IMA 2017-074 = hydroxynatropyrochlore

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Marco E. Ciriotti
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IMA 2017-074 = hydroxynatropyrochlore

Messaggio da Marco E. Ciriotti » ven 18 mag, 2018 18:00

Prossima pubblicazione.

Referenza:
▪ Ivanyuk, G.Yu., Yakovenchuk, V.N., Panikorovskii, T.L., Konoplyova, N., Pakhomovsky, Y.A., Bazai, A.V., Bocharov, V.N., Krivovichev, S.N. (2018): Hydroxynatropyrochlore, (Na,Ca,Ce)2Nb2O6(OH), a new member of the pyrochlore group from the Kovdor phoscorite-carbonatite pipe (Kola Peninsula, Russia). Mineralogical Magazine, 82, (in press).
Marco E. Ciriotti

«Things are interesting only in so far as they relate themselves to other things»

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Marco E. Ciriotti
Messaggi: 24892
Iscritto il: ven 25 giu, 2004 11:31
Località: via San Pietro, 55 I-10073 Devesi/Cirié TO - Italy
Contatta:

Re: IMA 2017-074 = hydroxynatropyrochlore

Messaggio da Marco E. Ciriotti » sab 10 nov, 2018 13:52

Referenza:
▪ Ivanyuk, G.Yu., Yakovenchuk, V.N., Panikorovskii, T.L., Konoplyova, N., Pakhomovsky, Y.A., Bazai, A.V., Bocharov, V.N., Krivovichev, S.N. (2018): Hydroxynatropyrochlore, (Na,Ca,Ce)2Nb2O6(OH), a new member of the pyrochlore group from the Kovdor phoscorite-carbonatite pipe (Kola Peninsula, Russia). Mineralogical Magazine, 82, (in press).

Abstract:
Hydroxynatropyrochlore, (Na,Сa,Ce)2Nb2O6(OH), is a new Na-Nb-OH-dominant member of the pyrochlore supergroup from the Kovdor phoscorite-carbonatite pipe (Kola Peninsula, Russia). It is cubic, Fd-3m, a = 10.3211(3) Å, V = 1099.46 (8) Å3, Z = 8 (from powder diffraction data) or a = 10.3276(5) Å, V = 1101.5(2) Å3, Z = 8 (from single-crystal diffraction data). Hydroxynatropyrochlore is a characteristic accessory mineral of low-carbonate phoscorite of the contact zone of the phoscorite-carbonatite pipe with host foidolite as well as of carbonate-rich phoscorite and carbonatite of the pipe axial zone. It usually forms zonal cubic or cubooctahedral crystals (up to 0.5 mm in diameter) with irregularly shaped relics of amorphous U-Ta-rich hydroxykenopyrochlore inside. Characteristic associated minerals include rock-forming calcite, dolomite, forsterite, hydroxylapatite, magnetite,and phlogopite, accessory baddeleyite, baryte, barytocalcite, chalcopyrite, chamosite–clinochlore, galena, gladiusite, juonniite, ilmenite, magnesite, pyrite, pyrrhotite, quintinite, spinel, strontianite, valleriite, and zirconolite. Hydroxynatropyrochlore is pale-brown, with an adamantine to greasy lustre and a white streak. The cleavage is average on {111}, the fracture is conchoidal. Mohs hardness is about 5. In transmitted light, the mineral is light brown, isotropic, n = 2.10(5) (delta = 589 nm). The calculated and measured densities are 4.77 and 4.60(5) g·cm-3, respectively. The mean chemical composition determined by electron microprobe is: F 0.05, Na2O 7.97, CaO 10.38, TiO2 4.71, FeO 0.42, Nb2O5 56.44, Ce2O3 3.56, Ta2O5 4.73, ThO2 5.73, UO2 3.66, total 97.65 wt. %. The empirical formula calculated on the basis of Nb+Ta+Ti = 2 apfu is (Na1.02Ca0.73Ce0.09Th0.09 U0.05Fe2+0.02)Σ2.00 (Nb1.68Ti0.23Ta0.09)Σ2.00O6.03(OH1.04F0.01)Σ1.05. The simplified formula is (Na, Ca,Ce)2Nb2O6(OH). The mineral slowly dissolves in hot HCl. The strongest X-ray powder-diffraction lines [listed as (d in Å)(I)(hkl)] are as follows: 5.96(47)(111), 3.110(30)(311), 2.580(100)(222), 2.368(19)(400), 1.9875(6)(333), 1.8257(25)(440) and 1.5561(14)(622). The crystal structure of hydroxynatropyrochlore was refined to R1 = 0.026 on the basis of 1819 unique observed reflections. The mineral belongs to the pyrochlore structure type A2B2O6Y1 with octahedral framework of corner-sharing BO6 octahedra with A cations and OH groups in the interstices. The Raman spectrum of hydroxynatropyrochlore contains characteristic bands of the lattice, BO6, B–O and O–H vibrations and no characteristic bands of the H2O vibrations. Within the Kovdor phoscorite-carbonatite pipe, hydroxynatropyrochlore is the latest hydrothermal mineral of the pyrochlore supergroup, which forms external rims around grains of earlier U-rich hydroxykenopyrochlore and separated crystals in voids of dolomite carbonatite veins. The mineral is named in accordance with the pyrochlore supergroup nomenclature.
Marco E. Ciriotti

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