1501 The Canadian Mineralogist Vol. 43, pp. 1501-1510 (2005) OXYKINOSHITALITE, A NEW SPECIES OF MICA FROM FERNANDO DE NORONHA ISLAND, PERNAMBUCO, BRAZIL: OCCURRENCE AND CRYSTAL STRUCTURE LIA N. KOGARKO Vernadsky Institute of Geochemistry, Russian Academy of Sciences, Kosygin Street 19, Moscow 119991, Russia YULIA A. UVAROVA, ELENA SOKOLOVA § AND FRANK C. HAWTHORNE Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba R3T 2N2, Canada LUISA OTTOLINI Istituto di Geoscienze e Georisorse (IGG), Sezione di Pavia, via Ferrata, 1, I–27100 Pavia, Italy JOEL D. GRICE Research Division, Canadian Museum of Nature, P.O. Box 3443, Station D, Ottawa, Ontario K1P 6P4, Canada ABSTRACT Oxykinoshitalite, ideally Ba (Mg 2 Ti 4+ ) (Si 2 Al 2 ) O 10 O 2 , is a new species of mica from Fernando de Noronha Island, Pernambuco, Brazil; it is found in an olivine nephelinite with olivine, clinopyroxene, Fe–Ti oxide, nepheline, calcite, apatite and K-rich feldspar. Oxykinoshitalite forms corroded irregularly shaped grains 0.03 to 0.05 mm across, and its color varies from bright orange to brown. It is brittle, H = 2½, D obs = 3.3(1), D calc = 3.45 g/cm 3 , has a brown streak, vitreous luster and does not fluoresce in ultraviolet light; it has a perfect cleavage on {001} and a splintery fracture. In transmitted plane-polarized light, oxykinoshitalite is strongly pleochroic, X pale brown, Y ≈ Z deep red-brown; X  c = 2° (in  obtuse), Y = b, Z  a = 8° (in  obtuse), with absorption X < Y ≈ Z. It is biaxial positive,  1.708 ± 0.001,  1.710 ± 0.001,  1.719 ± 0.001, 2V obs = 56 ± 2°, 2V calc = 51°. Oxykinoshitalite is monoclinic, space group C2/m, a 5.3516(7), b 9.2817(11), c 10.0475(13) Å,  100.337(3)°, V 490.98(18) Å 3 , Z = 2. The strongest ten X-ray-diffraction lines in the powder pattern [d in Å(I)(hkl)] are: 2.637(10)( ¯ 131), 2.172(9)( ¯ 133), 3.646(7)( ¯ 112), 3.130(7)(112), 3.383(6)(002), 2.902(5)( ¯ 113), 2.435(5)(201), 1.988(5)(133), 1.661(5)( ¯ 135), 1.547(5)(312), and 1.526(5)( ¯ 206). Analysis by a combination of electron microprobe, SIMS and crystal-structure refinement gives SiO 2 28.92, Al 2 O 3 15.26, TiO 2 11.94, Cr 2 O 3 0.03, Fe 2 O 3 9.07, FeO 3.84, MnO 0.14, MgO 10.74, CaO 0.03, BaO 15.49, Na 2 O 0.39, K 2 O 4.05, F 0.91, H 2 O 0.55, sum 100.97 wt.%; V, Ni, Zn, Sr, Cl were not detected. The formula unit, calculated on the basis of 12(O,OH,F) is (Ba 0.50 K 0.43 Na 0.07 ) 1.00 (Mg 1.33 Fe 2+ 0.27 Mn 2+ 0.01 Fe 3+ 0.57 Ti 4+ 0.74 ) 2.92 (Si 2.40 Al 1.49 ) 3.89 O 10 [O 1.46 (OH) 0.30 F 0.24 ] 2.00 . Refinement of the crystal structure shows it to be the 1M polytype. The refinement converged to R 1 = 4.3% for 585 unique (F o > 4F o ) reflections, collected on a Bruker single-crystal P4 diffractometer with a CCD detector and MoK X-radiation. Electron-microprobe and SIMS analysis of the crystal used to collect the data on X-ray intensities gave the empirical formula (Ba 0.47 K 0.42 Na 0.06 Ca 0.01 ) 0.96 (Mg 1.35 Fe 2+ 0.28 Fe 3+ 0.59 Ti 4+ 0.75 Mn 0.01 ) 2.98 [Si 2.31 Al 1.56 ] 3.97 O 10 (O 1.44 OH 0.31 F 0.25 ) 2.00 . The oxygenian Ti-dominant analogue of kinoshitalite occurs in olivine nephelinite and is associated with olivine, clinopyroxene, Fe–Ti oxide, nepheline, calcite, apatite and K-rich feldspar. There is one tetrahedrally coordinated T site, <T–O> = 1.671 Å, occupied by (Si 2.31 Al 1.56 ). There are two octahedrally coordinated sites, M(1) occupied by (Mg 0.52 Fe 2+ 0.13 Fe 3+ 0.27 Ti 4+ 0.08 ) with <M(1)–O> = 2.110 Å, and M(2) occupied by (Mg 0.84 Fe 2+ 0.15 Fe 3+ 0.33 Mn 0.01 Ti 4+ 0.67 ) with <M(2)–O> = 2.065 Å. The interstitial [12]-coordinated I site is occupied by (Ba 0.47 K 0.42 Na 0.06 Ca 0.01 ), with <I–O> = 3.107 Å. There are two main coupled substitutions in this structure: (1) Ti 4+ for Mg, primarily at the M(2) site, and O 2– for (OH) – and F – at the O(4) site, and (2) K for Ba at the I site, and Si for Al at the T site. Keywords: oxykinoshitalite, new mineral species, crystal-structure refinement, oxygenian mica, Fernando de Noronha Island, Brazil. § E-mail address: elena_sokolova@umanitoba.ca