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| Titel |
Synthesis of quenchable high-pressure form of magnetite (h-Fe3O4) with composition [4](Fe0.732+ Mg0.26)[6](Fe0.713+ Cr0.14Al0.10 Si0.04)2O4 |
| VerfasserIn |
Monika Koch-Müller, Enrico Mugnaioli, Dieter Rhede, Sergio Speziale, Ute Kolb, Richard Wirth |
| Konferenz |
EGU General Assembly 2014
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250087582
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| Publikation (Nr.) |
 EGU/EGU2014-1639.pdf |
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| Zusammenfassung |
| Cubic inverse-spinel magnetite transforms under pressure to orthorhombic normal-spinel
magnetite, h-Fe3O4 ( e.g. Fei et al. 1999; Bengtson et al. 2013). The pressure at which the
transition takes place is still controversial. The high-pressure form is reported to be not
quenchable to ambient conditions. We report the synthesis of h-magnetite which incorporates
considerable amounts of additional cations (Cr, Mg, Al, Si) and is quenchable to ambient
conditions. Two experiments were performed at 18 GPa and 1800 °C in a multi-anvil press.
The run products were investigated by electron microprobe, transmission electron microscopy
and electron diffraction tomography. We observed the formation of h-magnetite in both
experiments. In experiment MA-367 we used an oxide mixture with a majoritic stoichiometry
Mg1.8Fe1.2(Al1.4 Cr0.2Si0.2Mg0.2)Si3O12 as starting material, with Si and Mg in excess.
The Fe-oxide phase forms elongated aggregates 10–30 μm in length, mutually
intergrown with majorite, the latter being the main phase of the run products coexisting
with small amounts of stishovite. The formula for h-magnetite in run MA-367 was
calculated as [4](Fe0.732+ Mg0.26)[6](Fe0.713+ Cr0.14Al0.10 Si0.04)2O4. In the second
experiment (MA-376) we used an oxide mixture corresponding to the composition of
h-magnetite obtained in MA-367. In this experiment the main phase was h-magnetite with
composition [4](Fe0.982+)[6](Fe0.683+ Cr0.17Al0.13 Si0.02)2O4coexisting with very small
amounts of wadsleyite. Interestingly no magnesium was incorporated into the Fe-oxide
in this experiment compared to MA-367 and no iron was found in the coexisting
wadsleyite. For the first time it was possible to perform electron diffraction on recovered
h-magnetite of both experiments and we observed that -at least in our case- the
h-magnetite structure can better be described in space group Amam than in space group
Bbmm as previously proposed. The substitution of Fe by Cr, Mg, Al and Si, all
smaller in atomic size, may have favored the survival of the high pressure form to
ambient conditions. We prove that the h-magnetite phase is also stable in chemical
systems more complex than the simple Fe-O. Based on our results obtained at 18 GPa
and 1800 oC in a system that is closely related to Fe-enriched oceanic lithospheric
material, we suggest that h-magnetite may be present in environments connected to
deeply subducted slabs. The strong enrichment of Cr in this oxide phase implies that
coexisting silicates may be depleted in Cr compared to Fe3O4-free assemblages. This
would significantly affect the chemical signature of melts produced in the deep
mantle.
References:
Fei et al. (1999) American Mineralogist, 84, 203 – 206
Bengtson et al. (2013) Physical Review B87, 155141 |
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