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

摘要

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 the B 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 mmacross. The measured density of an unheated sample is 6.580(4) g cm-3, and the calculated one 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 Fd3m and with unit-cell parameters similar those of other pyrochlore supergroup species: a = 10.420(6) ?, V = 1131.4(2) ?3. Electron microprobe analyses revealed the following composition of the mineral (in wt.%): Na2O 5.41, CaO 4.56, UO2 14.60, 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, F 1.04, 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)Σ2.00 (Ta1.58Nb0.35Si0.05Sn0.01Sb0.01)Σ2.00O6 (O0.43F0.31H2O0.26)Σ1.00, 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-2M1 (IMA2011-102) and a hydrated derivative of oxynatromicrolite.