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Waimirite-(Y), orthorhombic YF 3 , a new mineral from the Pitinga mine, Presidente Figueiredo, Amazonas, Brazil and from Jabal Tawlah, Saudi Arabia: description and crystal structure

2015; Cambridge University Press; Volume: 79; Issue: 3 Linguagem: Inglês

10.1180/minmag.2015.079.3.18

1471-8022

Daniel Atencio, Artur Cezar Bastos Neto, Vítor Paulo Pereira, JOSÉ TADEU MAXIMINO MIRRAS FERRON, M. Hoshino, T. Moriyama, Yasushi Watanabe, Ritsuro Miyawaki, José Moacyr Vianna Coutinho, Marcelo B. Andrade, Kenneth J. Domanik, Н. В. Чуканов, Koichi Momma, H. Hirano, Maiko Tsunematsu,

Inorganic Fluorides and Related Compounds

Abstract Waimirite-(Y) (IMA 2013-108), orthorhombic YF 3 , occurs associated with halloysite, in hydrothermal veins (up to 30 mm thick) cross-cutting the albite-enriched facies of the A-type Madeira granite (∼1820 Ma), at the Pitinga mine, Presidente Figueiredo Co., Amazonas State, Brazil. Minerals in the granite are 'K-feldspar', albite, quartz, riebeckite, 'biotite', muscovite, cryolite, zircon, polylithionite, cassiterite, pyrochlore-group minerals, 'columbite', thorite, native lead, hematite, galena, fluorite, xenotime-(Y), gagarinite-(Y), fluocerite-(Ce), genthelvite–helvite, topaz, 'illite', kaolinite and 'chlorite'. The mineral occurs as massive aggregates of platy crystals up to ∼1 μm in size. Forms are not determined, but synthetic YF 3 displays pinacoids, prisms and bipyramids. Colour: pale pink. Streak: white. Lustre: non-metallic. Transparent to translucent. Density (calc.) = 5.586 g/cm 3 using the empirical formula. Waimirite-(Y) is biaxial, mean n = 1.54–1.56. The chemical composition is (average of 24 wavelength dispersive spectroscopy mode electron microprobe analyses, O calculated for charge balance): F 29.27, Ca 0.83, Y 37.25, La 0.19, Ce 0.30, Pr 0.15, Nd 0.65, Sm 0.74, Gd 1.86, Tb 0.78, Dy 8.06, Ho 1.85, Er 6.38, Tm 1.00, Yb 5.52, Lu 0.65, O (2.05), total (97.53) wt.%. The empirical formula (based on 1 cation) is (Y 0.69 Dy 0.08 Er 0.06 Yb 0.05 Ca 0.03 Gd 0.02 Ho 0.02 Nd 0.01 Sm 0.01 Tb 0.01 Tm 0.01 Lu 0.01 ) Σ1.00 [F 2.54 〈 0.25 O 0.21 ] Σ3.00 . Orthorhombic, Pnma , a = 6.386(1), b = 6.877(1), c = 4.401(1) Å, V = 193.28(7) Å 3 , Z = 4 (powder data). Powder X-ray diffraction (XRD) data [ d in Å ( I ) ( hkl )]: 3.707 (26) (011), 3.623 (78) (101), 3.438 (99) (020), 3.205 (100) (111), 2.894 (59) (210), 1.937 (33) (131), 1.916 (24) (301), 1.862 (27) (230). The name is for the Waimiri-Atroari Indian people of Roraima and Amazonas. A second occurrence of waimirite-(Y) is described from the hydrothermally altered quartz-rich microgranite at Jabal Tawlah, Saudi Arabia. Electron microprobe analyses gave the empirical formula (Y 0.79 Dy 0.08 Er 0.05 Gd 0.03 Ho 0.02 Tb 0.01 Tm 0.01 Yb 0.01 ) Σ1.00 [F 2.85 O 0.08 〈 0.07 ] Σ3.00 . The crystal structure was determined with a single crystal from Saudi Arabia. Unit-cell parameters refined from single-crystal XRD data are a = 6.38270(12), b = 6.86727(12), c = 4.39168(8) Å, V = 192.495(6) Å3, Z = 4. The refinement converged to R 1 = 0.0173 and w R 2 = 0.0388 for 193 independent reflections. Waimirite-(Y) is isomorphous with synthetic SmF 3 , HoF 3 and YbF 3 . The Y atom forms a 9-coordinated YF 9 tricapped trigonal prism in the crystal structure. The substitution of Y for Dy, as well as for other lanthanoids, causes no notable deviations in the crystallographic values, such as unit-cell parameters and interatomic distances, from those of pure YF 3 .