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Hydroxynatropyrochlore, (Na,Sa,Ce) (sub 2) Nb (sub 2) O (sub 6) (OH), a new member of the pyrochlore group from the Kovdor phoscorite-carbonatite pipe, Kola Peninsula, Russia

Gregory Yu. Ivanyuk, Victor N. Yakovenchuk, Taras L. Panikorovskii, Nataliya Konoplyova, Yakov A. Pakhomovsky, Ayya V. Bazai, Vladimir N. Bocharov and Sergey V. Krivovichev
Hydroxynatropyrochlore, (Na,Sa,Ce) (sub 2) Nb (sub 2) O (sub 6) (OH), a new member of the pyrochlore group from the Kovdor phoscorite-carbonatite pipe, Kola Peninsula, Russia
Mineralogical Magazine and Journal of the Mineralogical Society (February 2019) 83 (1): 107-113

Abstract

Hydroxynatropyrochlore, (Na,Sa,Ce) (sub 2) Nb (sub 2) O (sub 6) (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, Fd3m, a = 10.3211(3) Aa, V = 1099.46(8) Aa (super 3) and Z = 8 (from powder diffraction data) or a = 10.3276(5) Aa, V = 1101.5(2) A (super 3) 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 approximately 5. In transmitted light, the mineral is light brown, isotropic and n = 2.10(5) (lambda = 589 nm). The calculated and measured densities are 4.77 and 4.60(5) g cm (super -3) , respectively. The mean chemical composition determined by electron microprobe is: F 0.05, Na (sub 2) O 7.97, CaO 10.38, TiO (sub 2) 4.71, FeO 0.42, Nb (sub 2) O (sub 5) 56.44, Ce (sub 2) O (sub 3) 3.56, Ta (sub 2) O (sub 5) 4.73, ThO (sub 2) 5.73, UO (sub 2) 3.66, total 97.65 wt.%. The empirical formula calculated on the basis of Nb + Ta + Ti = 2 apfu is (Na (sub 1.02) Ca (sub 0.73) Ce (sub 0.09) Th (sub 0.09) U (sub 0.05) Fe (sub 0.02) (super 2+) ) (sub Sigma 2.00) (Nb (sub 1.68) Ti (sub 0.23) Ta (sub 0.09) ) (sub Sigma 2.00) O (sub 6.03) (OH (sub 1.04) F (sub 0.01) ) (sub Sigma 1.0 5) . The simplified formula is (Na,Ca,Ce) (sub 2) Nb (sub 2) O (sub 6) (OH). The mineral dissolves slowly in hot HCl. The strongest X-ray powder-diffraction lines [listed as (d in A)(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 R (sub 1) = 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 BO (sub 6) octahedra with A cations and OH groups in the interstices. The Raman spectrum of hydroxynatropyrochlore contains characteristic bands of the lattice, BO (sub 6) , B-O and O-H vibrations and no characteristic bands of the H (sub 2) O 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.


ISSN: 0369-0148
Coden: MNLMBB
Serial Title: Mineralogical Magazine and Journal of the Mineralogical Society
Serial Volume: 83
Serial Issue: 1
Title: Hydroxynatropyrochlore, (Na,Sa,Ce) (sub 2) Nb (sub 2) O (sub 6) (OH), a new member of the pyrochlore group from the Kovdor phoscorite-carbonatite pipe, Kola Peninsula, Russia
Affiliation: Russian Academy of Sciences, Kola Science Centre, Apatity, Russian Federation
Pages: 107-113
Published: 201902
Text Language: English
Publisher: Mineralogical Society, London, United Kingdom
References: 35
Accession Number: 2019-026985
Categories: Mineralogy of non-silicates
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 4 tables, sketch map
N67°34'60" - N67°34'60", E30°19'60" - E30°19'60"
Secondary Affiliation: St. Petersburg State University, RUS, Russian Federation
Country of Publication: United Kingdom
Secondary Affiliation: GeoRef, Copyright 2019, American Geosciences Institute. Abstract, Copyright, Mineralogical Society of Great Britain and Ireland. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 201915
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