• journal_title：The Canadian Mineralogist
• Contributor：Lia N. Kogarko ; Yulia A. Uvarova ; Elena Sokolova ; Frank C. Hawthorne ; Luisa Ottolini ; Joel D. Grice
• Publisher：Mineralogical Association of Canada
• Date：2005-
• Format：text/html
• Language：en
• Identifier：10.2113/gscanmin.43.5.1501
• journal_abbrev：Can Mineral
• issn：0008-4476
• volume：43
• issue：5
• firstpage：1501
• section：Crystallography and Mineralogy

Oxykinoshitalite, ideally Ba (Mg₂ Ti⁴⁺) (Si₂ Al₂) O₁₀ O₂, 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/cm³, 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, 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, β100.337(3)°, V 490.98(18) ų, Z = 2. The strongest ten X-ray-diffraction lines in the powder pattern [d in Å(I)(hkl)] are: 2.637(10)(1̅31), 2.172(9)(1̅33), 3.646(7)(1̅12), 3.130(7)(112), 3.383(6)(002), 2.902(5)(1̅13), 2.435(5)(201), 1.988(5)(133), 1.661(5)(1̅35), 1.547(5)(312), and 1.526(5)(2̅06). Analysis by a combination of electron microprobe, SIMS and crystal-structure refinement gives SiO₂ 28.92, Al₂O₃ 15.26, TiO₂ 11.94, Cr₂O₃ 0.03, Fe₂O₃ 9.07, FeO 3.84, MnO 0.14, MgO 10.74, CaO 0.03, BaO 15.49, Na₂O 0.39, K₂O 4.05, F 0.91, H₂O 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 Fe²⁺ _0.27 Mn²⁺ _0.01 Fe³⁺ _0.57 Ti⁴⁺ _0.74)_∑2.92 (Si_2.40 Al_1.49)_∑3.89 O₁₀ [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 R₁ = 4.3% for 585 unique (F_o > 4σF_o) reflections, collected on a Bruker single-crystal P4 diffractometer with a CCD detector and MoKα 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 Fe²⁺ _0.28 Fe³⁺ _0.59 Ti⁴⁺ _0.75 Mn_0.01)_∑2.98 [Si_2.31 Al_1.56]_∑3.97 O₁₀ (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 Fe²⁺ _0.13 Fe³⁺ _0.27 Ti⁴⁺ _0.08) with <M(1)–O> = 2.110 Å, and M(2) occupied by (Mg_0.84 Fe²⁺ _0.15 Fe³⁺ _0.33 Mn_0.01 Ti⁴⁺ _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) Ti⁴⁺ for Mg, primarily at the M(2) site, and O²⁻ 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.