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, Y, 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) Å, β = 91.282(8)°, V = 293.87(14) Å3, space group I2/a] and has the empirical formula (Y 0.70Dy0.07Er0.05Ca0.05Gd 0.02U0.02Yb0.01 Tb0.01Th 0.01Nd0.01)Σ0.95 (Nb 0.68As0.275+W0.06Ta 0.01Si0.01)Σ1.03O4. The crystal structure of fergusonite-beta-(Y) has been refined using a thermally untreated single crystal to R1 = 6.6% for 441 observed reflections with Fo/σFo > 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.