HR: 14:04h
AN: MR23D-03    [Abstracts]
TI: Spin crossover and short-range order in lower mantle (Mg,Fe)O
AU: Kantor, I
EM: innokenty.kantor@uni-bayreuth.de
AF: Bayerisches Geoinstitut, Universitaet Bayreuth, Bayreuth, D-95440, Germany
AU: * McCammon, C
EM: catherine.mccammon@uni-bayreuth.de
AF: Bayerisches Geoinstitut, Universitaet Bayreuth, Bayreuth, D-95440, Germany
AU: Dubrovinsky, L
EM: leonid.dubrovinsky@uni-bayreuth.de
AF: Bayerisches Geoinstitut, Universitaet Bayreuth, Bayreuth, D-95440, Germany
AB: It is now well established that spin crossover occurs in (Mg,Fe)O at high pressure, but conflicting results have been reported of the pressure range over which the transition occurs. Since the spin state can influence the physical properties of (Mg,Fe)O and hence the properties and dynamics of the lower mantle, we made a detailed study of (Mg,Fe)O containing 5, 13 and 20 mole % FeO using Mössbauer spectroscopy in the diamond anvil cell. In the pressure range up to 20 GPa we observed a non-reversible change in quadrupole splitting in both hydrostatic and non-hydrostatic experiments that can be interpreted from an analysis of the quadrupole splitting distribution to indicate a rapid increase in short-range order with a tendency to Fe clusterisation. Further experiments varying the P,T path during synthesis suggest that (Mg,Fe)O likely exhibits significant short-range order under lower mantle conditions. We observed the onset of spin crossover near 50 GPa in the Mössbauer spectra of all samples, but with a transition width that is highly composition dependent. Analysis of literature data together with the current results suggest that spin crossover is a thermal equilibrium process without phase transition. The composition and temperature dependence of spin crossover in (Mg,Fe)O can be described relatively well using such a model, taking into account the local structure of the solid solution. The model further suggests that Fe ions in different local surroundings can have different critical pressures of spin crossover. The total fraction of low-spin Fe²⁺ depends therefore not only on pressure, temperature and composition, but also on short-range order. Consequences for the physical properties and dynamics of the lower mantle will be discussed.
DE: 1042 Mineral and crystal chemistry (3620)
DE: 3620 Mineral and crystal chemistry (1042)
DE: 3630 Experimental mineralogy and petrology
DE: 3929 NMR, Mossbauer spectroscopy, and other magnetic techniques
DE: 8124 Earth's interior: composition and state (1212, 7207, 7208, 8105)
SC: Mineral and Rock Physics [MR]
MN: 2007 Fall Meeting