Abstract

Nezilovite, ideally PbZn 2 (Mn (super 4+) ,Ti (super 4+) ) 2 Fe 8 O 19 , is a newly recognized member of the magnetoplumbite group from Nezilovo, 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, Sb 2 O 5 0.25, ZnO 13.23, Fe 2 O 3 41.96, Mn 2 O 3 7.13, MnO 2 11.01, TiO 2 2.84, Al 2 O 3 4.52, MgO 0.08, total 100.06 wt.%. The mineral is optically anisotropic and bireflectant. Reflectance values (R o %) obtained in air and oil are: 23.8, 10.0 (470 nm), 22.4, 8.8 (589 nm) and 20.7, 7.7. (650 nm). D calc = 5.69(1) g.cm (super -3) and VHN 25 = 735. The strongest reflections in the powder XRD pattern [d in Aa (I/I o )(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) Aa], hibonite [a 5.613(1), c 22.285(8) A//] and nezilovite [a 5.849(1), c 22.809(2) Aa] have been refined in the space group P6 3 /mmc to R indices of 6.9%, 5.3% and 4.5%, respectively, using Mokalpha single-crystal X-ray-diffraction data. The general formula for the magnetoplumbite-group minerals is A (super [6]) (M1) (super [5]) (M2) (super [4]) (M3) 2 (super [6]) (M2 (super [6]) (M4) 2 (super [6]) (M5) 6 O 19 , with Z = 2. An important common feature is the tendency of M (super 2+) and M (super 4+,5+) ions, hosted by these minerals, to order onto the M3 and M4 sites, respectively; for nezilovite, almost complete occupancy of M3 by Zn is demonstrated, which forms the basis for naming a new species. Al (super 3+) strongly prefers M1. Mossbauer spectral data show that nezilovite is paramagnetic at room temperature; they also support the present model of cation distribution.

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