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

摘要

Hydroxynatropyrochlore, (Na,Ca,Ce)(2)Nb2O6(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 (3) over barm, a = 10.3211(3) angstrom, V = 1099.46(8) angstrom(3) and Z = 8 (from powder diffraction data) or a = 10.3276(5) angstrom, V = 1101.5(2) angstrom(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 similar to 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(-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 U-0.05)(Sigma 2.00)(Nb1.68Ti0.23Ta0.09)(Sigma 2.00)O-6.03(OH1.04F0.01)(Sigma 1.05). The simplified formula is (Na,Ca,Ce)(2)Nb2O6(OH). The mineral dissolves slowly in hot HCl. The strongest X-ray powder-diffraction lines [listed as (d in angstrom)(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-1 = 0.026 on the basis of 80 unique observed reflections. The mineral belongs to the pyrochlore structure type A(2)B(2)O(6)Y(1) 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.