ALBERT

Beschreibung: 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 ) 1.00 [F 2.54 0.25 O 0.21 ] 3.00 . 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 ) 1.00 [F 2.85 O 0.08 0.07 ] 3.00 . 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) Å 3 , 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 .