Hydroxycalciopyrochlore, A New Mineral Species from Sichuan, China

Abstract

AbstractHydroxycalciopyrochlore, ideally (Ca,Na,U,□)2(Nb,Ti)2O6(OH), cubic, is a new mineral species (IMA2011–026) within the pyrochlore supergroup that was found occurring at the Maoniuping mine, Mianning County, Xichang prefecture, Sichuan Province, southwest China. The mineral is found in an alkali feldspar granite rare‐earth ore deposit (26–27 Ma). Associated minerals include calcite, barite, celestine, albite, aegirine, aegirine‐augite, fluorite, parasite‐(Ce), thorite, thorianite, zircon, galena, sphalerite, magnetite, and pyrite. Crystals occur mostly as octahedra, and less often as dodecahedra and tetrahexahedra or combinations thereof. Some occur with an allotriomorphic habit with a thick triangular tabular form. Crystals generally range from 0.1 to 1 mm in size. The mineral is brownish‐black, greenish‐black and black on fresh sections with a brown streak. The crystal is translucent, and has a greasy lustre on fresh sections. It is metamict without any observed parting or cleavage and with a conchoidal fracture. The Vickers microhardness is 572 kg/mm2 (5–6 on the Mohs hardness scale). The density measured by hydrostatic weighing is 5.10(3) g/cm3. The strongest four reflections in the X‐ray powder‐diffraction pattern [d in Å (I) hkl] are: 2.9657 (100) 2 2 2, 1.8142 (34) 0 4 4, 1.5463 (21) 2 2 6, 2.5688 (18) 0 0 4. The unit‐cell parameters are a = 10.381(4) Å, V = 1118.7(7) Å3, Z = 8. The structure was solved and refined in the space group Fd3m with R = 0.09. The empirical formula is (Ca0.74Na0.58U0.40Ce0.05Fe0.02□0.21)2.00(Nb1.15Ti0.80Ta0.03Al0.01Mg0.01)2.00O6.02 [(OH)1.01F0.09]1.10, on the basis of 2 atoms of B pfu; the simplified formula is (Ca,Na,U,□)2(Nb,Ti)2O6(OH). Type material is deposited in the Geological Museum of China, Beijing, People's Republic of China, catalogue number M11800.