Arsenic-rich fergusonite-beta-(Y) from Mount Cervandone (Western Alps, Italy): Crystal structure and genetic implications

An As-rich variety of fergusonite-beta-(Y) occurs as greenish yellow pseudo-bipyramidal crystals up to 1 mm in length in centimeter-sized secondary cavities within sub-horizontal pegmatite dikes at Mount Cervandone (Western Alps, Italy). The mineral is associated with quartz, biotite, potassium feldspar, and orange-yellow barrel-shaped hexagonal crystals of synchysite-(Ce) up to 2 mm in length. Fergusonite-beta-(Y) crystallized during the Alpine metamorphism under amphibolite-facies conditions, as a result of interaction between As-enriched hydrothermal fluids, circulating through the pegmatite dikes, and precursor accessory minerals in the pegmatites enriched in high-field-strength elements. These pegmatites are of NYF (niobium-yttrium-fluorine) geochemical type and served as the principal source of Be, 5(, Nb, Ta, and rare-earth elements (REE) that were liberated and redeposited as rare Be-As-Y-REE minerals, including the As-rich fergusonite-beta-(Y). The latter mineral crystallizes with monoclinic symmetry [a - 5.1794(14), b = 11.089(3), c = 5.1176(14) [Angstrom], [beta] = 91.282(8)[degrees], V= 293.87(14) [[Angstrom].sup.3], space group 12/a] and has the empirical formula [([Y.sub.0.70][Dy.sub.0.07][Er.sub.0.05] [Ca.sub.0.05][Gd.sub.0.02][U.sub.0.02][Yb.sub.0.01] [Tb.sub.0.01][Th.sub.0.01][Nd.sub.0.01])[SIGMA]0.95] [([Nb.sub.o.68][As.sup.5+.sub.0.27][W.sub.0.06] [Ta.sub.0.01][Si.sub.0.01]).sub.[SIGMA]1.03][O.sub.4]. The crystal structure of fergusonite-beta-(Y) has been refined using a thermally untreated single crystal to [R.sub.1] = 6.6% for 441 observed reflections with [F.sub.o]/[sigma][F.sub.o] > 4. The incorporation of As in the structure of monoclinic fergusonite-type phases is discussed in the context of the data available for synthetic analogs. Keywords: Fergusonite, single crystal, X-ray diffraction, EMPA DOI: 10.2138/am.2010.3239