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| Titel |
High-pressure behavior of cristobalite under quasi-hydrostatic conditions |
| VerfasserIn |
Ana Cernok, Elena Bykova, Hanns-Peter Liermann, Leonid Dubrovinsky |
| 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 |
250094941
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| Publikation (Nr.) |
 EGU/EGU2014-10377.pdf |
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| Zusammenfassung |
| Cristobalite is a high-temperature, low-pressure polymorph of SiO2. It is comprised of a
framework of SiO4 tetrahedra, like many other low-pressure silica polymorphs, e.g. quartz
tridymite, or coesite. Its cubic form known as β-cristobalite crystallizes above 1470 °C and
at atmospheric pressure. The tetragonal α-phase is a metastable polymorph which appears
upon cooling of the cubic β-cristobalite to ambient temperature and it can be found in natural
acidic volcanic rocks.
High-pressure behavior of α-cristobalite is of interest form the geological standpoint
because it was experimentally observed that under elevated pressures and at ambient
temperature this polymorph with 4-fold coordinated silicon can transform to seifertite
– a lower mantle phase with octahedrally coordinated Si. The transformational
sequence is as follows. The tetragonal α-cristobalite transforms to a monoclinic
cristobalite II near ~1.5 GPa upon hydrostatic compression. The structure of this
higher-pressure polymorph was recently solved. Above ~10 GPa cristobalite II is
found to transforms to cristobalite X-I. Apart from the recent determination of the
unit cell parameters, the structure of the cristobalite X-I still remains unknown.
Further compression of cristobalite X-I above pressures exceeding ~35 GPa leads to
formation of cristobalite X-II, which is the polymorph reported to have the structure
of seifertite. However, there is a large discrepancy among various experimental
observations regarding the pressure at which this transition takes place. Motivated by the
recent studies, we aim at understanding how the structure of this "bridging phase",
cristobalite X-I, is evolving under pressure and how it transforms to the seifertite
phase.
We performed in-situ single crystal Raman spectroscopy and X-ray diffraction up to
82 GPa in steps of 3-5 GPa. We used diamond-anvil cells with 250 μm diamond
culet size and neon as the pressure-transmitting medium. In-situ Raman spectra
indicated the phase transition to cristobalite II near 1.5 GPa and the formation of the
X-I phase above 10 GPa, in agreement with previous studies. This high-pressure
phase persists up the highest pressures achieved and it is not quenchable – upon
decompression it retransforms into a phase with Raman spectra very similar to that of
the starting α-cristabolate. Single crystal X-ray diffraction data were collected at
Extreme Conditions Beamline P02.2 at Petra III. We confirmed the phase transition to
cristobalite II and further above 10 GPa to the phase X-I. Indexing of the diffraction
patterns of the phase X-I collected at 17 GPa yields a primitive monoclinic unit cell
with a = 6.5899(9) Å, b = 4.0493(6) Å, c = 6.841(8) Å, β = 98.15(3)° and the
most likely space groups P21/c or P21/n. A preliminary structure analysis reveals
that all Si atoms are octahedrally coordinated at this pressure. Single-crystal X-ray
diffraction data from experiments at higher pressures are currently being analyzed. |
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