The New Mineral Neustädtelite, Bi₂Fe(Fe,Co)(O,OH)₂(OH)₂(AsO₄)₂

Werner Krause¹, Heinz-Jürgen Bernhardt², Catherine McCammon³ & Herta Effenberger⁴

¹ Henriette-Lott Weg 8, D-50354, Germany

² Institüt für Mineralogie, Rhur-Universität Bochum, Universitätsstraße 150, D-44780 Bochum, Germany

³ Bayerisches Geoinstitüt, Universität Bayreuth, D-95440 Bayreuth, Germany

⁴ Institüt für Mineralogie und Kristallographie, Universität Wien, Althanstraße 14, A-1090 Wien, Austria

Neustädtelite was found on dumps near Schneeberg-Neustädtel, Saxony, Germany. Associated minerals are bismutite, preisingerite and quartz. The strongly pleochroitic (brown to pale yellow) neustädtelite crystals are up to 0.1 mm in diameter. Electron-microprobe analyses and X-ray powder-investigations showed that neustädtelite is isostructural with medenbachite, Bi₂Fe(Cu,Fe)(O,OH)₂(OH)₂(AsO₄)₂ (Krause et al., 1996). The crystal structure was investigated by single-crystal X-ray diffraction: R1(F) = 0.083, wR2(F²) = 0.138 (872 unique data, 95 variable parameters). The cell parameters are: a = 4.551(1) Å, b = 6.146(1) Å, c = 9.002(2) Å, (alpha) = 95.41(2)°, ß = 99.28(2)°, (gamma) = 92.89(1)°, Z = 1, space group P.

The Bi atom is split onto two half occupied positions with [3+4] and [2+5] coordination, respectively. The separation of 0.488(2) Å is comparable to that of medenbachite which amounts 0.463(5) Å. Fe³⁺ (according to Mössbauer measurements) and Co2+ atoms occupy two Me positions. In neustädtelite both their coordination figures are moderately distorted octahedra. The bond distance <Me1­O> = 2.02 Å is slightly shorter than <Me2­O> = 2.07 Å indicating that Me1 = Fe³⁺ and Me2 = (Fe³⁺,Co²⁺). In medenbachite Me1O₆ = Fe³⁺O₆ forms a comparable octahedron, <Me1­O> = 2.01 Å. The Me2O₆ polyhedron in medenbachite features a tetragonal bipyramid predominantly occupied by Cu²⁺ atoms (<Me2­O> = 2.01 Å and 2.27 Å for the four nearest and two next-nearest neighbours, Jahn-Teller-effect). One of the two O atoms not belonging to AsO₄ is a hydroxyl group which links a Me1O₆ and a Me2O₆ polyhedron, Bi­O is 2.77 Å. The other one represents an oxo-oxygen atom or a hydroxyl group with two Bi­O bonds 2.21 Å. The mixed occupation O^2-:(OH)^- is required for charge balance according to the ratio Fe³⁺:Co²⁺. Layers in (001) are formed by corner connection of the MeO₄ chains running parallel [010] (formed by edge-sharing MeO₆ polyhedra) with the arsenate tetrahedra; they are linked by Bi atoms.

Krause W, Bernhardt HJ, Gebert W, Graetsch H, Balendorff K, Pettitjean K, Amer. Miner., 81, 505-512, (1996).

Resemblance and Difference between the Constitution of the Moon and Io

Oleg Kuskov (ol_kuskov@mail.ru) & Victor Kronrod

Vernadsky Institute of, Geochemistry, 117975 Moscow, GSP-1, Russia

The Moon and Io are the unique satellites in the Solar system. The mean Earth-Moon (384,000 km, 60 RE from the Earth) and Jupiter-Io (422,000 km, 6 RJ from Jupiter) distances are comparable. The Moon and Io are also comparable in the mean density, the mass and moment of inertia in spite of the principle differences in the masses of central planets. In this work, internally consistent models of the constitution of the Moon and Io based on the geophysical (the seismic velocities, the mass and moment of inertia) and geochemical (chemical and phase composition) constraints are compared. Thermodynamic modelling in the CaO-FeO-MgO-Al₂O₃-SiO2-Fe-FeS system was used to estimate the bulk composition, the density distribution in the mantle and core radii from the geophysical data (inverse problem). The atomic ratios of total iron to silicon for Io are estimated to be (Fe/Si)= 0.40-0.66 in comparison with 0.22-0.24 for the Moon, 0.51-0.59 for the LL and L chondrites, and 0.74-0.8 for CV3 and CM2 meteorites. The bulk composition of Io is estimated to be close to that of the L and LL chondrites having 8-13 wt.% of iron and iron sulfide. If Io does have such a composition, then the core is probably Fe or Fe-rich, whereas a large FeS core is excluded by the composition of the L and LL chondrites. A comparison of estimated bulk chemistry of the Moon and Io shows that Earth's and Jupiter's satellites have absolutely different bulk compositions.

Kinetics of Olivine and Pyroxenes Dissolution in Alkaline Basalts: Experimental Basis of the Websterite Model of the Upper Mantle

Vladislav A. Kutolin (lab02@uiggm.nsc.ru) & Valentina A. Shirokikh

Institute of Geology, Koptyuga pr. 3, Novosibirsk, 630090, Russia

Kutolin and Agafonov (1978) studied the kinetics of dissolution of olivine and pyroxenes in a melt of alkaline basalt at atmosphere pressure and came to the conclusion that the abundance of lherzolites among mantle-derived xenoliths is not accounted for by the predominance of these rocks in the mantle; the reason is the better preservation of lherzolites as compared with pyroxenites during the transport of the xenoliths to the surface, since olivine dissolves in a basalt melt 5-35 times as slowly as pyroxenes. With this taken into account, the composition of the upper mantle, calculated from the contents of various mantle xenoliths in basalts, will correspond to olivine websterite rather than to pyrolite (Ringwood, 1975). The results of Kutolin and Agafonov (1978) were tested by Scarfe et al. (1980) and Brearley and Scarfe (1986), who have studied the dissolution of olivine and pyroxenes in melts of alkaline basalts at high pressures. Scarfe et al. (1980) have established that at pressures of 12.5, 14, and 20 kbar pyroxenes dissolve more quickly than olivine. In the experiments of Brearley and Scarfe (1986) at 5 kbar pyroxenes dissolve more quickly than olivine. At 30 kbar pyroxenes appear to be more stable. At 12 kbar and 1400°C olivine dissolves more slowly than clinopyroxene but more quickly than orthopyroxene. At 1350°C, olivine appears to be more stable than pyroxenes. Thus, Scarfe et al. (1980) and Brearley and Scarfe (1986) confirmed the conclusions on the great stability of olivine as compared with pyroxenes during the transportation of mantle xenoliths to the surface by a basalt melt, but they did not support proposal of Kutolin and Agafonov (1978) to re-evaluate the composition of the upper mantle on this ground. The geophysical data (Duffy et al., 1995, Jeanloz, 1995) however conform with the websterite model rather than pyrolite.

Kutolin VA & Agafonov LV, Soviet Geol. Geophys, 19, 1-19, (1978).

Ringwood AE, Composition and Petrology of the Earth's Mantle, McGraw-Hill, New York, (1975).

Scarfe CM, Takahashi E & Yoder HC, Yb. Carnegie Instn. Wash, 79, 290-296, (1980).

Brearley M & Scarfe CM, J. Petrol, 27, 1157-1182, (1986).

Duffy TS, Zha C, Downs RT, Mao H & Hemley RJ, Nature, 378, 170-173, (1995).

Jeanloz R, Nature, 378, 130-131, (1995).