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| Titel |
The post-spinel transition in Fe3O4-Fe2SiO4 and Fe3O4- FeCr2O4 solid solutions |
| VerfasserIn |
Alan Woodland, Klaus Schollenbruch, Daniel Frost, Falko Langenhorst |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250042875
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| Zusammenfassung |
| Minerals with spinel structure are important phases in the Earth´ s mantle. Both magnetite
(mt, Fe3O4) and chromite (chr, FeCr2O4) are known to transform to denser orthorhombic
post-spinel phases at pressures-¥10 GPa and -¥12.5 GPa, respectively (Schollenbruch et al.
2009a; Chen et al. 2003). On the other hand, Fe2SiO4 decomposes to its constituent oxides,
FeO and SiO2 at high P and no post-spinel polymorph appears to be stable (e.g. Ito &
Takahashi 1989). An important question is how spinel solid solutions behave at high
pressures and temperatures since such compositions are arguably more petrologically
relevant. In addition, since h-Fe3O4 is apparently not quenchable, it is difficult to investigate
its structure. In contrast, two high-P polymorphs of FeCr2O4-rich compositions have been
found in a meteorite (Chen et al. 2003), suggesting that the addition of Cr might
allow us to recover the post-spinel phase of Fe3O4-bearing compositions from
experiments.
Building on recent results for the Fe3O4 end member (Schollenbruch et al. 2009a, 2009b), we
have begun a study of the high-pressure behaviour of solid solutions along the Fe3O4
-Fe2SiO4 and Fe3O4- FeCr2O4 joins. Multianvil experiments were performed at 10 and 13
GPa and 1200-1300°C on pre-synthesised spinels with compositions 85mt-15 Fe2SiO4,
50mt-50chr and 80mt-20chr. For the Si-bearing experiments, stishovite was present in the run
products. This occurrence, along with observed twinning in the Fe-oxide phase
(Schollenbruch et al. 2009a) allows us to conclude that the original spinel had transformed to
a high-P polymorph at a pressure and that Si is essentially excluded from this new structure.
However, the powder XRD data from the run products could not be indexed either to
magnetite (spinel structure) or to any other expected phase, including the known
post-spinel structures. Interestingly, these are the same reflections reported by Koch et al.
(2004) for an unidentified phase in their high-P (> 9 GPa) syntheses in the system
Fe3O4-Fe2SiO4-Mg2SiO4.
In the Cr-bearing experiments, the 80mt-20chr composition produced a single phase, while
the 50mt-50chr composition yielded several phases. In all samples, the same set of diffraction
peaks found in the Si-bearing experiments were present even though microprobe analysis
revealed that all phases present contained significant Cr. A first look at the 80mt-20chr
sample with TEM suggests a hexagonal structure, however, it is inconsistent with an
eskolaite-hematite solid solution. Positive identification of this phase awaits further analysis.
In addition to this "mystery" phase, the 50mt-50chr samples either contained a spinel or an
eskolaite-hematite solid solution. Thus, Si- and Cr-bearing spinels have also been
demonstrated to undergo a phase transition at essentially the same pressure as that observed
for the magnetite end member (Schollenbruch et al. 2009b), however, the resulting phase
appears to be different.
References
Chen et al. (2003) Proc Nat Acad Sci, DOI 10.1073/pnas.2136599100
Ito & Takahashi (1989) J Geophys Res, 94, 10637-10646
Koch et al. (2004) Phys Earth Planet Interiors, 143, 171-183
Schollenbruch et al. (2009a) High Pressure Research, 29, 520-524
Schollenbruch et al. (2009b) AGU abstract MR31B-1649, Fall meeting, Dec. 2009 |
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