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 NYE (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) angstrom, beta = 91.282(8)degrees, V = 293.87(14) angstrom(3), space group I2/a] and has the empirical formula (Y(0.70)Dy(0.07)Er(0.05)Ca(0.05)Gd(0.02)U(0.02)Yb(0.01)Tb(0.01)Th(0.01)Nd(0.01))(Sigma 0.95)(Nb(0.68)As(0.27)(5+)W(0.06)Ta(0.01)Si(0.01))(Sigma 1.03)O(4). The crystal structure of fergusonite-beta-(Y) has been refined using a thermally untreated single crystal to R(1) = 6.6% for 441 observed reflections with F(0)/sigma F(0) > 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.
Arsenic-rich beta-fergusonite-(Y) from Mount Cervandone (Western Alps, Italy): crystal structure and genetic implications
CAMARA ARTIGAS, Fernando;
2010-01-01
Abstract
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 NYE (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) angstrom, beta = 91.282(8)degrees, V = 293.87(14) angstrom(3), space group I2/a] and has the empirical formula (Y(0.70)Dy(0.07)Er(0.05)Ca(0.05)Gd(0.02)U(0.02)Yb(0.01)Tb(0.01)Th(0.01)Nd(0.01))(Sigma 0.95)(Nb(0.68)As(0.27)(5+)W(0.06)Ta(0.01)Si(0.01))(Sigma 1.03)O(4). The crystal structure of fergusonite-beta-(Y) has been refined using a thermally untreated single crystal to R(1) = 6.6% for 441 observed reflections with F(0)/sigma F(0) > 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.
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