IMA 2017-009b = laverovite

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Marco E. Ciriotti
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IMA 2017-009b = laverovite

Messaggio da Marco E. Ciriotti » mer 27 mar, 2019 20:27

Referenza:
▪ Sokolova E., Day M.C., Hawthorne F.C., Kasatkin A.V., Downs R.A., Horváth L., Pfenninger-Horváth E. (2019): Laverovite, K2NaMn7Zr2(Si4O12)2O2(OH)4F, a New Astrophyllite-supergroup Mineral from Mont Saint-hilaire, Québec, Canada. Canadian Mineralogist, 57, 201-213.

Abstract:
Laverovite (IMA 2017-009b), ideally K2NaMn7Zr2(Si4O12)2O2(OH)4F, is a kupletskite-group (astrophyllite-supergroup) mineral from Mont Saint-Hilaire, Québec, Canada. Associated minerals are zircophyllite, kupletskite, astrophyllite, aegirine, analcime, orthoclase, and albite. Laverovite is brown, transparent in thin grains, and has a vitreous luster. Mohs hardness is 3, Dcalc. = 3.367 g/cm3. Laverovite is biaxial (–) with refractive indices (λ = 589 nm) α = 1.670(2), β = 1.710(5), γ = 1.740(5); 2Vmeas. = 82(2)°, 2Vcalc. = 80°, strong dispersion: r > v. Cleavage is perfect parallel to {001}. Chemical analysis by electron microprobe gave Nb2O5 0.56, ZrO2 9.78, TiO2 4.69, SiO2 33.52, Al2O3 0.94, SrO 0.13, ZnO 0.07, FeO 13.94, MnO 20.51, CaO 0.48, MgO 0.76, Cs2O 0.05, K2O 6.00, Na2O 2.28, F 1.80, H2Ocalc. 2.57, sum 97.32 wt.%; H2O was calculated from crystal-structure analysis. The empirical formula based on 31.15 (O + F) apfu with [OH + F = 5 pfu and H2O = 0.15 pfu] is (K1.78Sr0.02Cs0.01□0.19)Σ2(□1.85Na0.15)Σ2(Na0.88Ca0.12)Σ1(Mn4.03Fe2+2.71Mg0.25Zn0.01)Σ7(Zr1.11Ti0.82Nb0.06Mg0.01)Σ2[(Si7.78Al0.26)Σ8.04O24]O2[(OH)3.68F0.32]Σ4F[□1.85(H2O)0.15]Σ2, Z = 1. The simplified formula is K2Na(Mn,Fe2+)7(Zr,Ti)2(Si4O12)2O2 (OH)4F. Laverovite is triclinic, space group P-1⁠, a 5.4329(1), b 11.9232(3), c 11.7491(3) Å, α 112.905(2), β 94.696(1), γ 103.178(1)°, V 670.14(5) Å3. The six strongest lines in the X-ray powder diffraction data [d (Å)(I)(hkl)] are: 3.452(92)(003,111); 2.788(97) (-1-31, 1-42), 2.680(68) (-211), 2.589(100) (130, 1-43), ⁠; 2.504(44) (-212)⁠, and 1.590(50) (3-51, -3-22)⁠. The crystal structure has been refined to R1 = 3.26% for 3757 observed (Fo > 4σF) reflections. In the crystal structure of laverovite, there are four [4]T sites, with <T–O> = 1.621 Å, occupied mainly by Si, with minor Al. TO4 tetrahedra constitute the T4O12 astrophyllite ribbon. The Zr-dominant [6]D site is occupied mainly by Zr and Ti and minor Nb and Mg, with <D–φ> = 2.002 Å (φ = O, F). The T4O12 astrophyllite ribbons and D octahedra constitute the H (Heteropolyhedral) sheet. In the O (Octahedral) sheet, there are four Mn2+-dominant [6]M(1–4) sites, with <M(1–4)–φ> = 2.187, 2.174, 2.161, and 2.146 Å (φ = O, OH). Two H sheets and the central O sheet form the HOH block, and adjacent HOH blocks link via a common XPD anion of two D octahedra. In the I (Intermediate) block between adjacent HOH blocks, there are two interstitial cation sites, A and B, and a WA site, partly occupied by H2O. The A site splits into two partly occupied sites, [13]A(1) and [6]A(2), with A(1)–A(2) = 1.108 Å. The [13]A(1) site is occupied at 90.5%: mainly by K, with minor Sr and Cs, <A(1)–φ> = 3.326 Å; the [6]A(2) site is occupied at 7.5% by Na: [□1.85Na0.15], <A(2)–φ> = 2.29 Å (φ = O, F, H2O). The aggregate content of the A site is (K1.78Sr0.02Cs0.01Na0.15□0.04)Σ2, ideally K2apfu. The [10]B site is occupied by (Na0.88Ca0.12), <B–φ> = 2.646 Å. The WA site is occupied at 7.5% by H2O: [□1.85(H2O)0.15] pfu. The presence of OH and H2O groups in the laverovite structure was confirmed by infrared spectroscopy. The mineral is named “laverovite” after Professor Nikolay Pavlovich Laverov (1930–2016), Academician of the Russian Academy of Sciences, a prominent Russian ore geologist and an expert in uranium ore deposits and radiogenic waste disposal. Laverovite is a Mn-analogue of zircophyllite, K2NaFe2+7Zr2(Si4O12)2O2(OH)4F.
Marco E. Ciriotti

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