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Oxynatromicrolite, (Na,Ca,U)2Ta2O6(O,F), a new member of the pyrochlore supergroup from Guanpo, Henan Province, China

  • Fan Guang (a1), Ge Xiangkun (a1), Li Guowu (a2), Yu Apeng (a1) and Shen Ganfu (a3)...

A new mineral species of the pyrochlore supergroup, oxynatromicrolite (IMA2013-063), (Na,Ca,U)2Ta2O6O, was found in the No. 309 rare-metal granitic pegmatite vein, Guanpo, Lushi county, Henan Province, China, which is characterized by tantalum at theB site and oxygen at the Y site and is Na dominant at the A site. The mineral occurs as strongly metamict, and mostly euhedral octahedral crystals up to 0.05–0.20 mm across. The measured density of an unheated sample is 6.580(4) g cm–3, and the calculatedone is 6.506 g cm–3. Optically, the mineral is isotropic, with an index of refraction 1.999(5) and a reflectance of 11.88% (470 nm). When heated to 1000°C for 4 hours in N2, the mineral recrystallizes in the cubic system, with space group Fd3mand with unit-cell parameters similar those of other pyrochlore supergroup species: a = 10.420(6) Å, V = 1131.4(2) Å3. Electron microprobe analyses revealed the following composition of the mineral (in wt.%): Na2O 5.41, CaO 4.56, UO214.60, La2O3 0.16, Ce2O3 0.11, Nd2O3 0.13, PbO 0.62, Ta2O5 61.52, Nb2O5 8.21, Sb2O5 0.23, TiO2 0.05, SiO2 0.56, SnO2 0.29, F1.04, H2O 1.50 (calculated to correspond to 0.47 H2O pfu), F≡O –0.44, sum = 98.53%, which corresponds to the empirical formula (Na0.99Ca0.46U0.31Pb0.02La0.01H2O0.21)∑2.00(Ta1.58Nb0.35Si0.05Sn0.01Sb0.01)∑2.00O6 (O0.43F0.31H2O0.26)∑1.00, represented by the simplified formula (Na,Ca,U)2(Ta,Nb)2O6(O,F).Oxynatromicrolite crystallized during the late-stage of formation for the No. 309 pegmatite dyke and is associated with quartz, albite, potassium feldspar, muscovite, kaolinite, tantalite-Mn, stibiotantalite, pollucite, spodumene, montebrasite, Hf-rich zircon, a red tourmaline, polylithionite,trilithionite, luanshiweiite-2M 1 (IMA2011-102) and a hydrated derivative of oxynatromicrolite.

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