OXYKINOSHITALITE, A NEW SPECIES OF MICA FROM FERNANDO DE NORONHA ISLAND, PERNAMBUCO, BRAZIL: OCCURRENCE AND CRYSTAL STRUCTURE

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 β obtuse), Y = b , Z ∧ a = 8° (in β obtuse), with absorption X Y ≈ Z. It is biaxial positive, α 1.708 ± 0.001, β 1.710 ± 0.001, γ 1.719 ± 0.001, 2 V obs = 56 ± 2°, 2 V calc = 51°. Oxykinoshitalite is monoclinic, space group C 2/ m , a 5.3516(7), b 9.2817(11), c 10.0475(13) A, β100.337(3)°, V 490.98(18) A 3 , Z = 2. The strongest ten X-ray-diffraction lines in the powder pattern [ d in A( 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 1 M polytype. The refinement converged to R 1 = 4.3% for 585 unique ( F o > 4σ F o ) reflections, collected on a Bruker single-crystal P 4 diffractometer with a CCD detector and Mo K α 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 A, 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 A, 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 A. 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 A. 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.