Publication Date:
2018
Description:
〈div data-abstract-type="normal"〉〈p〉Hydroxynatropyrochlore, (Na,Сa,Ce)〈span〉2〈/span〉Nb〈span〉2〈/span〉O〈span〉6〈/span〉(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, 〈span〉Fd〈/span〉〈span〉〈span〉〈img data-mimesubtype="gif" data-type="simple" src="http://static.cambridge.org/resource/id/urn:cambridge.org:id:binary:20190313104343836-0311:S0026461X18001056:S0026461X18001056_inline1.gif"〉
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〈/span〉〈/span〉〈span〉m〈/span〉, 〈span〉a〈/span〉 = 10.3211(3) Å, 〈span〉V〈/span〉 = 1099.46(8) Å〈span〉3〈/span〉 and 〈span〉Z〈/span〉 = 8 (from powder diffraction data) or 〈span〉a〈/span〉 = 10.3276(5) Å, 〈span〉V〈/span〉 = 1101.5(2) Å〈span〉3〈/span〉 and 〈span〉Z〈/span〉 = 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 〈span〉n〈/span〉 = 2.10(5) (λ = 589 nm). The calculated and measured densities are 4.77 and 4.60(5) g cm〈span〉−3〈/span〉, respectively. The mean chemical composition determined by electron microprobe is: F 0.05, Na〈span〉2〈/span〉O 7.97, CaO 10.38, TiO〈span〉2〈/span〉 4.71, FeO 0.42, Nb〈span〉2〈/span〉O〈span〉5〈/span〉 56.44, Ce〈span〉2〈/span〉O〈span〉3〈/span〉 3.56, Ta〈span〉2〈/span〉O〈span〉5〈/span〉 4.73, ThO〈span〉2〈/span〉 5.73, UO〈span〉2〈/span〉 3.66, total 97.65 wt.%. The empirical formula calculated on the basis of Nb + Ta + Ti = 2 apfu is (Na〈span〉1.02〈/span〉Ca〈span〉0.73〈/span〉Ce〈span〉0.09〈/span〉Th〈span〉0.09〈/span〉 U〈span〉0.05〈/span〉〈span〉〈span〉〈img data-mimesubtype="gif" data-type="simple" src="http://static.cambridge.org/resource/id/urn:cambridge.org:id:binary:20190313104343836-0311:S0026461X18001056:S0026461X18001056_inline2.gif"〉
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〈/span〉〈/span〉)〈span〉Σ2.00〈/span〉(Nb〈span〉1.68〈/span〉Ti〈span〉0.23〈/span〉Ta〈span〉0.09〈/span〉)〈span〉Σ2.00〈/span〉O〈span〉6.03〈/span〉(OH〈span〉1.04〈/span〉F〈span〉0.01〈/span〉)〈span〉Σ1.05〈/span〉. The simplified formula is (Na,Ca,Ce)〈span〉2〈/span〉Nb〈span〉2〈/span〉O〈span〉6〈/span〉(OH). The mineral dissolves slowly in hot HCl. The strongest X-ray powder-diffraction lines [listed as (〈span〉d〈/span〉 in Å)(〈span〉I〈/span〉)(〈span〉hkl〈/span〉)] 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 〈span〉R〈/span〉〈span〉1〈/span〉 = 0.026 on the basis of 80 unique observed reflections. The mineral belongs to the pyrochlore structure type 〈span〉A〈/span〉〈span〉2〈/span〉〈span〉B〈/span〉〈span〉2〈/span〉O〈span〉6〈/span〉〈span〉Y〈/span〉〈span〉1〈/span〉 with octahedral framework of corner-sharing 〈span〉B〈/span〉O〈span〉6〈/span〉 octahedra with 〈span〉A〈/span〉 cations and OH groups in the interstices. The Raman spectrum of hydroxynatropyrochlore contains characteristic bands of the lattice, 〈span〉B〈/span〉O〈span〉6〈/span〉, 〈span〉B〈/span〉–O and O–H vibrations and no characteristic bands of the H〈span〉2〈/span〉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.〈/p〉〈/div〉
Print ISSN:
0026-461X
Electronic ISSN:
1471-8022
Topics:
Geosciences
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