formula: (Ca1.24Na0.17U0.49REE0.03)S=1.93 (Ti1.05Nb0.76Zr0.14Fe0.04Ta0.01)S=2.00O6(O,OH). Compared with other occurrences reported in the mineralogical literature, betafite from Le Carcarelle is extremely enriched in U and depleted in Th. Due to its young age of formation (~150 k.y.), this betafite sample is highly crystalline, thus allowing structure refinement of unheated material. Betafite from Le Carcarelle is cubic Fd3¯ m, with a = 10.2637(13) A ? , and V = 1081.21(35) A ? 3, and has a smaller A site (consistent with the higher U content), and a larger and more distorted B site (consistent with the higher Ti content) than calciobetafite from Campi Flegrei, Italy (Mazzi and Munno, 1983). Analysis of the atomic displacement parameters provides evidence for static disorder at the X site.