engleski

Nežilovite, a new member of the magnetoplumbite group, and the crystal chemistry of magnetoplumbite and hibonite

Nežilovite, ideally PbZn2(Mn4+, Ti4+)2Fe8O19, is a newly recognized member of the magnetoplumbite group from Nežilovo, FYR Macedonia. It forms black, magnetic crystals up to 1 mm with a tabular, hexagonal outline. Electron-microprobe analyses yielded PbO 19.01, CaO 0.03, Sb2O5 0.25, ZnO 13.23, Fe2O3 41.96, Mn2O3 7.13, MnO2 11.01, TiO2 2.84, Al2O3 4.52, MgO 0.08, total 100.06wt.%. The mineral is optically anisotropic and bireflectant. Reflectance values (Ro%) obtained in air and oil are: 23.8, 10.0 (470 nm), 22.4, 8.8 (546 nm) 21.7, 8.3 (589 nm)and 20.7, 7.7. (650 nm). Dcalc = 5.69(1) gcm-3 and VHN25= 735. The strongest reflections in the powder XRD pattern [d in Å (I/Io)(hkl) ] are: 3.811 (100)(006), 2.858 (75)(008), 2.745(50)(107), 11.39(45)(002), 2.605(40)(114) and 1.636(30)(0, 0, 14). The crystal structures of natural magnetoplumbite [a 5.902(2), c 23.185(8)Å ], hibonite [ a 5.613(1), c 22.285(8)Å ] and nežilovite [a 5.849(1), c 22.809(2)Å ] have been refined in the space group P63/mmc to R indices of 6.9%, 5.3% and 4.5%, respectively, using MoKα single-crystal X-ray-diffraction data. The general formula for the magnetoplumbite-group minerals is A[6](M1) [5](M2) [4](M3)2[6](M4)2[6](M5)6O19, with Z = 2. An important common feature is the tendency of M2+ and M4+, 5+ ions, hosted by these minerals, to order onto the M3 and M4 sites, respectively ; for nežilovite, almost complete occupancy of M3 by Zn is demonstrated, which forms the basis for naming a new species. Al3+ strongly prefers M1. Mössbauer spectral data show that nežilovite is paramagnetic at room temperature ; they also support the present model of cation distribution.

magnetoplumbite; hibonite; nežilovite; new mineral species; cation ordering; Mössbauer spectrum; FYR Macedonia

nije evidentirano