An arsenic-rich beta-fergusonite has been collected at Mount Cervandone (Verbano-Cusio-Ossola, Devero valley, Western Alps, Italy). Its crystal structure has been determined by single crystal X-ray diffraction, space group I2/a with unit-cell parameters a = 5.1794(14), b = 11.089(3), c = 5.1176(14) Å, β = 91.282(8)°, and V = 293.87(14) Å3. Its chemical composition analyzed by Electron Microprobe is (Y0.70Dy0.07Er0.05Ca2+0.05Gd0.02Tb0.01Yb0.01Th4+0.01U4+0.02)Σ=0.94 (Nb5+0.68As5+0.27W6+0.06Ta5+0.01 Si4+0.01)Σ=1.03O4. It occurs as pseudo-bipyramidal crystals up to 1 mm length, green-yellow in colour. Beta-fergusonite-(Y) shows an extended solid solution of arsenic in the beta-fergusonite structure, while the complete substitution leads to a different structure in chernovite-(Y). The study of the crystal structure also sheds light on the stability of the monoclinic polymorph of fergusonite and formanite-(Y). Beta-fergusonite-(Y)l is associated with orange-yellow barrel-shaped pseudohexagonal synchysite-(Ce) [Ca(Ce, La)(CO3)2F] crystals, up to 2 mm in length. Beta-fergusonite-(Y) occurs in alpine fissures which intersect pegmatite dikes, hosted in two-mica fine-grained leucocratic gneisses of Mount Cervandone, which can be described as metamorphosed leucogranitic-aplitic rocks. The pegmatites show an unequivocally strong NYF (niobium-yttrium-fluorine) geochemical signature represented by the occurrence of an unique mineralogy including aeschynite-(Y), agardite-(Y), Nb-rich anatase, cervandonite-(Ce), chernovite-(Y), crichtonite-senaite group minerals, fergusonite-(Y), fluorite, gadolinite-(Y), monazite-(Ce), paraniite-(Y), Nb-rich rutile, synchysite-(Ce), and xenotime-(Y)(Guastoni et al., 2006). Alessandro Guastoni is Museum Curator at the Department of Geoscience at the University of Padova. His field of research is focused on mineralogy and geochemistry of pegmatites mainly occurring in the Alps. Reference Guastoni Alessandro, Pezzotta Federico, Vignola Pietro. (2006) Characterization, paragenesis and genetic inferences of arsenates, sulfates and vanadates of Fe, Cu, Pb, Zn from Mount Cervandone (Western Alps, Italy). Periodico di Mineralogia, 76, 2, 141-150.
Crystal-chemical relationships between beta-fergusonite-(Y) and related phases from Mount Cervandone and genetic implications
CAMARA ARTIGAS, Fernando;
2008-01-01
Abstract
An arsenic-rich beta-fergusonite has been collected at Mount Cervandone (Verbano-Cusio-Ossola, Devero valley, Western Alps, Italy). Its crystal structure has been determined by single crystal X-ray diffraction, space group I2/a with unit-cell parameters a = 5.1794(14), b = 11.089(3), c = 5.1176(14) Å, β = 91.282(8)°, and V = 293.87(14) Å3. Its chemical composition analyzed by Electron Microprobe is (Y0.70Dy0.07Er0.05Ca2+0.05Gd0.02Tb0.01Yb0.01Th4+0.01U4+0.02)Σ=0.94 (Nb5+0.68As5+0.27W6+0.06Ta5+0.01 Si4+0.01)Σ=1.03O4. It occurs as pseudo-bipyramidal crystals up to 1 mm length, green-yellow in colour. Beta-fergusonite-(Y) shows an extended solid solution of arsenic in the beta-fergusonite structure, while the complete substitution leads to a different structure in chernovite-(Y). The study of the crystal structure also sheds light on the stability of the monoclinic polymorph of fergusonite and formanite-(Y). Beta-fergusonite-(Y)l is associated with orange-yellow barrel-shaped pseudohexagonal synchysite-(Ce) [Ca(Ce, La)(CO3)2F] crystals, up to 2 mm in length. Beta-fergusonite-(Y) occurs in alpine fissures which intersect pegmatite dikes, hosted in two-mica fine-grained leucocratic gneisses of Mount Cervandone, which can be described as metamorphosed leucogranitic-aplitic rocks. The pegmatites show an unequivocally strong NYF (niobium-yttrium-fluorine) geochemical signature represented by the occurrence of an unique mineralogy including aeschynite-(Y), agardite-(Y), Nb-rich anatase, cervandonite-(Ce), chernovite-(Y), crichtonite-senaite group minerals, fergusonite-(Y), fluorite, gadolinite-(Y), monazite-(Ce), paraniite-(Y), Nb-rich rutile, synchysite-(Ce), and xenotime-(Y)(Guastoni et al., 2006). Alessandro Guastoni is Museum Curator at the Department of Geoscience at the University of Padova. His field of research is focused on mineralogy and geochemistry of pegmatites mainly occurring in the Alps. Reference Guastoni Alessandro, Pezzotta Federico, Vignola Pietro. (2006) Characterization, paragenesis and genetic inferences of arsenates, sulfates and vanadates of Fe, Cu, Pb, Zn from Mount Cervandone (Western Alps, Italy). Periodico di Mineralogia, 76, 2, 141-150.
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