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Hydroxynatropyrochlore, (Na,Сa,Ce)2Nb2O6(OH), a new member of the pyrochlore group from the Kovdor phoscorite–carbonatite pipe, Kola Peninsula, Russia

Published online by Cambridge University Press:  15 May 2018

Gregory Yu. Ivanyuk
Affiliation:
Kola Science Centre, Russian Academy of Sciences, 14 Fersman Street, Apatity 184200, Russia
Victor N. Yakovenchuk
Affiliation:
Kola Science Centre, Russian Academy of Sciences, 14 Fersman Street, Apatity 184200, Russia
Taras L. Panikorovskii
Affiliation:
Kola Science Centre, Russian Academy of Sciences, 14 Fersman Street, Apatity 184200, Russia Department of Crystallography, St. Petersburg State University, 7–9 University Emb, St. Petersburg 199034, Russia;
Nataliya Konoplyova
Affiliation:
Kola Science Centre, Russian Academy of Sciences, 14 Fersman Street, Apatity 184200, Russia
Yakov A. Pakhomovsky
Affiliation:
Kola Science Centre, Russian Academy of Sciences, 14 Fersman Street, Apatity 184200, Russia
Ayya V. Bazai
Affiliation:
Kola Science Centre, Russian Academy of Sciences, 14 Fersman Street, Apatity 184200, Russia
Vladimir N. Bocharov
Affiliation:
Geo Environmental Centre “Geomodel”, St. Petersburg State University, Ul'yanovskaya Str., St. Petersburg 198504, Russia
Sergey V. Krivovichev*
Affiliation:
Kola Science Centre, Russian Academy of Sciences, 14 Fersman Street, Apatity 184200, Russia Department of Crystallography, St. Petersburg State University, 7–9 University Emb, St. Petersburg 199034, Russia;
*
Corresponding author: Sergey V. Krivovichev, Email: s.krivovichev@spbu.ru

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$\bar{3}$m, a = 10.3211(3) Å, V = 1099.46(8) Å3 and Z = 8 (from powder diffraction data) or a = 10.3276(5) Å, V = 1101.5(2) Å3 and Z = 8 (from single-crystal diffraction data). Hydroxynatropyrochlore is a characteristic accessory mineral of the low-carbonate phoscorite in the contact zone of the phoscorite–carbonatite pipe with host foidolite as well as in the 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} and the fracture is conchoidal. Mohs hardness is ~5. In transmitted light, the mineral is light brown, isotropic and n = 2.10(5) (λ = 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 U0.05${\rm Fe}_{{\rm 0}{\rm. 02}}^{2 +} $)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 dissolves slowly 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 80 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.

Type
Article
Copyright
Copyright © Mineralogical Society of Great Britain and Ireland 2018 

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Footnotes

Associate Editor: G. Diego Gatta

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