Oxygen-Deficient Silicate and Ferrite Perovskites: Structure and Physical Properties

Ana Isabel Becerro (ana-isabel.becerro @uni-bayreuth.de)¹, Stefan Lauterbach (stefan.lauterbach@uni-bayreuth.de)¹, Ross J. Angel (ross.angel@uni-bayreuth.de)¹, Falko Langenhorst (falko.langenhorst @uni-bayreuth.de)¹, Stefan Marion (stefan.marion@kjemi.uio.no)², Catherine McCammon (catherine.mccammon @uni-bayreuth.de)¹, Nancy Ross (n.ross@ucl.ac.uk)³ & Friedrich Seifert (friedrich.seifert@uni-bayreuth.de)¹

¹ Bayerisches Geoinstitüt, Universität Bayreuth, D-95440 Bayreuth, Germany

² Centre for Materials Science, University of Oslo, N-0349 Oslo, Norway

³ Dept. Geological Sciences, University College London, London WC1E 6BT, United Kingdom

Physical and chemical properties of silicate perovskite close in composition to (Mg,Fe)SiO₃ largely determine the properties and dynamic processes in the Earth's lower mantle because it is the dominant (ca. 80%) phase at these pressures and temperatures. Such properties may be affected significantly by the incorporation of even minor concentrations of cations other than Mg²⁺, Fe²⁺ and Si into the perovskite structure. Incorporation of trivalent cations (Al³⁺, Fe³⁺) into silicate perovskite is governed by the combination of two substitutions (Mg²⁺,Fe²⁺) + Si⁴⁺ ­> 2(Al³⁺, Fe³⁺) and 2Si⁴⁺ + O^2- ­> 2(Al³⁺,Fe³⁺) + V where V represents an anion vacancy on the oxygen sublattice. The latter substitution is particularly important in low-SiO₂ environments as expected in the Earth's lower mantle.

The system CaTiO₃-CaFeO_2.5 provides a more easily tractable model for studying the effect of oxygen vacancies on the variations of structural, physical and chemical properties in perovskites. The following sequence of vacancy clusters is observed with increasing vacancy concentration and/or decreasing temperature: isolated vacancies, finite (110) chains, ordering of chains into tetrahedral layers and finally ordering into a regular sequence of tetrahedral and octahedral layers. Defect perovskites with finite defect chains exhibit short-range order only and have cubic lattice symmetry at high temperature, whereas the long-range ordered phases are orthorhombic. Electrical conductivity is essentially ionic at oxygen fugacities corresponding to the Earth's mantle and conductivity values decrease more than tenfold when oxygen vacancies are pinned by the formation of infinite chains in tetrahedral sheets. In the CaFeO_2.5 end member, consisting of a 1:1 sequence of tetrahedral and octahedral sheets, the bulk modulus is more than 10% less than would be expected from the volume-bulk modulus systematics in stoichiometric perovskites, indicating the strong influence of oxygen defects on elastic properties.

Ar and H₂O Diffusion in Rhyolitic Glasses and Melts

Harald Behrens (h.behrens @mineralogie.uni-hannover.de)¹ & Youxue Zhang (youxue@umich.edu)²

¹ Institut für Mineralogie, Universität Hannover, Welfengarten 1, D-30167 Hannover, Germany

² Department of Geological Siences, 2534 XC.C. Little Building, 425 East University, Ann Arbor, MI 48109-1063, USA

Knowledge of Ar diffusion in rhyolitic melts is important to understand the behavior of noble gases during degasing of magmas. Furthermore, Ar diffusion data may provide insight to the mechanisms of water diffusion in the melt because Ar, in contrast to H₂O, is an inert component without chemical interaction with the melt. The T-dependence of Ar diffusion in water poor rhyolitic glasses was studied at 500 MPa in the T-range 480-1102°C. After exposing rhyolitic plates to an Ar atmosphere in CSPV (<800°C) or IHPV (>800°C), Ar sorption profiles were measured perpendicular to the original surface by electron microprobe. The obtained Arrhenius law D = 0.0219 cm²/s * exp(-8225 K/T) is consistent with data of Carroll (1991) at lower pressure (110-250 MPa). However, variation of pressure at 600°C (50-810 MPa) and 1000°C (100-500 MPa) indicate that the apparent activation volume for Ar diffusion is not constant as assumed by Carroll (1991) but decreases with temperature and, at least at low T, with pressure. The effect of water on Ar diffusion was investigated by diffusion couples using Ar bearing and Ar-free glasses pre-equilibrated with water. In contrast to viscosity and total water diffusion, an exponential dependence on water content was observed in the whole range of water contents (1-5 wt% H₂O). The Ar data are consistent with a constant increase of diffusivity by about 0.25 log unit per 1 wt% H₂O in the whole investigated T-range (900-1200°C). The data support the interpretation that the strong increase of total water diffusivity and decrease of viscosity by adding water to dry melts is due to the dissociation reaction of H₂O and that the diffusivity of molecular H₂O exponentially varies with water content as assumed in modelling of total H₂O diffusion profiles by Zhang and Behrens (2000).

Carroll M, Earth Planet. Sci. Lett, 103, 156-168, (1991).

Zhang Y & Behrens H, Chem. Geol, in press, (2000).