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

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.

Three-D Mineralogical Mapping of the Kovdor Phoscorite-Carbonatite Complex, NW Russia: III. Pyrochlore Supergroup Minerals

“…Carbonate-rich phoscorite and carbonatites enriched in PSM form an intensive radioactive anomaly (about 200 m in diameter and >900 m in depth), known as the "Anomalous Zone" [1]. Content of pyrochlore in rocks of the "Anomalous Zone" gradually increases with depth at the account of baddeleyite [27].…”

“…Carbonate-rich phoscorite and carbonatites enriched in PSM form an intensive radioactive anomaly (about 200 m in diameter and >900 m in depth), known as the "Anomalous Zone" [1]. Content of pyrochlore in rocks of the "Anomalous Zone" gradually increases with depth at the account of baddeleyite [27]. The pyrochlore group minerals, namely oxycalciopyrochlore (OCP), oxynatropyrochlore (ONP), hydroxycalciopyrochlore (HCP), hydroxynatropyrochlore (HNP), hydroxykenopyrochlore (HKP), fluorcalciopyrochlore (FCP), fluornatropyrochlore (FNP) and fluorkenopyrochlore (FKP), are common in foidolite, diopsidite, phlogopitite, phoscorite and carbonatite (OCP, HCP and HNP are predominant).…”

“…In calcite carbonatite veins, there are reddish-brown to creamy and yellow octahedral, cubic, truncated octahedral and cubooctahedral crystals of pyrochlores (see Figure 2c), and their close intergrowth with baddeleyite and zirconolite [1,4,29,30]. In vein dolomite carbonatite, yellow to brown truncated octahedral to cubic pyrochlore crystals (see Figure 2d) occur in voids in typical association with zircon and endemic phosphates [1,27,31,32]. Betafite group minerals occur as separate irregularly shaped, drop-like and ellipsoidal grains (up to 100 µm) or form marginal zones of pyrochlore crystals (see Figure 2d) and rims around baddeleyite grains.…”

Видовое Разнообразие Минералов Супергруппы Пирохлора В Ковдорском Фоскорит-Карбонатитовом Комплексе

“…Доминирующий в позиции A компонент (с учетом валентных групп аналогично позиции В) определяет второй префикс в названии мине-рального вида (Ca -кальцио, Na -натро, -кено, Pb -плюмбо, H 2 O -гидро и т.д.). Для позиции X характерен O, но могут также присутствовать OH и F.Минералы супергруппы пирохлора являются обычными акцессорными минералами всех ти-пов фоскоритов и карбонатитов Ковдорского массива [2][3][4][6][7][8]. При проведении объёмного мине-ралогического картирования магнетит-апатит-бадделеитового месторождения мы отобрали 540 об-разцов керна в интервале абсолютных отметок от -80 до -650 метров [8].…”

“…Минералы супергруппы пирохлора являются обычными акцессорными минералами всех ти-пов фоскоритов и карбонатитов Ковдорского массива [2][3][4][6][7][8]. При проведении объёмного мине-ралогического картирования магнетит-апатит-бадделеитового месторождения мы отобрали 540 об-разцов керна в интервале абсолютных отметок от -80 до -650 метров [8].…”