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| Titel |
Compression of α-cristobalite under different hydrostatic conditions |
| VerfasserIn |
Ana Cernok, Katharina Marquardt, Elena Bykova, Hanns-Peter Liermann, Leonid Dubrovinsky |
| Konferenz |
EGU General Assembly 2015
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 17 (2015) |
| Datensatznummer |
250104798
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| Publikation (Nr.) |
 EGU/EGU2015-4236.pdf |
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| Zusammenfassung |
| The response of α-cristobalite to high-pressure has been a subject of numerous experimental
and theoretical studies for more than two decades. The results indicated prolific
polymorphism under high pressures, yet no consensus has emerged on what is the
sequence of these pressure-induced transformations. In particular, the structure of
the high-pressure polymorph that appears above ~10 GPa (hereafter cristobalite
X-I), which is believed to be a direct link between the low-pressure (silicon in SiO4
tetrahedra) and the high-pressure (SiO6 octahedra) forms of silica remained elusive. This
study examined the response of α-cristobalite when compressed at different levels
of hydrostaticity, with the special focus on formation and stability of cristobalite
X-I.
The structural behavior of cristobalite under pressure was investigated up to ~80
GPa and at ambient temperature. We investigated behavior of single crystals and
powders, in either (quasy)-hydrostatic or non-hydrostatic environment. In situ high
pressure transformation path and structural behavior was studied by means of Raman
spectroscopy and synchrotron X-ray diffraction (XRD). The samples recovered after
pressure release were additionally investigated by transmission electron microscopy
(TEM).
Low- or α-cristobalite responds differently to high pressure depending on the degree of
the hydrostaticity. The highest attainable hydrostaticity preserves the initial structure
of cristobalite at least up to ~15 GPa. When the crystal experiences even slight
stresses during an experiment, transformation sequence leads to cristobalite X-I – a
monoclinic polymorph with silicon in octahedral coordination. This polymorph
belongs to the family of the high-pressure silica phases that are comprised of distorted
close-packed array of oxygen ions in which silicon atoms fully or partially occupy
octahedral sites. The reflections collected on a single crystal at ~11 GPa can be
indexed by a monoclinic unit cell a=6.658(9) Å, b=4.1077(6) Å, c=6.8947(11) Å,
β=98.31(4)Ë , V=186.6(3) Å3 (Z=8 and Ï=4.28 g/cm3). The structure was solved
in P 21/n space group and refined at this pressure with the final R1 indices of 9%
for 209 unique reflections. The increase in coordination number of silicon from
cristobalite to its six-fold coordinated polymorph does not require any thermal
activation; however the high-pressure polymorph cannot be preserved at ambient
conditions. No other silica polymorph was found to transform to an octahedra-based
structure on cold compression at such low pressures (~11 GPa). This structure could
be accommodated in (quasi)-hydrostatic environment where temperature is not
sufficient to form stishovite. In non-hydrostatic conditions in the presence of uniaxial
stress, cristobalite eventually transforms to seifertite-like SiO2, which is quenchable.
Presence of seifertite might not always require the minimum shock pressures equal to
that of thermodynamic equilibrium (~80 GPa) as it can be clearly formed at much
lower pressures in an environment of uniaxial compression (e.g. dynamic event). |
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