An arsenic-rich β-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 using a Bruker-AXS diffractometer equipped with a CCD detector: it is monoclinic I2/a, a = 5.1794(14), b = 11.089(3), c = 5.1176(14) Å, β = 91.282(8)°, V = 293.87(14) Å3. The chemical composition has been analyzed on the same crystal studied by X-ray diffraction giving (Y0.70Dy0.07Er0.05Ca2+ 0.05Gd0.02Tb0.01Yb0.01Th4+ 0.01U4+ 0.02)Σ=0.94 (Nb5+ 0.68As5+ 0.27W6+ 0.06Ta5+ 0.01Si4+ 0.01)Σ=1.03O4. β-fergusonite occurs as pseudo-bipyramidal crystals up to 1 mm length (Fig. 1), green-yellow in colour under incandescent light. The mineral is associated with orange-yellow barrel-shape hexagonal synchysite-(Ce) [Ca (Ce, La)(CO3)2F] crystals, up to 2 mm in length. Both are found in a cavity of Alpine type quartz veins which intersect pegmatite dikes, in the two-mica fine grained leucocratic gneisses of Mount Cervandone, which can be described as metamorphosed leucogranitic-aplitic rocks. The pegmatites show a unequivocal strong NYF (niobium-yttrium-fluorine) geochemical signature represented by the occurrence of an unique mineralogy that includes 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). Isovalent substitution of Nb by As occurs in the 4-fold coordinated site, which decreases mean bond length from 1.894 Å in pure β-fegusonite [1] to 1.807 Å in the sample studied, corresponding to a 10% reduction of the polyhedral volume and a significant reduction of polyhedral distortion (TAV decreases from 121.8 to 46.9). The changes observed in the tetrahedron are not paralleled in the large 8-fold coordinated polyhedron hosting Y in β-fegusonite, which shows a limited 3% increase of polyhedral volume, mostly due to the solution of Ca and Dy in this site. Although infrequent, there seems that there is not a structural limitation in As solid solution in β-fegusonite. However, chernovite-(Y) [(AsO4)Y] structure is tetragonal I41/amd, and thus at a certain degree of As substitution a structural phase transition may be expected to tetragonal fergusonite/chernovite-(Y)
As-Nb solid solution in β-fergusonite-(Y) from Mount Cervandone
CAMARA ARTIGAS, Fernando;
2008-01-01
Abstract
An arsenic-rich β-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 using a Bruker-AXS diffractometer equipped with a CCD detector: it is monoclinic I2/a, a = 5.1794(14), b = 11.089(3), c = 5.1176(14) Å, β = 91.282(8)°, V = 293.87(14) Å3. The chemical composition has been analyzed on the same crystal studied by X-ray diffraction giving (Y0.70Dy0.07Er0.05Ca2+ 0.05Gd0.02Tb0.01Yb0.01Th4+ 0.01U4+ 0.02)Σ=0.94 (Nb5+ 0.68As5+ 0.27W6+ 0.06Ta5+ 0.01Si4+ 0.01)Σ=1.03O4. β-fergusonite occurs as pseudo-bipyramidal crystals up to 1 mm length (Fig. 1), green-yellow in colour under incandescent light. The mineral is associated with orange-yellow barrel-shape hexagonal synchysite-(Ce) [Ca (Ce, La)(CO3)2F] crystals, up to 2 mm in length. Both are found in a cavity of Alpine type quartz veins which intersect pegmatite dikes, in the two-mica fine grained leucocratic gneisses of Mount Cervandone, which can be described as metamorphosed leucogranitic-aplitic rocks. The pegmatites show a unequivocal strong NYF (niobium-yttrium-fluorine) geochemical signature represented by the occurrence of an unique mineralogy that includes 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). Isovalent substitution of Nb by As occurs in the 4-fold coordinated site, which decreases mean bond length from 1.894 Å in pure β-fegusonite [1] to 1.807 Å in the sample studied, corresponding to a 10% reduction of the polyhedral volume and a significant reduction of polyhedral distortion (TAV decreases from 121.8 to 46.9). The changes observed in the tetrahedron are not paralleled in the large 8-fold coordinated polyhedron hosting Y in β-fegusonite, which shows a limited 3% increase of polyhedral volume, mostly due to the solution of Ca and Dy in this site. Although infrequent, there seems that there is not a structural limitation in As solid solution in β-fegusonite. However, chernovite-(Y) [(AsO4)Y] structure is tetragonal I41/amd, and thus at a certain degree of As substitution a structural phase transition may be expected to tetragonal fergusonite/chernovite-(Y)
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