Hydroxycalciopyrochlore, A New Mineral Species from Sichuan, China

Hydroxycalciopyrochlore, ideally (Ca,Na,U,square)(2)(Nb,Ti)(2)O-6(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/mm(2) (5-6 on the Mohs hardness scale). The density measured by hydrostatic weighing is 5.10(3) g/cm(3). The strongest four reflections in the X-ray powder-diffraction pattern [d in angstrom (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) angstrom, V= 1118.7(7) angstrom(3), 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 square(0.21))(2.00)(Nb1.15Ti0.80Ta0.03Al0.01Mg0.01)(2.00)O-6.02 [(OH)(1.01)F-0.09](1.10), on the basis of 2 atoms of B pfu; the simplified formula is (Ca,Na,U,square)(2)(Nb,Ti)(2)O-6(OH). Type material is deposited in the Geological Museum of China, Beijing, People's Republic of China, catalogue number M11800.