• journal_title：American Mineralogist
• Contributor：Alessandro Guastoni ; Fernando Cámara ; Fabrizio Nestola
• Publisher：Mineralogical Society of America
• Date：2010-
• Format：text/html
• Language：en
• Identifier：10.2138/am.2010.3239
• journal_abbrev：American Mineralogist
• issn：0003-004X
• volume：95
• issue：4
• firstpage：487
• section：Articles

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) ų, space group I2/a] and has the empirical formula (Y_0.70Dy_0.07Er_0.05Ca_0.05Gd_0.02U_0.02Yb_0.01Tb_0.01Th_0.01Nd_0.01)_∑0.95(Nb_0.68As⁵⁺_0.27W_0.06Ta_0.01Si_0.01)_∑1.03O₄. 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 F_o/σF_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.