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

Non-metamict betafite, a pyrochlore-group mineral with general formula A(2-m)B(2)(X)O(6)(Y)(O,OH,F)(1-n).pH(2)O 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 ejecrum 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 rnicroprobe analyses gave the following crystal-chemical formula: (Ca1.24Na0.17U0.49REE0.03)(Sigma=1.93) (Ti1.05Nb0.76Zr0.14Fe0.04Ta0.01)(Sigma=2.00)O-6(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 (similar to150 k.y.,). this betafite sample is highly crystalline, thus allowing structure refinement of unheated material. Betafite from Le Carcarelle is cubic Fd3m, with a = 10.2637(13) Angstrom, and V = 1081.21(35) Angstrom(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.