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Selivanovaite, NaTi3 (Ti, Na, Fe, Mn)4 [(Si2 O7)2 O4 (OH, H2 O)4nH2 O, a new rock-forming mineral from the eudialyte-rich malignite of the Lovozero alkaline massif (Kola Peninsula, Russia)

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Selivanovaite, NaTi3 (Ti, Na, Fe, Mn)4 [(Si2 O7)2 O4 (OH, H2 O)4· n H2 O, is a new titanosilicate of the murmanite group (seidozerite supergroup). It is triclinic, P1 , a = 8.673(5), b = 8.694(3), c = 12.21(1) Å, α = 92.70(5), β =108.46(7), γ = 105.40(4)°, V = 833(1) Å3, Z = 2 (from single-crystal diffraction data); a = 8.673(5), b = 8.694(1), c = 12.21(1) Å, α = 92.70(5), β = 108.52(1), γ = 105.42(1) °, V = 833(1) Å3, Z = 2 (from powder diffraction data). The mineral was found in drill cores of medium-grained trachytoid eudialyte-rich malignite of the Lovozero eudyalite complex at Mt. Alluaiv, at the horizon 900–1000 m above the Baltic Sea level. The rock consists mainly of euhedral microcline-perthite, nepheline and manganoeudialyte crystals cemented by fine-acicular aegirine. Minor rock-forming minerals include sodalite, natrolite, magnesioarfvedsonite, and selivanovaite (up to 10 modal %). Characteristic accessory minerals are lamprophyllite, murmanite, loparite-(Ce), a pyrochlore-group mineral, thorite, anatase, baryte, rhabdophane-(Ce), pyrrhotite, chalcopyrite, pyrite, chlorbartonite, djerfisherite, sphalerite, and löllingite. Selivanovaite forms platy metacrysts (up to 8 mm in diameter) with numerous poikilitic inclusions of aegirine and magnesioarfvedsonite. It is dark-orange, with a vitreous lustre and a brownish-white streak. The cleavage is perfect on {0 0 1} and weak on {1 1 0}, the fracture is stepped. Mohs hardness is 3. In transmitted light, the mineral is brown; pleochroism and dispersion were not observed. Selivanovaite is biaxial (+), a 1.79(1), β 1.81(1), γ 1.87(1) (589 nm), 2 V meas = 40(5)°, 2 V calc = 57.3°. The optical orientation is Z^c = 5–10°. The calculated and measured densities are 3.34 and 3.15(3) g·cm3, respectively. The mean chemical composition determined by electron microprobe is: Na2 O 5.45, MgO 0.59, Al2 O3 0.04, SiO2 25.55, K2 O 0.63, CaO 1.68, TiO2 31.17, MnO 2.64, FeO 6.63, ZrO2 2.31, Nb2 O5 6.69, with H2 O 17.0 wt% (determined by the Penfield method), giving a total of 100.38 wt%. The empirical formula calculated on the basis of Si = 4 atoms per formula unit (apfu) is (Na1.65 Mn0.35 Ca0.28 Zr0.18 Mg0.14 K0.13)σ2.73 (Ti3.67 Fe3+ 0.78Nb0.47Al0.01)σ4.93 [Si4 O19.72 ]· 8.87H2 O. The simplified formula based upon the crystal-structure refinement is NaTi3 (TiNaFeMn0.5 []0.50)σ4 [(Si2 O7)2 O4 (OH)2 (H2 O)2 n H2 O or, taking into account chemical variations, NaTi3 (Ti, Na, Fe, Mn)4 [(Si2 O7)2 O4 (OH, H2 O)4 n H2 O. The mineral slowly dissolves in cold 10% HCl. The strongest X-ray powder-diffraction lines [listed as d in A (I) (hkl)] are: 11.43(1 0 0)(0 0 1), 6.37(25)(- 1 1 1), 5.73 (15)(0 0 2), 4.208(16)( 2 1 1), 3.108(35)(2 2 1), 3.043(20)( 1 0 4), and 2.596(17)(0 1 4). The crystal structure of selivanovaite is closely related to those of murmanite-group minerals and consists of the HOH-layers with the composition [N 2 M5 Si4 O18 (OH)3 ] (M = Ti, Nb, Fe and Mn, N = Na, K, Ca, Mn, Mg and Zr) connected by additional N -octahedra with H2 O molecules between them. The mineral is named in honour of Dr. Ekaterina A. Selivanova (b. 1967), from the Kola Science Centre of the Russian Academy of Sciences, for her contribution to the mineralogy of alkaline complexes.
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Keywords: CRYSTAL STRUCTURE; EUDIALYTE; KOLA PENINSULA; LOVOZERO; MALIGNITE; MASSIF; NEW MINERAL; SEIDOZERITE SUPERGROUP; SELIVANOVAITE; TITANOSILICATE

Document Type: Research Article

Publication date: June 1, 2018

This article was made available online on June 22, 2018 as a Fast Track article with title: "Selivanovaite, NaTi3(Ti,Na,Fe,Mn)4[(Si2O7)2O4(OH,H2O)4nH2O, a new rock-forming mineral from the eudialyte-rich malignite of the Lovozero alkaline massif (Kola Peninsula, Russia)".

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